Source: Scientific American
Date: August, 2003

Planet of the Apes

During the Miocene epoch, as many as 100 species of apes roamed throughout the Old World. New fossils suggest that the ones that gave rise to living great apes and humans evolved not in Africa but Eurasia

"It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; as these two species are now man’s closest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere."

So mused Charles Darwin in his 1871 work, The Descent of Man. Although no African fossil apes or humans were known at the time, remains recovered since then have largely confirmed his sage prediction about human origins. There is, however, considerably more complexity to the story than even Darwin could have imagined. Current fossil and genetic analyses indicate that the last common ancestor of humans and our closest living relative, the chimpanzee, surely arose in Africa, around six million to eight million years ago. But from where did this creature’s own forebears come? Paleoanthropologists have long presumed that they, too, had African roots. Mounting fossil evidence suggests that this received wisdom is flawed.

Today’s apes are few in number and in kind. But between 22 million and 5.5 million years ago, a time known as the Miocene epoch, apes ruled the primate world. Up to 100 ape species ranged throughout the Old World, from France to China in Eurasia and from Kenya to Namibia in Africa. Out of this dazzling diversity, the comparatively limited number of apes and humans arose. Yet fossils of great apes - the large-bodied group represented today by chimpanzees, gorillas and orangutans (gibbons and siamangs make up the so-called lesser apes) - have turned up only in western and central Europe, Greece, Turkey, South Asia and China. It is thus becoming clear that, by Darwin’s logic, Eurasia is more likely than Africa to have been the birthplace of the family that encompasses great apes and humans, the hominids. (The term "hominid" has traditionally been reserved for humans and protohumans, but scientists are increasingly placing our great ape kin in the definition as well and using another word, "hominin", to refer to the human subset. The word "hominoid" encompasses all apes - including gibbons and siamangs - and humans.) Perhaps it should not come as a surprise that the apes that gave rise to hominids may have evolved in Eurasia instead of Africa: the combined effects of migration, climate change, tectonic activity and ecological shifts on a scale unsurpassed since the Miocene made this region a hotbed of hominoid evolutionary experimentation. The result was a panoply of apes, two lineages of which would eventually find themselves well positioned to colonize Southeast Asia and Africa and ultimately to spawn modern great apes and humans.

Paleoanthropology has come a long way since Georges Cuvier, the French natural historian and founder of vertebrate paleontology, wrote in 1812 that "l’homme fossile n’existe pas" ("fossil man does not exist"). He included all fossil primates in his declaration. Although that statement seems unreasonable today, evidence that primates lived alongside animals then known to be extinct - mastodons, giant ground sloths and primitive ungulates, or hoofed mammals, for example - was quite poor. Ironically, Cuvier himself described what scholars would later identify as the first fossil primate ever named, Adapis parisiensis Cuvier 1822, a lemur from the chalk mines of Paris that he mistook for an ungulate. It wasn’t until 1837, shortly after Cuvier’s death, that his disciple Édouard Lartet described the first fossil higher primate recognized as such. Now known as Pliopithecus, this jaw from southeastern France, and other specimens like it, finally convinced scholars that such creatures had once inhabited the primeval forests of Europe. Nearly 20 years later Lartet unveiled the first fossil great ape, Dryopithecus, from the French Pyrénées.

In the remaining years of the 19th century and well into the 20th, paleontologists recovered many more fragments of ape jaws and teeth, along with a few limb bones, in Spain, France, Germany, Austria, Slovakia, Hungary, Georgia and Turkey. By the 1920s, however, attention had shifted from Europe to South Asia (India and Pakistan) and Africa (mainly Kenya), as a result of spectacular finds in those regions, and the apes of Eurasia were all but forgotten. But fossil discoveries of the past two decades have rekindled intense interest in Eurasian fossil apes, in large part because paleontologists have at last recovered specimens complete enough to address what these animals looked like and how they are related to living apes and humans.

