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Masters of the Planet
The Search for Our Human Origins
By Ian Tattersall St. Martin's Press
Copyright © 2012 Ian Tattersall
All rights reserved.
ISBN: 978-1-137-00038-5
CHAPTER 1
ANCIENT ORIGINS
Among the most important influences not only on how ancient creatures evolved, but on their preservation as fossils, has been the geography and topography of the Earth itself. This has been as true for our group as for any other, so it's worth giving a bit of background here. During the Age of Mammals that followed the demise of the dinosaurs some 65 million years ago, much of the African continent was a flattish highland plateau. This slab of the Earth's crust lay over the roiling molten rocks of the Earth's interior like a great thick blanket, trapping the heat below. Heat must rise, and eventually ascending hot rock began to swell the rigid surface above.
Thus began the formation of the great African Rift, the "spine of Africa," that formed as a series of more or less independent but ultimately conjoined areas of uplift known as "domes." These blistered and split apart the continent's surface along a line that started in Syria, proceeded down the Red Sea, then south from Ethiopia through East Africa to Mozambique. The Rift's major feature, the Great East African Rift Valley, formed as a complex chain of sheer-sided depressions when the swelling below cracked the inflexible rock at the surface. As the continent continued to rise with the injection of more hot rock from below, erosion by water and wind began to deposit sediments in the valley floors — sediments that contain an amazingly rich assortment of fossils. As a category, fossils technically include any direct evidence of past life, but the overwhelming majority of them consist of the bones and teeth of dead animals that were luckily — for paleontologists — covered and protected by marine or lake or river sediments before they could be obliterated by scavengers and the elements. And, as fate would have it, the sedimentary rocks of the Rift Valley include the most remarkable fossil record we have, from anywhere in the world, of the long history of mankind and its early relatives.
In eastern Africa, Rift sediments began to be deposited in the Ethiopian Dome about 29 million years ago, and similar deposits mark the initiation of the Kenya Dome only a few million years later, at about 22 million years. This occurred during the period known to geologists as the Miocene epoch, and it happens to have been an exceptionally interesting time in primate evolution, as the fossil record shows. It was what you might call "the golden age of the apes," and it set the stage for the evolution of the human family, which appeared toward its end.
Today's Great Apes — the chimpanzees, bonobos, gorillas, and orangutans — constitute a mere handful of forest species now restricted to tiny areas of Africa and a couple of southeast Asian islands. But the Miocene was the apes' heyday, and over its 18-million-year extent, scientists have named more than 20 genera of extinct apes from sites scattered all around the Old World, though mostly in East Africa. The earliest of these ancient apes are known as "proconsuloids." They scampered along the tops of large branches in the humid forests of the eastern African early Miocene in search of fruit, some 23 to 16 million years ago. Like today's apes, they already lacked tails; but in many ways they were more monkeylike, with less flexible forelimbs than those their descendants eventually acquired.
Around 16 million years ago, African climates seem to have become drier and more seasonal, changing the character of the forests. True monkeys began to flourish in the new habitat, and the proconsuloids themselves yielded to "hominoid" apes that more closely resembled their modern successors. Most notably, the apes of the later Miocene developed mobile arms that they could freely rotate at the shoulder joint, allowing efficient suspension of the body beneath tree branches and imparting all-around greater agility. These early hominoids also typically had molar teeth with thick enamel that were set in robust jaws, allowing them to tackle a broad range of seasonally available forest foods as they began spreading beyond the Afro-Arabian region into Eurasia.
In both Eurasia and Africa, paleontologists have found the remains of several different hominoid genera that date back between about 13 and 9 million years ago. These probably represent the group that gave rise to the first members of our own "hominid" family (or "hominin" subfamily; for most purposes the distinction is merely notional). Most of the genera concerned are known principally from teeth and bits of jaw and cranium; but one of them, the 13-million-year-old Pierolapithecus, is well known from a fairly complete skeleton discovered not long ago in Spain. Pierolapithecus was clearly a tree climber, but it also showed a host of bony characteristics that suggest it habitually held its body upright. Such a posture — in the trees, at least — may actually have been typical for many hominoids of the time (as it is for orangutans today). However, the skull and teeth of Pierolapithecus are different from those of any of the putative early hominids that we'll read about in a moment.
WILL THE EARLIEST HOMINID PLEASE STAND UP?
