Part 7: Summary of Fossil Creatures
Much of the fossil material that will now be discussed can be seen in photos in the National Geographic article "The Search for Our Ancestors."166
Anthropoids (humans, apes and monkeys) and prosimians (lemurs and tarsiers) are classed as primates. "Anthropoid" means manlike and "prosimian" means premonkey. Prosimians are said to be the earliest form of primates and the stem from which the anthropoids evolved. The prosimians arose in the early Cenozoic Era about 60 million years ago from a certain class of small insectivores resembling Asian tree shrews of today. In Oligocene time, about 30 million years ago, the prosimians gave rise to three primate groups: the New World monkeys, the Old World monkeys, and primitive apelike hominoids. This evolution is poorly documented in the fossil record. What does show up is a pattern gradually leading to the physical structure of modern apes and humans: the development of binocular vision, the big toe and opposable thumb, a large increase in brain size relative to body size, and a tendency to hold the body erect.
The earliest humanlike, or hominid, fossils come from East Africa. These have generally been lumped into a species called Australopithecus afarensis, although not all paleontologists agree they belong to the same creature. The Hadar region of Ethiopia has yielded several hundred fossil fragments, including the 40% complete skeleton known as "Lucy." Other fossil fragments have been found at Laetoli and in Olduvai Gorge in Tanzania. Laetoli is noted for the 3.6 million year old trail of footprints two hominids left in volcanic ash that has since hardened and been buried. These have been attributed to A. afarensis. The finds range from 2.9 to 3.7 million years old according to potassium-argon dating.167 A. afarensis has both humanlike and apelike features. Many paleontologists agree that the pelvic and overall skeletal structure show it habitually walked erect, although the skeleton was not quite like a modern human and was in some respects somewhat apelike. The fossil bones are consistent with the fossil footprints of Laetoli, and together these provide strong evidence for upright walking. The angle between the upper and lower leg bones, the femur and the tibia, is similar to that in humans but different from that in most apes, possibly indicating bipedality. The footbones have the platform style of modern humans and lack the opposable big toe of apes. Some researchers feel that the overall skeletal structure is compatible with both bipedality and arboreality, and feel that A. afarensis walked with a bent-hip, bent-knee gait, somewhat like the modern gibbon. The hand bones look more apelike than human. The arms were somewhat longer and the legs somewhat shorter compared to overall body size than in modern humans. The brain size was in the range of a modern chimpanzee, about 400 cm3, but the skull is quite different from a chimp's.168 A. afarensis had a ratio of brain to body size significantly higher than apes. The position of the foramen magnum, the aperture through which the spinal cord leaves the skull, is positioned midway between that of a chimp and a human, indicating a more or less upright posture.169 The upper dental arcade shows features like both humans and apes, but has its own unique features as well. The canine teeth are small, as in humans, in contrast with the large ones of chimps, gorillas and fossil apes. Tooth wear patterns suggest that it ate mostly fruit. Adult weights ranged from about 50 to 120 pounds, and it appears to have stood three to five feet tall. The overall impression from a photo of the Lucy skeleton is of an ape's head on top of a humanlike body. Lucy stood about three to three and a half feet tall.170 There is broad agreement that A. afarensis was ancestral, or at least a close cousin to the ancestor, of later hominids.171 This is, of course, disputed by some paleontologists.
The next hominids in the fossil sequence seem to consist of several varieties of australopithecines. They showed up between two and three million years ago, and disappeared about one million years ago. They had brain sizes somewhat larger than A. afarensis, about 450 to 550 cm3 (within the range of gorillas) and were somewhat larger overall, up to about 150 pounds in the larger robust specimens. The position of the foramen magnum, the spinal cord aperture in the skull, is located more toward the front of the skull than in A. afarensis, again indicating an upright posture. The angle between the femur and the tibia is similar to that in humans, again indicating bipedality, and the overall skeleton is much like A. afarensis. The hands were much more humanlike than those of A. afarensis.172 Tooth wear patterns again suggest a diet of fruit. The dental arcade is similar to that in humans and is quite different from A. afarensis. There apparently were several forms of australopithecines, which included the lightly built, or gracile form called A. africanus, and forms that had a more robust skeleton and skull, which are called A. robustus and A. boisei. There is some disagreement as to whether these represent different species or are large variations between male and female in a single species. Paleontologists do not agree on whether A. afarensis was an ancestor of A. africanus,173 although the 1985 discovery at West Turkana of a nearly complete skull, which has been termed the "Black Skull," seems to have features of both A. boisei and A. afarensis174, 175 and has been termed an intermediate between them. The australopithecines as a whole have been described as "essentially bipedal apes with modified dentition."176
A third category of hominid is called Homo habilis. It had a cranial capacity of about 650 to 800 cm3 and is the first creature whose skull looks more human than apelike to non-specialists. Its remains are the most fragmentary of all the hominid categories, and so are the most in dispute. A 1986 find of leg and arm bones, in addition to some cranial parts, suggests that it was about the size of A. afarensis and A. africanus,177 although some paleontologists would assign the find to other species.178 H. habilis appeared in the fossil record from about two million years to 1.8 million years ago and is not generally considered a descendant of A. africanus. It was contemporary with one of the australopithecines, A. boisei, which lived until about one million years ago. It walked upright, but its hand, leg and foot bones had somewhat apelike characteristics as well. It apparently ate mostly fruit, but may have scavenged carcasses for bone178a marrow. While some paleoanthropologists have considered that H. habilis may have been a variety or descendant of A. africanus, this view seems to have been dropped by the late 1980s. Coincident with the appearance of H. habilis in the fossil record is the beginning of the archaeological record: crude stone tools are often found in association with its remains, sometimes in association with animal bones.179, 180 C. Loring Brace said that stone tools were "found in association with the partially butchered skeleton of an extinct hippopotamus all silted over in the deposits of an ancient river delta."181
A fourth category of hominid is called Homo erectus, which first showed up in the fossil record about 1.6 million years ago and disappeared about 200,000 years ago. Its cranial capacity apparently ranged from 800 to 900 cm3 in some specimens from the Zoukoutien Cave in China to 1200 cm3 in other populations. By comparison, modern humans have an extreme range of 1000 to 2000 cm3, with an average of about 1360 cm3. The jaw was robust and had no prominent chin, unlike modern humans. The forehead was extremely low and sloping, and above the eyes were large brow ridges. Below the neck its skeleton was similar to modern humans, except usually much more massive in the limbs and muscle attachment points. During the time H. erectus existed, there appeared a number of important "firsts," including evidence of systematic and cooperative hunting, use of fire, systematic toolmaking, firm evidence of frequently formed and seasonal home bases and camp sites, evidence of habitation outside Africa, reduction of sexual dimorphism, and evidence of extended childhood. The use of fire is documented at Zoukoutien Cave, with an ash accumulation at one hearth site 20 feet thick. The sophistication of stone tools and campsites, and evidence of systematic hunting and butchering of animals, increases through time.182 As evidenced by tooth wear patterns H. erectus apparently ate a variety of foods, including meat and underground tubers, in contrast to the australopithecines and H. habilis, who were fruit eaters.183 They seem to have practiced cannibalism,184 although this is disputed.