The First Apes

TO DATE, RESEARCHERS have identified as many as 40 genera of Miocene fossil apes from localities across the Old World - eight times the number that survive today. Such diversity seems to have characterized the ape family from the outset: almost as soon as apes appear in the fossil record, there are lots of them. So far 14 genera are known to have inhabited Africa during the early Miocene alone, between 22 million and 17 million years ago. And considering the extremely imperfect nature of the fossil record, chances are that this figure significantly underrepresents the number of apes that actually existed at that time.

Like living apes, these creatures varied considerably in size. The smallest weighed in at a mere three kilograms, hardly more than a small housecat; the largest tipped the scales at a gorillalike heft of 80 kilograms. They were even more diverse than their modern counterparts in terms of what they ate, with some specializing in leaves and others in fruits and nuts, although the majority subsisted on ripe fruits, as most apes do today. The biggest difference between those first apes and extant ones lay in their posture and means of getting around. Whereas modern apes exhibit a rich repertoire of locomotory modes - from the highly acrobatic brachiation employed by the arboreal gibbon to the gorilla’s terrestrial knuckle walking - early Miocene apes were obliged to travel along tree branches on all fours.

To understand why the first apes were restricted in this way, consider the body plan of the early Miocene ape. The bestknown ape from this period is Proconsul, exceptionally complete fossils of which have come from sites on Kenya’s Rusinga Island [see The Hunt for Proconsul, by Alan Walker and Mark Teaford; Scientific American, January 1989]. Specialists currently recognize four species of Proconsul, which ranged in size from about 10 kilograms to possibly as much as 80 kilograms. Proconsul gives us a good idea of the anatomy and locomotion of an early ape. Like all extant apes, this one lacked a tail. And it had more mobile hips, shoulders, wrists, ankles, hands and feet than those of monkeys, presaging the fundamental adaptations that today’s apes and humans have for flexibility in these joints. In modern apes, this augmented mobility enables their unique pattern of movement, swinging from branch to branch. In humans, these capabilities have been exapted, or borrowed, in an evolutionary sense, for enhanced manipulation in the upper limb - something that allowed our ancestors to start making tools, among other things. At the same time, however, Proconsul and its cohorts retained a number of primitive, monkeylike characteristics in the backbone, pelvis and forelimbs, leaving them, like their monkey forebears, better suited to traveling along the tops of tree branches than hanging and swinging from limb to limb. (Intriguingly, one enigmatic early Miocene genus from Uganda, Morotopithecus, may have been more suspensory, but the evidence is inconclusive.) Only when early apes shed more of this evolutionary baggage could they begin to adopt the forms of locomotion favored by contemporary apes.

Passage to Eurasia

MOST OF THE EARLY Miocene apes went extinct. But one of them - perhaps Afropithecus from Kenya - was ancestral to the species that first made its way over to Eurasia some 16.5 million years ago. At around that time global sea levels dropped, exposing a land bridge between Africa and Eurasia. A mammalian exodus ensued. Among the creatures that migrated out of their African homeland were elephants, rodents, ungulates such as pigs and antelopes, a few exotic animals such as aardvarks, and primates.

The apes that journeyed to Eurasia from Africa appear to have passed through Saudi Arabia, where the remains of Heliopithecus, an ape similar to Afropithecus, have been found. Both Afropithecus and Heliopithecus (which some workers regard as members of the same genus) had a thick covering of enamel on their teeth - good for processing hard foods, such as nuts, and tough foods protected by durable husks. This dental innovation may have played a key role in helping their descendants establish a foothold in the forests of Eurasia by enabling them to exploit food resources not available to Proconsul and most earlier apes. By the time the seas rose to swallow the bridge linking Africa to Eurasia half a million years later, apes had ensconced themselves in this new land.