The earliest representatives of our own group lived at the end of the Miocene and at the beginning of the following Pliocene epoch, between about six and 4.5 million years ago. And they appear just as the arrival of many new open-country mammal genera in the fossil record signals another major climatic change. Oceanic cooling affected rainfall and temperatures on continents worldwide, giving rise in tropical regions to an exaggerated form of seasonality often known as the "monsoon cycle." In Europe this cooling led to the widespread development of temperate grasslands, while in Africa it inaugurated a trend toward the breakup of forest masses and the formation of woodlands into which grasslands intruded locally. This episode of climatic deterioration furnished the larger ecological stage on which the earliest known hominids made their debut.
Before we look at the varied cast of contenders for the title of "most ancient hominid," perhaps we should pause for a moment to consider just what an early hominid should look like. How would we recognize the first hominid, the earliest member of the group to which we belong to the exclusion of the apes, if we had it? The question seems straightforward, but the issue has proven to be contentious, especially since members of related lineages — such as our own and that of the chimpanzees — should logically become more similar to each other, and thus harder to distinguish, as they converge back in time toward their common ancestor. But while the characteristics that define modern groups should even in principle lose definition back in the mists of the past, attempts to recognize very early hominids have paradoxically been dominated by the search for the early occurrence of those features that mark out their descendants today.
When the Dutch physician Eugene Dubois discovered the first truly ancient human fossil in Java in 1891, he called his new find Pithecanthropus erectus ("upright ape-man"). His choice of species name emphasized the importance he attached to the erect stature of this hominid (indicated by the structure of its thighbone) in determining its human (or at least close-to-human) status. But soon thereafter the emphasis changed, at least temporarily. Modern people are perhaps most remarkable for their large brains; and in the early years of the twentieth century, brain size expansion replaced uprightness as the key criterion for any fossil seriously considered for inclusion in the hominid family. Indeed, its big human braincase (which was matched with an ape jawbone) was the basis for recognizing the famously fraudulent English Piltdown "fossil" as a human ancestor in 1912. The fraud was only officially uncovered some 40 years later, although many scientists were suspicious of it from the start; and as time passed the Piltdown specimens became increasingly ignored, which had the effect of bringing the big-brain criterion into disfavor. In its place came a behavioral yardstick rather than an anatomical one: manual dexterity and the manufacture of stone tools became the key to human status, as the notion of "Man the Toolmaker" took hold.
But this too had its difficulties. Eventually and inevitably, attention refocused on anatomy, and various potentially diagnostic morphological features of hominids were touted. Teeth, which are coated with the toughest biological material and thus preserve particularly well in the fossil record, received particular attention. One dental characteristic that many noticed among potential early hominid fossils was thick molar enamel — although, as we have seen, this indicator of a tough diet is also found widely among Miocene apes. Another hominid dental feature that has perennially attracted attention is the reduction in size of the canine teeth. This occurs in conjunction with the loss of honing of the large upper canine against the front premolar of the lower jaw with which it occludes. Large-bodied male apes typically have fearsome upper canine teeth with razor-sharp back edges — although in small females these teeth can be dainty. But again, a tendency toward canine reduction is not unique to hominids. It is also found in various Miocene apes, most famously the bizarre late-Miocene Oreopithecus, an insular form that additionally showed a distinct tendency toward postural uprightness. What is more, the remarkable Oreopithecus was recently reported to have had "precision-grip capability" — something else that was once thought unique to tool-making hominids.
Part of the problem of spotting features that are unique to hominids stems from the nature of evolutionary diversification. As we look farther back into hominid history, every feature indicative of modern hominids is likely to become less distinctive — and more reminiscent of its counterparts in members of related lineages. Given this reality, it is hardly realistic to expect that we'll ever find an anatomical "silver bullet" that will by itself tell us infallibly if an ancient fossil is a hominid or not. Every effort to do this has foundered on one technicality or another. Take, for example, the early-twentieth-century attempt of the English anatomist Sir Arthur Keith to set a "cerebral Rubicon" of 750 cubic centimeters (cc) minimum brain volume for membership in the genus Homo. Any smaller than this, Keith said, and you didn't belong to the club. This was certainly a convenient and easily measurable criterion; and, at a time when very few hominid fossils were known, perhaps it was even a workable one. But predictably, as the hominid fossil sample increased, problems arose. Brain size is notably variable within populations (modern human brains range in size from about 1,000 to 2,000 cc, with no indication that people with larger brains are necessarily smarter), so that even in principle this standard might have admitted an ancient hominid to our genus while excluding his or her parents or offspring. Accumulating fossil finds predictably forced later authors to lower Keith's figure several times, until it became obvious that the entire "Rubicon" idea was misguided.