A fifth category of hominid is Neanderthal man, who was very similar to fully modern humans and is usually classed as a variety of Homo sapiens. Neanderthals were, on average, much more robustly built, but still fall in the upper limit of the modern range. The cranial size averaged slightly larger than in modern humans, about 1400 cm3. The skull tended to be longer and flatter than modern ones, with a low, sloping forehead and prominent brow ridges, even though the height of the top of the skull above the eyes was often the same. The impression is of a massive, long head atop a bull neck. The overall skeletal structure has features much like H. erectus. This has given rise to continuing debate about whether Neanderthals were their descendants. Neanderthals show up in the fossil record from about 150,000 years to 32,000 years ago. According to last appearances in the fossil record, they disappeared in a wave flowing east to west, between 45,000 and 32,000 years ago.185 They apparently buried their dead. Some paleoanthropologists speculate that Neanderthals may have been an odd variant of man local to Europe and the Middle East, where most of the remains have been found.186, 187 See Scientific American188 for a good discussion of Neanderthals, with photos of several skulls.
Fully modern humans first appear in the fossil record about 40,000 years ago, in Cro-Magnon, France, although recent interpretations of some fossils and molecular biological evidence push that back to over 100,000 years.189 Art forms first appear in association with their remains, as does a tool complex that rapidly increases in sophistication. Included among modern humans are forms called archaic Homo sapiens, but there is good reason to believe that this categorization is in error. As a result, there is much speculation about the nature of the sequence of creatures from Homo erectus to modern humans, but that an overall sequence can be seen in the fossil record is clear. Starting about 500,000 years ago, more modern cranial features show up with increasing frequency in the fossil specimens. The difficulty for classification is that these specimens have many clearly modern human features but have other features that are far outside the norm for modern humans.190 A living person who had these ancient skull features would be considered deformed. Various species of sabretooth cats, which became extinct about one million years ago, as well as many other extinct animals, were contemporary with all the hominids except modern man.191 For a slightly out of date overview of the above material see "The Descent of Hominoids and Hominids" in Scientific American.192
Overall, the views of hominid evolution are not very clear, and are subject to revision as new fossil evidence forces new interpretations. However, there is a pattern to be seen in the dating of the fossils. This pattern can be clearly seen in a chart in Human Evolution: An Illustrated Introduction, Second Edition,193 which shows the overlap in time of some species and the discontinuity in time between others. The view is that the fossils that have been found represent only a small subset of the total number of species that existed, and that the evolutionary history is "bushy." As Roger Lewin said:194
The typical evolutionary pattern is that, once a new lineage is established there follows a quite rapid radiation of species: in other words, the group, or clade, is bushy from the beginning, not just half way through.
This is one of the ideas of "punctuated equilibrium."
Let us now consider some specific areas relevant to human evolution that Creation ignores.
Increase of Brain Size
Humans are obviously by far the most intelligent creatures on earth. Evolutionists see a progression in intelligence, as measured by brain size, from the earliest hominids to man. Human Evolution: An Illustrated Introduction195 describes the evidence and conclusions:
A major question about hominid history is why we became so very intelligent, particularly over the past two million years. The primate order as a whole is the most generously endowed of all animal orders as far as brain capacity relative to body size is concerned. And within the order there is, in general, an ascending scale of relative brain capacity that runs from the prosimians through the monkeys to the apes. The human brain is three times as big as an ape's would be if it had the same body size.
Increase in brain size is a persistent theme of evolutionary history in two particular respects. First, the progression through more and more advanced animal groups -- from amphibians through reptiles to mammals -- is marked at each step by a substantial leap in the degree of encephalization displayed by each group as a whole. These stepwise mental increments between the major animal classes reflect gestalt jumps in the complexity of neural processing involved in the animal's daily lives. Each increment has been accompanied by an ever greater learning capacity as opposed to genetically determined fixed action patterns.
The second pattern of brain size increase in evolution is seen within individual lineages through time. This, typically, is associated with a parallel tendency for an increase in body size through evolutionary time. Some of the growth in brain size from around 400 cm3 in the earliest hominids to an average of 1360 cm3 today can be accounted for in terms of an increase in the size of the body, but most cannot. There was a real and dramatic enhancement of encephalization in hominids that is not matched, nor even approached, by any other animal lineage.
Science and Earth History196 presents a graph which plots the average body weight versus average brain size for the great apes, the australopithecines, and Homo, i.e., the Pygmy chimpanzee, chimpanzee, orangutan, gorilla, A. africanus, A. robustus, A. boisei, H. habilis, H. erectus and H. sapiens. The result is that a straight line can be drawn through the points for each of the three classes, with the line for the australopithecines intersecting the line for Homo at A. africanus, and the great apes on a line by itself. This means that the brains of australopithecines are always larger relative to body size than the apes. The same holds true for Homo with respect to the australopithecines and the apes, except that the rate of increase of brain size with body size within the group is much faster. This graph shows relationships similar to what have been inferred from comparative anatomy, and is interpreted as strongly suggestive of evolutionary relationships.
Another area of comparison is in the gross organization of the brain. The brain has various lobes (frontal, parietal, occipital, etc.) that perform special functions such as vision, motor control and the higher mental functions, and which leave an impression on the inside of the skull. The impression can be seen in an inside cast (endocast) of the skull. It is relatively easy to compare this overall organization in man and the apes, but it can only be done using endocasts in fossil creatures. This work has been done for the australopithecines, and Science and Earth History describes the results:197
Anthropologist Ralph L. Holloway made a comprehensive comparative study of the endocasts of all available fossil hominid brains (1974). Of special interest to us are his findings on the brain of Australopithecus, which is classified by mainstream paleontology as a hominid, but considered an ape by the creationists... Holloway's conclusion is highly positive:
Fortunately, no matter what controversy may surround the question of how these early African hominids are related to one another it has very little bearing on the question of their neurological development. The reason is that in each instance where an endocast is available, whether the skull is less than a million years old or more than two million years old, the brain shows the distinctive pattern of hominid neurological organization. (p. 109)
Holloway's description of the differences in the two brains is quite technical in spots, but he notes that the australopithecine frontal lobe is larger and more convoluted than in pongid endocasts. Brain height and form of the temporal lobes show the hominid configuration. The lunate sulcus, when it can be found, lies in the human position... the australopiths had brains much like those of modern humans, but unlike those of pongids. Creationists take note! This is strong corroborative evidence contrary to your assertions that the australopiths were actually apes.
Holloway's conclusions have, of course, been disputed by some paleontologists:198
Just recently Holloway's conclusions have been challenged by Dean Falk of Purdue University. Although she agrees with Holloway that brains of Homo species are reorganized in the human direction, she contends that australopithecine brains are essentially apelike. The precise location of some of the key fissures and divisions between lobes is often very difficult and open to interpretation. In this case the differences of opinion continue unresolved.
Dental Eruption Patterns
Teeth in apes and humans erupt from the gums in a distinctive order that is different in each species. The following information from Human Evolution: An Illustrated Introduction, Second Edition,199 shows the significance of this pattern:
The pattern of eruption of permanent teeth in modern apes and humans is distinctive, as is the overall timing. Just recently anthropologists have been debating this aspect of hominoid dentition, specifically asking how early hominids fit in: were they more like humans or more like apes? Although the issue remains to be fully resolved, there are indications that until rather late in hominid history, dental development was in many ways rather apelike, particularly in its overall timing...
University of Michigan anthropologist Holly Smith recently analyzed tooth eruption patterns in a series of fossil hominids and concluded that most of the early species were distinctly apelike. For Homo erectus, which lived from 1.5 million until about 400,000 years ago, the results were somewhat equivocal, but with strong apish overtones. The human pattern was apparent in a Neanderthal child who died about 60,000 years ago...