The movement of organisms into new environments drives speciation, and the arrival of apes in Eurasia was no exception. Indeed, within a geologic blink of an eye, these primates adapted to the novel ecological conditions and diversified into a plethora of forms - at least eight known in just 1.5 million years. This flurry of evolutionary activity laid the groundwork for the emergence of great apes and humans. But only recently have researchers begun to realize just how important Eurasia was in this regard. Paleontologists traditionally thought that apes more sophisticated in their food-processing abilities than Afropithecus and Heliopithecus reached Eurasia about 15 million years ago, around the time they first appear in Africa. This fit with the notion that they arose in Africa and then dispersed northward. New fossil evidence, however, indicates that advanced apes (those with massive jaws and large, grinding teeth) were actually in Eurasia far earlier than that. In 2001 and 2003 my colleagues and I described a more modern-looking ape, Griphopithecus, from 16.5-million-year-old sites in Germany and Turkey, pushing the Eurasian ape record back by more than a million years.

The apparent absence of such newer models in Africa between 17 million and 15 million years ago suggests that, contrary to the long-held view of this region as the wellspring of all ape forms, some hominoids began evolving modern cranial and dental features in Eurasia and returned to Africa changed into more advanced species only after the sea receded again. (A few genera - such as Kenyapithecus from Fort Ternan, Kenya - may have gone on to develop some postcranial adaptations to life on the ground, but for the most part, these animals still looked like their early Miocene predecessors from the neck down.)

Rise of the Great Apes

BY THE END of the middle Miocene, roughly 13 million years ago, we have evidence for great apes in Eurasia, notably Lartet’s fossil great ape, Dryopithecus, in Europe and Sivapithecus in Asia. Like living great apes, these animals had long, strongly built jaws that housed large incisors, bladelike (as opposed to tusklike) canines, and long molars and premolars with relatively simple chewing surfaces- a feeding apparatus well suited to a diet of soft, ripe fruits. They also possessed shortened snouts, reflecting the reduced importance of olfaction in favor of vision. Histological studies of the teeth of Dryopithecus and Sivapithecus suggest that these creatures grew fairly slowly, as living great apes do, and that they probably had life histories similar to those of the great apes - maturing at a leisurely rate, living long lives, bearing one large offspring at a time, and so forth. Other evidence hints that were they around today, these early great apes might have even matched wits with modern ones: fossil braincases of Dryopithecus indicate that it was as large-brained as a chimpanzee of comparable proportions. We lack direct clues to brain size in Sivapithecus, but given that life history correlates strongly with brain size, it is likely that this ape was similarly brainy.

Examinations of the limb skeletons of these two apes have revealed additional great ape-like characteristics. Most important, both Dryopithecus and Sivapithecus display adaptations to suspensory locomotion, especially in the elbow joint, which was fully extendable and stable throughout the full range of motion. Among primates, this morphology is unique to apes, and it figures prominently in their ability to hang and swing below branches. It also gives humans the ability to throw with great speed and accuracy. For its part, Dryopithecus exhibits numerous other adaptations to suspension, both in the limb bones and in the hands and feet, which had powerful grasping capabilities. Together these features strongly suggest that Dryopithecus negotiated the forest canopy in much the way that living great apes do. Exactly how Sivapithecus got around is less clear. Some characteristics of this animal’s limbs are indicative of suspension, whereas others imply that it had more quadrupedal habits. In all likelihood, Sivapithecus employed a mode of locomotion for which no modern analogue exists - the product of its own unique ecological circumstances.

The Sivapithecus lineage thrived in Asia, producing offshoots in Turkey, Pakistan, India, Nepal, China and Southeast Asia. Most phylogenetic analyses concur that it is from Sivapithecus that the living orangutan, Pongo pygmaeus, is descended. Today this ape, which dwells in the rain forests of Borneo and Sumatra, is the sole survivor of that successful group.

In the west the radiation of great apes was similarly grand. From the earliest species of Dryopithecus, D. fontani, the one found by Lartet, several other species emerged over about three million years. More specialized descendants of this lineage followed suit. Within two million years four new species of Dryopithecus would evolve and span the region from northwestern Spain to the Republic of Georgia. But where Dryopithecus belongs on the hominoid family tree has proved controversial. Some studies link Dryopithecus to Asian apes; others position it as the ancestor of all living great apes. My own phylogenetic analysis of these animals - the most comprehensive in terms of the number of morphological characteristics considered - indicates that Dryopithecus is most closely related to an ape known as Ouranopithecus from Greece and that one of these two European genera was the likely ancestor of African apes and humans.