Similar objections apply to any touchstone of this kind for membership in the genus Homo or the family Hominidae. But the temptation to see matters from the "key criterion" perspective is nevertheless always there. Indeed, in recent years paleoanthropologists have come full circle back to Dubois' view, so that the most notable common factor uniting all currently touted "earliest hominids" is the claim that each had walked bipedally on the ground. This seemingly straightforward standard for membership in our family is particularly attractive given that in the latest Miocene the eastern African forests were beginning to yield to patches of more open territory. This would have obliged at least some ape populations to spend more time on the ground (though extinction was, as always, the easier option for steadfastly arboreal types). Still, if this environmental change forced one ape lineage to stand upright, why not others? Several likely did; but only one of them can have been the hominid progenitor.
A further confounding factor is that all of the known "very early hominid" fossils have been found in contexts indicating thickly wooded habitats, or at least mixed ones. The earliest hominids were thus not obliged to walk upright on the ground by the disappearance of their ancestral habitat. We humans have rather reductionist minds, and are beguiled by clear, straightforward explanations. But where murky Mother Nature is concerned, beware of excessively simple stories.
THE CAST OF CHARACTERS
Until close to the turn of this century, the known hominid fossil record extended back in time to only about three to four million years ago. But a remarkable series of finds has since turned up a variety of contenders for the mantle of "earliest" hominid that are significantly older than this. The oldest of them come from around the time that DNA studies suggest our ancestors parted company with our closest ape relatives, believed to be the chimpanzees and bonobos.
"Toumaï" andOrrorin
The most ancient of the "earliest hominids" on offer today is the close-to-seven-million-year-old species Sahelanthropus tchadensis, discovered in 2001 in the central-western African country of Chad (well to the west of the Rift Valley). What has so far been published of this form consists of a badly crushed cranium (informally dubbed "Toumaï" — "hope of life" in the local language) and some partial mandibles. These fossils caused a stir when discovered, because nobody had anticipated an ancestral hominid like this. What was particularly strange about Toumaï was that it combined a small (hence rather apelike) braincase with a large, flattish face that was distinctly unlike the more protruding snouts of younger fossil hominids (or apes, for that matter). Two things caused its describers to classify this form as a hominid: first, the teeth. The molars had moderately thick enamel, the canines were reduced, and there was no lower premolar honing mechanism. So far, so good; but as we've seen, both thick enamel and the reduced canine-premolar complex can be matched outside Hominidae. So the key finding was in the base of the crushed cranium, where the foramen magnum, the large hole through which the spinal cord exits the cranium, appeared to be shifted underneath the skull to face largely downward. This is significant in that you would expect to find this setup in an upright biped like us: a skull balanced atop an erect spine. In a quadrupedal chimpanzee, the skull hangs on the front of a horizontal spine, so the foramen magnum has to be at the rear of the skull, facing backward. Unfortunately, though, the skull of Sahelanthropus was badly crushed, so the crucial claim about its foramen magnum was inevitably disputed.
In response, researchers took CT-scans of the crushed skull in a medical scanning machine, and produced a computerized virtual reconstruction that eliminated the distortions. Now, no matter how high-tech the procedure is, there's always an element of human judgment involved in making any reconstruction. But the resulting model of the pristine Sahelanthropus skull gave its creators substantial grounds for viewing Toumaï as plausibly — if not definitively — the skull of a biped. There are still some skeptics; but although the bipedality question will never be finally settled until key parts of the body skeleton of Sahelanthropus are announced, the reconstruction does appear to give this form the benefit of the doubt.
If Toumaï was a hominid — or even if he wasn't — what can we say about his way of life? Fossils found in the same deposits suggest that Sahelanthropus lived in an environment that was well watered, with forest in the close vicinity. This doesn't tell us much directly, but it does say something about the kind of resources that were available to this presumed ancestor. Put this information together with its posture, its habitat, and the general form of its teeth, and it seems reasonable to suggest that Sahelanthropus was at least a part-time biped that subsisted on a fairly generalized plant-based diet that would have included fruit, leaves, nuts, seeds, and roots, and probably extended to insects and small vertebrates such as lizards. For the moment it's probably unwise to say too much beyond that, though we'll speculate a bit about such things as the nature of early hominid sociality in a little while.
(Continues...)
Excerpted from Masters of the Planet by Ian Tattersall. Copyright © 2012 Ian Tattersall. Excerpted by permission of St. Martin's Press.
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