If Smith and [others] are correct, it seems that until relatively recently in evolutionary history, hominids followed a distinctly apelike pattern of dental development. This is important for its implication about the period of infant care. Once infant care becomes prolonged, which becomes necessary when postnatal brain growth is significant (see unit 27), then social life becomes greatly intensified. The dental evidence indicates that this prolongation may have begun with Homo erectus, which is in accord with data on increased brain size.
Almost all paleontologists agree that early hominids were bipedal, or walked erect. They disagree, however, on exactly how they walked -- whether they were fully erect, in the manner of modern humans, or walked in a bent-leg, bent-hip style. The flavor of the disagreement on australopithecine locomotion is illustrated by the following, from Science and Earth History:200
But not all the experts agree that A. afarensis had forsaken the forest for open ground. Anthropologist Randall L. Susman and anatomist Jack Stern interpret Lucy's bone structure as indicating good adaptation for climbing but that she also had developed efficient locomotion... In rebuttal to Susman and Stern, C. Owen Lovejoy, an anatomist, argues that Lucy's hip is "beautifully adapted for bipedality and poorly adapted for climbing."
At the Institute of Human Origins a conference called "The Evolution of Human Locomotion" was held in 1983, where a group of scientists compared notes. An interesting account appears in Lucy's Child201 of the argument between Susman and Stern, and Lovejoy, and shows how when Susman and Stern were presented with unpublished information at the conference they conceded that that they had "much reduced faith" in some of their conclusions. They had used casts of the Lucy pelvis and other bones to do their evaluations, whereas Lovejoy had the original fossils. The consensus of the conference was that a form of bipedality was the major mode of locomotion, but there remained a number of unanswered questions.
Another book described the results of a later symposium:202
In a recent symposium on the fossil record of hominoids (Delson, 1985) there was a lot of discussion about Australopithecus afarensis. It was generally (but not unanimously) concluded that the fossil material presently referred to as A. afarensis did in fact represent a single variable species. Further, it appears on the basis of detailed study of nearly complete skeletons to have been a species that '...spent a significant amount of time in the trees without being as adept as an ape and that also lived on the ground without being as quick and agile on two legs as are humans' (Susman et al., 1985: 189). In sum, its features (within single skeletons) are intermediate between those of apes and humans.
A National Geographic article203 showed side by side photos of the femoral condyle (bottom end of the femur) of a chimpanzee, Lucy and a modern human. According to the caption, the chimpanzee's rounded condyle is not specially adapted to any single leg position, while Lucy's and the modern human's show a much larger surface that reduces pressure on the knee when the leg is extended. Interestingly, the extended surface in Lucy's femur is significantly smaller and more rounded than in the modern human's, and looks structurally intermediate between the chimp's and the human's.
Lovejoy published an article in Scientific American in 1988 explaining his opinion on why some of Lucy's anatomical features showed she was indeed bipedal.204 The accompanying photographs and drawings comparing human and chimpanzee pelvises and other bones with those of Lucy are most enlightening. Here are some excerpts:
[Lucy's] pelvis bears all the hallmarks of bipedality seen in our own. Its ilia are much shorter than those in the pelvis of an ape. The shortening would have lowered the trunk's center of mass and made it easier to keep upright. The ilia have also become bent around to provide lateral attachment for the abductor muscles that stabilize the bipedal pelvis when it is supported on one leg. The attachment points for the gluteus maximus, abductors and quadricepts can be seen, and they indicated that in Lucy these muscles had attained a size and disposition remarkably similar to our own arrangement... In one respect Lucy seems to have been even better designed for bipedality than we are. Her ilia flare outward more sharply than those of a modern pelvis and her femoral necks are longer. Her abductor muscles thus enjoyed a greater mechanical advantage than these muscles do in modern females...
Why should a three-million-year-old hominid have had this mechanical advantage over her descendants? The answer lies in the accelerated growth of the human brain during the past three million years. Lucy's pelvis was almost singularly designed for bipedality. The flairing ilia and long femoral necks increased her abductors' lever arm, but they yielded a pelvis that in top view was markedly elliptical, resulting in a birth canal that was wide but short from front to back. The constriction was tolerable because Lucy predated the dramatic expansion of the brain; her infant's cranium would have been slightly more complex than in an ape, but much easier than the modern human birth process... As human ancestors evolved a larger brain, the pelvic opening had to become rounder. The pelvis had to expand from front to back, but at the same time it contracted slightly from side to side. In the process the flare of the ilia was reduced, leaving us with a somewhat shorter abductor lever arm than Lucy's...
The close resemblance of Lucy's pelvis to that of a modern human and its dramatic contrast to the pelvis of a chimpanzee make it clear that she walked fully upright. But was her bipedal progression truly habitual? Had she forsaken all other kinds of locomotion? The muscular rearrangements that enabled her to walk upright would not have allowed efficient quadrupedal movement on the ground. Perhaps, however, she often took to the trees and climbed, as most primates do, using all four limbs... For natural selection to have so thoroughly modified for bipedality the skeleton Lucy inherited, her ancestors must already have spent most of their time on the ground, walking upright. Analysis of the Lucy fossil, however, can yield more direct evidence.
The analysis focuses on the neck of the femur, where much of the stress of locomotion is concentrated. When the leg is bearing weight, the hip joint transmits the weight of the torso to the femoral neck. The neck acts as a cantilevered beam: a beam that is anchored at one end to a supporting structure (the shaft of the femur) and carries a load at the other end. Cantilevering results in high bending stresses at the beam's anchorage -- compression along the bottom of the beam and tension along the top -- and the stresses increase with the length of the beam. A long femoral neck such as Lucy's reduces pressure on the hip joint by improving the leverage of the abductors, but the neck itself is subject to higher bending stresses.
The femoral neck of the chimpanzee is much shorter than the modern human one; nonetheless, it is robustly engineered to withstand the loads imposed by the animal's terrestrial and arboreal acrobatics. A cross section of the bone reveals a central marrow-filled channel surrounded by a thick layer of dense bone. Dense bone is weaker under tension than it is under compression, and so the upper surface of the structure, which will be subjected to tension when the neck is bent, carries a markedly thicker layer of bone. With this ridge of thick bone (a bone "spike" in cross section), the chimpanzee femoral neck imitates the principle of an I beam: material is placed where it can best resist bending stresses.
Because the human femoral neck is longer than the chimpanzee's and must resist the combined force of body weight and abductor contraction, one would expect it to be even more robustly constructed. A cross section of the human bone reveals a surprise: the outer ring of solid bone is thick only at the bottom, and the rest of the neck is bounded by a thin shell of bone and filled in by a lattice of fine bone plates called trabeculae. Such porous bone, as one might expect, is weaker than solid material. The upper part of the femoral neck, where tensile stresses are presumably the highest, actually contains less bone than any other part of the structure. How can our femoral neck survive the greater stresses imposed by its length and function when it seems so much less sturdy than the femoral neck of the chimpanzee?
The answer lies in the action of muscles that operate only in bipedal locomotion: the abductors. These muscles have lines of action that are not vertical but are sharply inclined, which makes them roughly parallel to the femoral neck. When they contract, they push the femoral neck into the hip socket, compressing the neck along its length. This compressive stress combines with the stresses that result from bending (tension on the top of the femoral neck and compression on the bottom). The effect is to eliminate tension at the top of the femoral neck and create a gradient of increasing stress running from the top of the femoral neck, where stress is now minimal, to the bottom, where stress is very high but purely compressive. The bottom of the human femoral neck has a robust layer of solid bone, and even the porous bone that fills in the rest of the section is reasonably strong as long as it remains under compression.