A Dryopithecus skull from Rudabánya, Hungary, that my colleagues and I discovered in 1999 bolsters that argument. Nicknamed Gabiafter its discoverer, Hungarian geologist Gabor Hernyák, it is the first specimen to preserve a key piece of anatomy: the connection between the face and the braincase. Gabi shows that the cranium of Dryopithecus, like that of African apes and early fossil humans, had a long and low braincase, a flatter nasal region and an enlarged lower face. Perhaps most significant, it reveals that also like African apes and early humans, Dryopithecus was klinorhynch, meaning that viewed in profile its face tilts downward. Orangutans, in contrast - as well as Proconsul, gibbons and siamangs - have faces that tilt upward, a condition known as airorhinchy. That fundamental aspect of Dryopithecus’s cranial architecture speaks strongly to a close evolutionary relationship between this animal and the African apes and humans lineage. Additional support for that link comes from the observation that the Dryopithecus skull resembles that of an infant or juvenile chimpanzee - a common feature of ancestral morphology. It follows, then, that the unique aspects of adult cranial form in chimpanzees, gorillas and fossil humans evolved as modifications to the ground plan represented by Dryopithecus and living African ape youngsters.

One more Miocene ape deserves special mention. The best-known Eurasian fossil ape, in terms of the percentage of the skeleton recovered, is seven-millionyear-old Oreopithecus from Italy. First described in 1872 by renowned French paleontologist Paul Gervais, Oreopithecus was more specialized for dining on leaves than was any other Old World fossil monkey or ape. It survived very late into the Miocene in the dense and isolated forests of the islands of Tuscany, which would eventually be joined to one another and the rest of Europe by the retreat of the sea to form the backbone of the Italian peninsula. Large-bodied and smallbrained, this creature is so unusual looking that it is not clear whether it is a primitive form that predates the divergence of gibbons and great apes or an early great ape or a close relative of Dryopithecus. Meike Köhler and Salvador Moyà-Solà of the Miquel Crusafont Institute of Paleontology in Barcelona have proposed that Oreopithecus walked bipedally along tree limbs and had a humanlike hand capable of a precision grip. Most paleoanthropologists, however, believe that it was instead a highly suspensory animal. Whatever Oreopithecus turns out to be, it is a striking reminder of how very diverse and successful at adapting to new surroundings the Eurasian apes were.

So what happened to the myriad species that did not evolve into the living great apes and humans, and why did the ancestors of extant species persevere? Clues have come from paleoclimatological studies. Throughout the middle Miocene, the great apes flourished in Eurasia, thanks to its then lush subtropical forest cover and consistently warm temperatures. These conditions assured a nearly continuous supply of ripe fruits and an easily traversed arboreal habitat with several tree stories. Climate changes in the late Miocene brought an end to this easy living. The combined effects of Alpine, Himalayan and East African mountain building, shifting ocean currents, and the early stages of polar ice cap formation precipitated the birth of the modern Asian monsoon cycle, the desiccation of East Africa and the development of a temperate climate in Europe. Most of the Eurasian great apes went extinct as a result of this environmental overhaul. The two lineages that did persevere - —those represented by Sivapithecus and Dryopithecus - did so by moving south of the Tropic of Cancer, into Southeast Asia from China and into the African tropics from Europe, both groups tracking the ecological settings to which they had adapted in Eurasia.

The biogeographical model outlined above provides an important perspective on a long-standing question in paleoanthropology concerning how and why humans came to walk on two legs. To address that issue, we need to know from what form of locomotion bipedalism evolved. Lacking unambiguous fossil evidence of the earliest biped and its ancestor, we cannot say with certainty what that ancestral condition was, but researchers generally fall into one of two theoretical camps: those who think twolegged walking arose from arboreal climbing and suspension and those who think it grew out of a terrestrial form of locomotion, perhaps knuckle walking.