Other muscles work with the abductors to keep the femoral neck under compression when it is loaded. The most important of them is the piriformis, which originates on the front of the sacrum and extends to the outer end of the femoral neck. That orientation enables the muscle to increase the femoral neck's level of compression. The synchronized action of all these muscles when body weight is supported on one leg makes it possible for this seemingly fragile bone to cope with its load.
Because of its distribution of bone, however, the femoral neck is indeed vulnerable if the abductors and other muscles do not act in the proper synchrony. The femoral neck is a primary site of fracture in old age, and not just because bone quality is reduced in old people. These "broken hips" are also a product of reduced muscular coordination. Thus the design of the human femoral neck requires the muscular action of bipedal walking. The bone is poorly engineered for climbing and arboreal acrobatics, where it would be frequently subjected to bending stresses without being compressed at the same time by the abductors.
The femoral neck in Australopithecus, because it was even longer than that of modern humans, was subject to even greater bending stresses. If these human ancestors had often taken to the trees, stressing their femoral neck without coordinated compression by the abductors, the bone would have had to have been even more robust than it is in the apes. Was it? The same site where Lucy was found also yielded several femurs that had broken during their long burial, affording a view of the neck's internal structure. Each specimen clearly shows the human feature of thin bone on the upper part of the femoral neck. Lucy's femoral neck, then, was suited exclusively for bipedality. She was not just capable of walking upright; it had become her only choice.
Lovejoy then lists the other skeletal characteristics he feels favor bipedality and rule out other modes of locomotion.
Concerning the outcome of the above mentioned 1983 conference Donald Johanson, finder of Lucy and coauthor of Lucy's Child said:205
I was convinced that Lovejoy and Co. had made a convincing case, at least with respect to anatomy below the waist. When it came to upper-body adaptations, I was not quite so sure. What bothered me were Lucy's long arms and strong, curved fingers. Both would be a big advantage in moving about in the trees, regardless of whatever commitments a species had made toward bipedality from the waist down. On the other hand, Lucy's long arms and curved fingers might be examples of what Lovejoy calls "evolutionary baggage" -- traits left over from a more distant ancestor, not yet lost but no longer needed. I, for one, cannot imagine long arms somehow being a disadvantage to a full bipedal hominid.
A good summary of the arguments among scientists about australopithecine bipedality can be found in Human Evolution: An Illustrated Introduction, Second Edition.206 It points up the arguments of many scientists who disagree with parts of Lovejoy's analysis. It said:
Superficially, A. afarensis does indeed appear to be essentially apelike above the neck and essentially humanlike below the neck. This is a good example of mosaic evolution, in which different parts of the body change at different rates and at different times. In fact, mosaicism is even more pervasive and detailed in this species, because, throughout the postcranial skeleton, anatomy associated with bipedal locomotion is developed to different degrees in different places. One of the continuing debates over this species concerns the interpretation of the various primitive aspects of the postcranial anatomy: do they imply that, like most hominoids, A. afarensis still spent a significant amount of time in the trees? Or, were these primitive aspects of the anatomy simply genetic holdovers from an earlier adaptation, having no particular behavioral significance in A. afarensis? And while individuals were on the ground, was their bipedalism significantly different from or essentially the same as that in modern humans?...
The differences of opinion in the A. afarensis locomotor debate stem partly from a lack of agreement over what exactly the anatomy is in certain instances, and differences in functional interpretation of other aspects of the anatomy. The opposing views were aired on an equal footing at a scientific symposium organized by the Institute of Human Origins in Berkeley in 1983. Since that time most publications have favored the partially arboreal, bent-hip, bent-knee bipedal locomotor posture.
The question of australopithecine bipedality has been discussed at length by creationist authors such as Duane Gish,207 who typically cite only references saying australopithecines did not walk erect. The issue is important to creationists because they cannot resolve the problem of why God would create species that hardly differed at all from modern man except in body size and the size and shape of the skull. They must discredit fossil interpretations because they cannot dispute that bipedality seems to be an exclusively human characteristic. If they admitted that the australopithecines could have walked on two legs they would have to admit that these might well be an example of the dreaded apeman or "missing link." For some reason they do not seem to like the other alternative -- that australopithecines were bipedal apes but had nothing to do with man's ancestry. The Watchtower Society generally has an attitude similar to that of the creationists, but rather than discussing the issue straightforwardly, it treats it as it does all other difficult issues -- it ignores it.
A wide variety of stone tools has been discovered from as far back as 2.5 million years. The fossil record of tools is strongly biased toward stone because tools made from animal or plant materials are perishable. Human Evolution: An Illustrated Introduction gives a description of the findings:208
The earliest putative stone artifacts discovered so far come from Ethiopia and are dated at around 2.5 million years. They are a collection of extremely crude scrapers, choppers and flakes, each the product of a very few blows with a hammerstone. In archaeological terms, they are described as an example of the Oldowan industry. Looking forward through time from this earliest example of toolmaking, one gains two powerful impressions.
First, there is a striking continuity through vast tracks of time. Tools such as these earliest artifacts represent the dominant form of stone tool technology for more than a million years. About 1.5 million years ago a new industry emerges, which is known as the Acheulian. This industry represents only a modest advance over the Oldowan, and is characterized by the presence of tear-drop shaped handaxes. The Acheulian did not replace the Oldowan immediately, but merely accompanied it through half a million years of human history, after which it became the dominant form. Even so, tools that can be described as Oldowan in type were still to be found in Eastern Asia right up to 200,000 years ago and less. In Africa and Europe the Acheulian continued as the main tool industry, until it too began to be replaced around 150,000 years ago.
The second powerful impression of stone tool technologies up to about 150,000 years ago is their essentially opportunistic nature. Although there is a gradual imposition of form and style through that great swath of time, it is really rather minimal. Only after 150,000 years ago is there a strong sense of stylistic order.
The characteristics of the Oldowan technology have been meticulously studied by Mary Leakey through her many decades of excavation at Olduvai Gorge, the site after which the industry is named. It is a collection of perhaps half a dozen main forms in which the so-called pebble chopper predominates. Discoids, spheroids, polyhedrons, core scrapers, flake scrapers and hammerstones are some of the items in the industry, together of course with a large representation of debitage, which includes small sharp flakes. Mary Leakey is careful to point out that debitage does not necessarily mean useless waste, as the small flakes very probably were deliberately struck as stone 'knives'. The principal raw material was lava cobbles, although chert and other similar rocks were sometimes used.
The oldest levels at Olduvai date back to almost two million years ago, and it is here that the Oldowan industry begins. The industry continues for one million years and more, but becomes a little more refined, adding a few more tool categories, such as awls and protobifaces. These advances, which appear about 1.5 million years ago, are recognized as the Developed Oldowan. At about the same time, a new industry, the Acheulian, appears in the record. The handaxe, as has been noted, is the hallmark of this new industry, and it represents the first tool in which a predetermined shape has been imposed on a piece of raw material.
The principle invention of early stone tool technologies was that of concoidal fracture: strike a core at an angle and a flake, large or small, is removed. The resultant tool is very much determined by the shape of the starting material. However, the bifacial symmetry of the handaxe, with its two sharp converging edges, required the shape to be 'seen' within the lump of stone, which is then worked towards with a series of careful striking actions.
Some handaxes of later times were aesthetically pleasing products of hours of skilled labour. Exactly what they were used for is still something of a mystery, but the combination of a long sharp edge with bulk and weight makes them exceedingly efficient at slicing through even the toughest hide, including that of elephants and rhinoceroses. Cleavers, which were also part of the Acheulian technology, had similar properties, but are cruder, less elegant implements.
The Acheulian industry, which had about ten principle implements, continued for more than a million years, before it was replaced by the wide range of much more refined tools of the Mousterian culture: the products of Homo sapiens neanderthalensis. During that long period of time the best examples of the Acheulian technology became increasingly more elegant, but throughout this time there were crude examples, similar to those from the beginning of the record.
Glynn Isaac points out that from the very beginning of stone tool-making the range of implements produced does not increase significantly. What does change through time, however, is the degree of standardization, the frequency of producing certain forms against a background of 'noise'. Ad hoc stone knapping gives way to deliberate imposition of preconceived order.
The duration of the Acheulian saw certain idiosyncratic expression, for example in details of the shape of tools and their size. Differences in availability of suitable stone, different specific technological needs and an element of individual style would have contributed to this. There was, however, a certain homogeneity at any particular time. There was, in a sense, just one Acheulian culture. From 150,000 years onwards, this pattern of culture began to change, at an ever accelerating pace. It is, as Isaac says, as if some threshold was passed: 'a critical threshold in information capacity and precision of expression.'
Between 250,000 and 150,000 years ago the pace of change of tool technologies began to accelerate. Whereas continuity was the hallmark of tool-making prior to this turning point, change began to dominate thereafter. Moreover, each succeeding culture contained a larger array of finer implements than the last. Bone, antler and ivory became increasingly important raw materials for tool-making, particularly for fine, flexible and sharp implements. And, most striking of all, there began to emerge a previously unseen degree of variability in the form of tool-kits found in neighboring sites, a variability that has been explained variously as discrete functional differentiation or cultural expression through style.
The major post-Acheulian tool culture was the Mousterian, associated with the origin of archaic Homo sapiens: this was known in Europe as the Middle Palaeolithic. The Mousterian continued through to around 35,000 to 40,000 years ago, which coincided with the emergence of fully modern humans, Homo sapiens sapiens. The 100,000 year tenure of the Mousterian was overtaken by a succession of tool-kits, known as the Upper Palaeolithic, that displayed the ever increasing virtuosity of the tool-makers, so much so that some implements lost any function they might have had and instead assumed some kind of abstract symbolism. (In Africa the equivalent periods are called the Middle Stone Age and the later Stone Age.)
Towards the end of the Acheulian era, some 100,000 years ago, there arose in South Africa a new technique for the production of large flakes that was to foreshadow later developments in tool technology. Known as the Levallois technique, this new development involved a much more intensive preparation of a core than had hitherto been the practice. Virtually complete flakes could then be struck from the core at a single blow, although they were typically retouched to give the final desired shape. It was principally a refinement and development of the Levallois technique that formed the basis of Mousterian tool technology of the Middle Palaeolithic.
One immediate practical consequence of careful core preparation is a greater efficiency in the use of raw materials. For example, the basic Acheulian method yielded just 5.1-20.3 cm of cutting edge from 0.45 kg of flint, whereas a Mousterian tool-maker could strike 10.2 m of cutting edge from the same amount of starting material. This trajectory of greater efficiency soared following the origin of modern humans, 40,000 years ago, who could manufacture 12 m of cutting edge from 0.45 kg of flint, struck in the form of long sharp blades.
The book then goes on to describe details of the tools of the later paleolithic tool industries. It is clear that evidence of ancient toolmaking provides a much broader base for evolutionary claims than does the evidence based on skeletal remains alone. It also provides a basis for claims that the evolution of man was continuous, as the following from anthropologist C. Loring Brace shows:209
Once stone tools appear in the prehistoric strata 2 million years ago, they continue to provide us with an unbroken record of the activities of our ancestors. The chances of fossilization and preservation of any given individual of a creature who lived at such a low population density as the early hominids are pretty small, and it is not surprising that hominid fossils are such rare and spotty phenomena. Stone tools, however, are made of imperishable materials, and once they become a regular part of the hominid cultural repertoire, the archaeologists who study them can trace those activities to which they pertain and tell us what regions of the world their makers were occupying and what changes they made in their way of life through time.
The occasional glimpses we get of the fossilized fragments of the makers of that unbroken cultural tradition enable us to check on the course of human evolution with the conviction that the earlier fossil hominids are indeed the ancestors of the ones who come later and show those modifications that foretell the emergence of modern form. The unbroken continuity of stone tools from the levels at the bottom of Bed I in Olduvai Gorge to the time five to six thousand years ago when written accounts begin to provide us with an articulate picture of human doings serves as an abundant check and confirmation of the anatomical continuity provided by the interrupted sequence visible in the course of the fossil record.
There is another side to the story. One author, after describing the well known fact that chimpanzees use a sort of tool by using a twig to extract termites from rotting logs, says:210
It is even more interesting to hear what orangutans can do with a little instruction. They have been taught in an hour or two to make crude stonecutting tools like those attributed to man's "ancestors." Although apes have never been observed making stonecutters by themselves in the wild, the experience does show that a specialized primate of their limited cranial capacity (350-400 cubic centimeters) can manage this task. And it raises the distinct possibility that over the last few million years there could well have been slightly brainier apes not ancestral to man who had learned to chip a few flakes off a stone without human instruction. Toolmaking remains an important indicator of man, but with the simplest stone and bone implements it is well not to assume too much.
Excavation of a Campsite
The following are some excerpts from a description of the excavation of a 1.5 million year old African campsite designated Site 50:211
... what evidence is there that hominids 1.5 million years ago transported bones and stones to a favoured location where both would be processed, the stone to provide tools and the bones food?
When hominids were living in the area, Site 50 was located in the middle of a large floodplain on the eastern shore of Lake Turkana. The landscape was typical open savannah, with short Acacia and Commiphera trees scattered over the terrain, while thicker groves of taller trees lined the water course that laced the floodplain. Plains animals such as giraffe, zebra, antelope and baboons, lived there. Site 50 was formed on a sandy bank in the crook of a winding river, a location that would have offered access to water, shade from the sun, a supply of fruit and berries from nearby bushes, and ready access to lava cobbles suitable for tool-making.
During the two-year long excavation, 1405 stone fragments and 2100 pieces of bone were recovered, distributed in a thin layer over an area of about 200 m2. The density of bones and stones within the site was more than ten times higher than outside the putative campsite area. Bones and stones within this area were concentrated into two distinct spots, suggesting two locations of particular activity.
Less than one-half of the bone fragments could be positively identified, but it was clear from those that could be that a wide range of animals was represented at the site, albeit by just a few bones in most cases. Remains of every major group apart from carnivores, rhinoceroses and elephants were present. One particularly interesting specimen was the shattered shaft of a leg bone (humerus) of an eland-sized antelope, Megalotragus. By careful reconstruction, Henry Bunn was able to fit seven large fragments together, which revealed fracture damage of the sort inflicted by hunter-gatherers when in search of marrow from long bones. Moreover, Bunn found that the end of a leg bone from the same type of animal (and possibly the same individual) bore a set of short narrow marks such as would be made by a sharp instrument used to deflesh the bone. Experiments with sharp stone flakes and modern bones, together with microscopy of the fossil bones, confirmed the presence of 'cut marks' on at least half a dozen major bones at Site 50...
Among the putative stone artifacts were 59 items that, according to traditional classification, would be labelled choppers, polyhedrons, discoids, core scrapers, scrapers or flakes and flaked core/cobble fragments. In addition, there were more than 1300 flakes and flake fragments. Although the numbers sound large, a practised stone knapper could have produced this type of assemblage in about an hour...
Another member of the research team... spent many patient hours trying to fit fragments of stone back together, a project rewarded by the assembly of 53 sets of two or more pieces [p. 62 illustrates the reassembly of six fragments]. The conjoining of a series of flakes, sometimes together with the core from which they were struck, sometimes without, gave an important coherence to an otherwise apparently jumbled heap of broken stones. The distribution of the conjoining sets and their concentration into two distinct activity areas is strong testimony to deliberate hominid activity.
The natural presumption is that the stones were gathered nearby, taken to the site and then struck in order to produce sharp flakes with which to process the pieces of carcass that were also brought to the river bank site. In a few instances, this presumption has been confirmed about as strongly as it could be with present techniques. [Two workers] have examined the sharp edges of some of the flakes and they detect on two of them the unmistakable evidence of meat-slicing. Both of these flakes had been found within a metre of the cut-marked leg bone. Moreover, another flake from the site shows evidence of use on soft plant tissue, which is a rare signal of the presumed common use of vegetable foods by early hominids.
The size and material accumulation of the site indicated it was used by a small group of individuals for a relatively short time, perhaps just a few days. It is a reasonable conclusion from the data accumulated in the excavation and their subsequent analyses that the hominids of the time were transporting stones, parts of carcasses and plant foods to the site, where they were then processed. The range of bones on the site indicated scavenging rather than hunting as the primary source of meat for these hominids.
A photograph of cut marks on fossil bone can be seen on page 32 of the book from which this quotation was taken.
The Molecular Clock
Molecular biology seems to provide strong evidence for a close relation between apes and man. It has been used as a sort of clock to try to determine when various species branched apart from one another in the evolutionary sequence. Human Evolution: An Illustrated Introduction describes the general idea and presents some conclusions:212
The idea of using molecules as phylogenetic clocks rests on one simple assumption: once two species separate in evolution the genetic material (DNA) in the two lines accumulates changes or mutations. The longer the separation time, the greater will be the sum of accumulated mutations. If the rate of accumulation remains steady through the ages -- and there is a good deal of discussion on this point -- a measure of the biochemical differences between the two species can be converted into a measure of the time since they derived from a common ancestor.
Morris Goodman, of Wayne State University, kindled modern interest in molecular clocks when in 1962 he published data on the immunological properties of the protein albumin, which showed chimpanzees, gorillas and humans to be closely related to each other while the gibbon and orangutan were more distant cousins from this trio. According to the comparison of albumin from these three species, humans are as close genetically to chimpanzees and gorillas as these two apes are to each other. This was quite a shock, as chimpanzees and gorillas are morphologically rather similar to each other and to the orangutan, whereas humans seem unquestionably distinct from them.
Two biochemists at the University of California at Berkeley, Vincent Sarich and Allan Wilson, then published a landmark paper in 1967 that put a date on the chimpanzee-gorilla-human divergence. Again, using immunological properties to measure the differences in structure between the same protein from the three species, Sarich and Wilson concluded that the African great apes and humans last shared a common ancestor five million years ago. The resolution allowed by the technique was insufficient to determine whether the divergence was a three-way split or, for example, the two apes shared a common ancestor briefly following the separation of the hominid line.
Sarich and Wilson's conclusion was not embraced with enthusiasm by the palaeoanthropological community. At the time, the community's favourite candidate for the first hominid in the fossil record was Ramapithecus, specimens of which from Asia, Europe and Africa showed that it lived at least 14 million years ago. A divergence between apes and humans just five million years ago, as Wilson and Sarich contended, was therefore considered far too recent.
In the 15 years following the publication of Wilson and Sarich's provocative paper, immunological data on at least six independent proteins has been accumulated, and each appears to tell the same story. Many new potential clock techniques have been developed over the years, some in the Berkeley laboratory, others elsewhere. Some, such as the analysis of the nucleotide sequence of segments of DNA, have the advantage of giving a much greater resolution of genetic differences than is possible from the gross protein structure techniques of the original immunological methods. Others, such as the physical matching of one strand of DNA with a counterpart from another species (DNA hybridization), attempt to average out complicating differences that arise from the exceedingly complex nature of genes and other genetic elements in the DNA.
The simple assumption of a regularly ticking molecular clock turns out to be an over-simplification. A species' DNA is a patchwork of many different types of sequences, each of which might be susceptible to change in different ways. Ironically, however, in spite of this tremendous complication there are clock-like features in genetic change through time, even though they cannot yet be explained or modelled. The clock may be sloppy, but it tells the time nevertheless. The upshot of all this is that there remains the expectation that the molecules will be able to pinpoint the branch times in human history, but there is still a degree of disagreement over the data that are already available and their interpretation.
Wilson and Sarich, for example, argue that the conclusions from all the various molecular techniques consistently indicate five million years (plus or minus one million years) as the ape-human branch point. Others point to the DNA hybridization data in particular and claim a somewhat earlier divergence, between ten and seven million years. There is, however, consensus on the overall sequence of events: the gibbon diverged first, followed quickly by the orangutan; much later, the gorilla-chimpanzee-human split occurred. With the greater time resolution allowed by the modern molecular clock techniques it is now beginning to seem more than likely that the gorilla and chimpanzee briefly shared a common ancestor after the hominid line split off.
Meanwhile, the palaeoanthropological community has been reassessing its position. Fossil finds of recent years have dislodged Ramapithecus from its status as the putative first hominid and much more recent divergence data -- close to Sarich and Wilson's original five million year figure -- are now being considered.
The small degree of genetic distance that separates Homo sapiens from the African apes -- just one per cent in the genes that code for proteins -- is the same as that often recognized in sibling species, that is, species that are barely separate in evolutionary terms. And yet humans and apes are assigned to different families, a much higher taxonomic division than species or even genera. Pressure is rising to address this issue and to recognize that, in many ways, Homo sapiens is really just a rather unusual African ape.
It is interesting how the evidence from molecular biology is generally consistent with the relationships between apes and man earlier derived from morphology, i.e., they are closely related, and all are much less closely related to other kinds of animals. Blueprints has some comments about this:213
When Sarich got around to making further comparisons, that assumption [that evolution is responsible for the serum albumin difference between humans and chimps] also turned out to be true. Since then hundreds of cross matchings have been made and the evolutionary distance between many species calculated. Now the difference measurement that was begun between men, apes, and monkeys has been extended to include dogs, sheep, camels, elephants, and goes on to birds, amphibians, fishes, and insects. This is an absolutely astounding development. It means that a worker sitting in a laboratory, surrounded by a hundred unlabeled samples of serum albumin from a hundred different animals, can sort them out into sensible relationships without knowing what they are. If enough cross-checks, simple measurements of the amount of difference between the samples are made, a web of relationships that can fit logically in only one framework will result. If those relationships -- those positions in the web -- are marked down on a large sheet of paper, they will fall together into what emerges as a family tree. Only then -- after it is done -- need the investigator look at the names of the animals whose serum has been used, to see how closely that tree resembles one made by study of the animals themselves.
It is at this point that paleontologists must sit up and take notice, for the family tree drawn by serum albumin studies is a virtually exact match with one that would have been made by examining the bones, skin, size, shape, and behavior of living animals.
The tree drawn from molecular evidence differs from that drawn by paleontologists only in that it is more precise. It is able to tell us things that they cannot. Are humans more closely related to mice or rabbits? Paleontologists cannot say for sure, but the molecules can. The answer is mice. The molecules also tell us that pigs are more closely related to whales than they are to horses -- and so on. What is more, those relationships hold up when other molecular measuring methods are used.
There are problems with interpreting the results, and no one really knows why it works. The book Evolution: A Theory In Crisis214 gives an interesting description of the difficulties, although many biologists have strongly disputed the author's thesis.
Geology, Climate and the Appearance of New Species
Much interdisciplinary research has been done in the last thirty years attempting to correlate the sciences of geology, climate and paleontology. Paleontologist Roger Lewin describes what has been going on:215
Geologists, climatologists, and paleontologists have combined their different datasets -- and are beginning to see a distinct pattern emerge from the points. Briefly, that pattern indicates that episodes of significant global cooling are accompanied by pulses of extinctions and speciations among the world's biota. And it appears that hominids are no exception to this evolutionary pattern.
NeoDarwinism has always held the environment to be central to evolutionary change: different environments demand different adaptations. A species' environment is basically of two kinds: the physical world and the resources within it; and other species with which it interacts, specifically with which it competes in 'the struggle for existence' (Darwin's phrase)... Most investigators regard the physical environment as an important -- if not the prime -- engine initiating evolutionary change.
Changes in physical environment has influenced the history of life on at least three levels of scale, some of which are interlinked: extraterrestrial, global geography, and local climate.
Geologists have long known that Earth history is punctuated by mass extinction, the biggest being the Permian extinction 220 million years ago, when about 95 per cent of all species apparently perished. Five such mass dyings are known, together with a series of somewhat smaller ones... In recent years the scientific community has taken very seriously the suggestion that the Cretaceous/Tertiary extinction 65 million years ago, which drew an end to the age of dinosaurs, may have been caused by the impact of a comet or asteroid with the earth.
Lewin next discusses plate tectonics, which has been discussed in the essay "The Flood" in this series. He describes some of the tectonic effects seen in the fossil record:
On the next level of scale is tectonics, the constant movement of the dozen or so major plates that constitute the earth's crust and upon which the continents ride. Biotas that were once united have been divided, and previously independent biota have been brought together...
Whenever landmasses become isolated as a result of plate tectonics, the environment -- and therefore the evolutionary fate -- of the indigenous species was influenced simply by the fact of isolation. More dramatic, however, was the effect of uniting previously separated landmasses, because it brought a combination of new opportunities and the hazard of new competition to the biotas. Some groups diversified in these circumstances, as did the apes as they spread out of Africa, while others succumbed to extinction, the fate of many South American mammals during the Great American Interchange.
In addition to influencing evolution by shuffling landmasses, plate tectonics can also modify the environment within continents, a prime example of which occurred in Africa. Crudely speaking, 20 million years ago the continent was carpeted west to east with tropical forest. Today, however, East Africa is a mosaic of savannah and open woodland, separated from the still continuous forest to the west by the Great Rift Valley.
A minor tectonic plate margin runs south to north under East Africa, the first consequence of which was 'doming' that began 15 million years ago, producing tremendous lava-driven uplifts reaching 1000 meters high and centered near Nairobi in Kenya and Addis Ababa in Ethiopia. Then, weakened by the separating plates, the continental rock collapsed in a long, vertical fault, snaking several thousand kilometers from Tanzania in the south to Ethiopia in the north. The effect of all this was to throw the eastern part of the continent into rain shadow, thus dramatically altering the vegetation. These tectonic processes were accompanied by episodes of global cooling, which accentuated the replacement of forest by more open environments. The combination must have been key to early hominid evolution, which appears to have taken place there.
Climate represents a third level of scale influencing species environment. Clearly, the nutrient resources exploited by a species will be influenced by the prevailing climate, and any change may affect a species' ability to survive in a particular locality. But, in terms of major evolutionary change, more important than resources is the integrity of the species' habitat as a whole. Specifically, any fragmentation of a species' habitat range as a result of significant climate shift may lead to speciation in some cases and extinction in others... This, briefly stated, is the basis of the 'turnover-pulse' hypothesis advanced by Elisabeth Vrba, of Yale University.
The most common response of a species to changing climate is to migrate, following the conditions to which it is adapted: in the northern hemisphere, southward during times of cooling, and northward when conditions warm up. However, migration is not always possible, being prevented by physical barriers such as mountain ranges and rivers, or biological barriers, such as the absence of food resources and water. In such cases, populations may become fragmented and perhaps subject to different prevailing conditions. Too great a change, and extinction is likely. Moderate change, and speciation is possible.
Because episodes of significant cooling are likely to make northern latitudes less habitable, speciations will be concentrated in equatorial zones during such times. African lineages are therefore likely to enjoy a higher rate of speciation than those in Eurasia, because of the continent's equatorial location. 'Modern global patterns of species diversity in Bovidae are in accord with this prediction', notes Vrba. 'Subsaharan Africa alone has roughly twice as many endemic species as all of Eurasia.' Other considerations aside, Africa was therefore statistically more likely to have been the 'cradle of mankind' than was any other continent, simply by virtue of its position on the globe.
The turnover-pulse hypothesis states, therefore, that when lineages experience extinctions and speciations, they will do so synchronously and in coincidence with major climatic change, particularly cooling episodes. The hypothesis is still being tested, but so far the results appear to support it. The task is to identify major climatic shifts; to look for pulses of extinctions and speciations in the fossil record; and if such pulses exist, to see if they coincide with the climatic episodes. Because of the nature of the fossil record the best data come from marine sediments, while continental sequences lamentably are still fragmentary.
First, the climatic record, which centers on the formation of the polar ice caps as an indication of cooling episodes. During the first half of the Cenozoic period (65 million years ago to the present), which encompasses the history of the primate order, the globe appears to have been consistently free of significant polar ice. The first appearance of polar ice was the formation of the East Antarctic ice sheet a little after 35 million years ago, the Arctic remaining ice-free. The next climatic step was the formation of the West Antarctic ice sheet, beginning soon after 15 million years ago.
Antarctic ice appears to have been a permanent global feature after this point, although it fluctuated in extent, with major advances at around 5 million and 2.4 million years, the latter being brief but massive. Significantly, 2.4 million years ago was also the date of the first appearance of Arctic ice. Another climatic pulse occurred 0.9 million years ago, which set in train the main Pleistocene glaciations.
Probably the best dataset of continental vertebrates is that of the African bovids (various kinds of antelope), much of which has been collected by Vrba. Although the data are not suitable to test the 15 million year climatic pulse, they clearly show spikes of extinctions and speciations at 5 million, 2.4 million, and something less than 1 million. The 2.4 million year spike is especially pronounced, and there are mammalian fossil data from Europe and Asia that also reflect what apparently was an extreme climatic episode.
Note that between 5 and 6 million years ago the Mediterranean Sea dried up a number of times, when the African continent moved northward and blocked the Strait of Gibraltar. This would have had a tremendous effect on the surrounding climate. Also, the Pleistocene Ice Age began between 2.5 and 3.0 million years ago, with alternating cycles of cold and warm about every hundred thousand years. Both of these sets of events are recorded in marine sediments. Lewin continues:
What of the hominoids, including human ancestors? The 15 million year climatic episode does appear to coincide with a diversification of hominoids in Africa and Eurasia. But this expansion of species diversity also coincided with an expansion of geographic range following the joining of Africa with Eurasia. This one is therefore difficult to call. Towards the Late Miocene -- 8 to 5 million years ago -- hominoids became extinct in Eurasia, coinciding both with local indications of changing environments and with the onset of the West Antarctic ice sheet advance. This was also the period during which, according to molecular biological evidence, the chimpanzee and hominid lineages differentiated.
The 2.4 million year event is close to the point of origin of pygmy chimpanzees, but is right on the mark for some people's estimate for the beginning of the genus Homo, and possibly the origin of two lineages of robust australopithecine. The robust australopithecines became extinct about a million years ago, which is close to the 0.9 million year climatic event, as too is the first expansion of hominids out of Africa.
The turnover-pulse hypothesis clearly cannot be tested by data from hominid history, because for that one needs speciose groups, such as the antelopes. But, inasmuch as certain climatic events appear to be real and appear to be tracked by speciations and extinctions in some mammalian groups, some light can be thrown on the initiation of speciation within the hominid group. Of the data available so far, some degree of confidence can be placed in the 2.4 million year event, both in climatic and evolutionary terms. In decreasing degrees of confidence come the events at 5, 0.9 and 15 million years.
Paleontologist Donald Johanson added a few interesting details about the climate event at 2.5 million years ago:216
I've talked about the changes wrought upon Africa by the global temperature plunge in the late Miocene. Recently, Elisabeth Vrba drew my attention to a rash of new evidence pointing toward another climatic catastrophe later on. Deep-sea sediment cores record a surge in polar ice between 2.5 and 2.4 million years, perhaps marking the first glacial advance in the Northern Hemisphere. Pollen studies on land deposits tell the same story: In Holland, palm forests give way to open steppes; in Colombia mountain forests wither into plain. Pollen samples from the Omo in Ethiopia show an abrupt shift from woody plants to grasses and low shrubs. The Omo's record of microfauna -- rodents and the like -- neatly parallels the vegetative switch, with forest-living forms giving way to arid-adapted types between 2.5 and 2.4 million years ago. Most recently, deep-sea cores from the ocean off West Africa are coming up full of dust at the same point in time. From modern studies, we know that dust settles in the ocean where there is desert on the adjacent land.
Elisabeth's own work on African bovids comes up with the same conclusion, and in spades. Bovids are especially good indicators of evolutionary change in Africa simply because there are so many of them. Two and a half million years ago, the bovid family underwent an explosion of new species well adapted to savanna conditions: the hartebeest and wildebeest, the gazelles, impala, springbok, and similar forms.
"All the continents are saying the same thing," Elisabeth told me, with obvious delight. "The climate is changing in some places, with dramatic force. The cause of the change is not clear -- perhaps tectonic movements closed the Isthmus of Panama, reshuffling the circulation of currents in the Pacific and Atlantic oceans, which in turn affected temperature and precipitation patterns. But that it did happen is almost beyond doubt."
According to her "turnover-pulse" theory, this sudden shift in the global climate might have sent a surge of extinction and speciation through the food chain. The pulse would hit the early hominids too, possibly triggering the appearance of Homo. We do not have any direct evidence for Homo that early in the fossil record -- not yet -- but we do have the next best thing. The discovery of those very primitive stone tools near Hadar matches the date exactly. Equally important is the absence of tools any earlier. Stone tools are not time-fragile like fossils. If we look for them, and we can't find them, then they probably weren't there to begin with. If Elisabeth Vrba is right, and I think she is, the Homo emerged in East Africa some 2.5 million years ago, one among many species struggling to adapt to radically altered conditions.
Clearly something major happened to climate at various times in the past. It is recorded in marine sediments and in the fossil record of animals. All creationist publications seem to ignore the findings.
166 National Geographic Magazine, p. 593, Washington, D.C., November, 1985.
167 Roger Lewin, Human Evolution: An Illustrated Introduction, Second Edition, p. 74, Blackwell Scientific Publications, Boston, 1989.
168 Roger Lewin, op cit, pp. 38-41, 1984.
169 Arthur N. Strahler, Science and Earth History -- The Evolution/Creation Controversy, p. 484, Prometheus Books, Buffalo, New York, 1987.
170 Roger Lewin, op cit, pp. 38-41, 1984.
171 Roger Lewin, op cit, p. 92, 1989.
172 ibid, p. 83.
173 Roger Lewin, op cit, pp. 44-46, 1984.
174 Donald Johanson & James Shreeve, Lucy's Child, p. 130, Avon Books, New York, 1989.
175 Roger Lewin, op cit, p. 84, 93, 1989.
176 ibid, p. 80.
177 Johanson & Shreeve, op cit, p. 208.
178 Roger Lewin, op cit, p. 88, 1989.
178a See C. Loring Brace, "Humans in Time and Space," pp. 245-282, in Scientists Confront Creationism, edited by Laurie R. Godfrey, referenced elsewhere in this document.
179 Roger Lewin, op cit, pp. 23, 47-46, 1984.
180 Edey and Johanson, op cit, pp. 348-353.
181 Laurie R. Godfrey, Scientists Confront Creationism, p. 261, W. W. Norton & Company, New York, 1983.
182 Roger Lewin, op cit, pp. 53-56, 72, 1984.
183 ibid, p. 31.
184 Bjorn Kurten, Not From the Apes, p. 109, Vintage Books, New York, 1972.
185 Roger Lewin, op cit, p. 105, 1989.
186 Roger Lewin, op cit, pp. 71-73, 1984.
187 Edey and Johanson, op cit, p. 328.
188 Erik Trinkaus and William W. Howells, "The Neanderthals," Scientific American, pp. 118-133, New York, December, 1979.
189 Roger Lewin, op cit, p. 104, 1989.
190 Roger Lewin, op cit, pp. 74-77, 1984.
191 ibid, p. 51.
192 David Pilbeam, "The Descent of Hominoids and Hominids," Scientific American, pp. 84-96, New York, March, 1984.
193 Roger Lewin, op cit, p. 89, 1989.
194 ibid, p. 91.
195 Roger Lewin, op cit, pp. 81-83, 1984.
196 Arthur N. Strahler, op cit, p. 492.
197 ibid, pp. 490-491.
198 Roger Lewin, op cit, p. 128, 1989.
199 ibid, pp. 70-71.
200 Arthur N. Strahler, op cit, p. 485.
201 Johanson & Shreeve, op cit, pp. 195-201.
202 D. R. Selkirk and F. J. Burrows, editors, Confronting Creationism: Defending Darwin, p. 95, New South Wales University Press, Kensington NSW Australia, 1988.
203 "The Search for Our Ancestors," National Geographic Magazine, p. 593, Washington, D.C., November, 1985.
204 C. Owen Lovejoy, "Evolution of Human Walking," Scientific American, pp. 118-125, New York.
205 Johanson & Shreeve, op cit, p. 200.
206 Roger Lewin, op cit, pp. 75-79, 1989.
207 Duane T. Gish, Evolution: The Challenge of the Fossil Record, pp. 154-163, Creation-Life Publishers, El Cajon, California, 1985.
208 Roger Lewin, op cit, pp. 74-70, 1984.
209 Laurie R. Godfrey, Scientists Confront Creationism, p. 263, W. W. Norton & Company, New York, 1983.
210 William R. Fix, The Bone Peddlers: Selling Evolution, p. 7, Macmillan Publishing Company, New York, 1984.
211 Roger Lewin, op cit, pp. 60-63, 1984.
212 Roger Lewin, op cit, pp. 18-21, 1984.
213 Edey and Johanson, op cit, p. 358.
214 Michael Denton, Evolution: A Theory In Crisis, Adler & Adler, Publisher, Inc., Bethesda, Maryland, 1985.
215 Roger Lewin, op cit, pp. 19-23, 1989.
216 Donald Johanson & James Shreeve, Lucy's Child, p. 259, Avon Books, New York, 1989.