Your Great, Great Grand Ape

THE EURASIAN FOREBEAR of African apes and humans moved south in response to a drying and cooling of its environs that led to the replacement of forests with woodlands and grasslands. I believe that adaptations to life on the ground - knuckle walking in particular - were critical in enabling this lineage to withstand that loss of arboreal habitat and make it to Africa. Once there, some apes returned to the forests, others settled into varied woodland environments, and one ape - the one from which humans descended - eventually invaded open territory by committing to life on the ground.

Flexibility in adaptation is the consistent message in ape and human evolution. Early Miocene apes left Africa because of a new adaptation in their jaws and teeth that allowed them to exploit a diversity of ecological settings. Eurasian great apes evolved an array of skeletal adaptations that permitted them to live in varied environments as well as large brains to grapple with complex social and ecological challenges. These modifications made it possible for a few of them to survive the dramatic climate changes that took place at the end of the Miocene and return to Africa, around nine million years ago. Thus, the lineage that produced African apes and humans was preadapted to coping with the problems of a radically changing environment. It is therefore not surprising that one of these species eventually evolved very large brains and sophisticated forms of technology.

As an undergraduate more than 20 years ago, I began to look at fossil apes out of the conviction that to understand why humans evolved we have to know when, where, how and from what we arose. Scientists commonly look to living apes for anatomical and behavioral insights into the earliest humans. There is much to be gained from this approach. But living great apes have also evolved since their origins. The study of fossil great apes gives us both a unique view of the ancestors of living great apes and humans and a starting point for understanding the processes and circumstances that led to the emergence of this group. For example, having established the connection between European great apes and living African apes and humans, we can now reconstruct the last common ancestor of chimps and humans: it was a knuckle-walking, fruit-eating, forest-living chimplike primate that used tools, hunted animals, and lived in highly complex and dynamic social groups, as do living chimps and humans.

Tangled Branches

WE STILL HAVE MUCH to learn. Many fossil apes are represented only by jaws and teeth, leaving us with little or no idea about their posture and locomotion, brain size or body mass. Moreover, paleontologists have yet to recover any remains of ancient African great apes. Indeed, there is a substantial geographic and temporal gap in the fossil record between representatives of the early members of the African hominid lineage in Europe (Dryopithecus and Ouranopithecus) and the earliest African fossil hominids.

Moving up the family tree (or, more accurately, family bush), we find more confusion in that the earliest putative members of the human family are not obviously human. For instance, the recently discovered Sahelanthropus tchadensis, a six-million- to seven-million-year-old find from Chad, is humanlike in having small canine teeth and perhaps a more centrally located foramen magnum (the hole at the base of the skull through which the spinal cord exits), which could indicate that the animal was bipedal. Yet Sahelanthropus also exhibits a number of chimplike characteristics, including a small brain, projecting face, sloped forehead and large neck muscles. Another creature, Orrorin tugenensis, fossils of which come from a Kenyan site dating to six million years ago, exhibits a comparable mosaic of chimp and human traits, as does 5.8-million-year-old Ardipithecus ramidus kadabba from Ethiopia. Each of these taxa has been described by its discoverers as a human ancestor [see An Ancestor to Call Our Own, by Kate Wong; Scientific American, January]. But in truth, we do not yet know enough about any of these creatures to say whether they are protohumans, African ape ancestors or dead-end apes. The earliest unambiguously human fossil, in my view, is 4.4-million-year-old Ardipithecus ramidus ramidus, also from Ethiopia.

The idea that the ancestors of great apes and humans evolved in Eurasia is controversial, but not because there is inadequate evidence to support it. Skepticism comes from the legacy of Darwin, whose prediction noted at the beginning of this article is commonly interpreted to mean that humans and African apes must have evolved solely in Africa. Doubts also come from fans of the aphorism "absence of evidence is not evidence of absence." To wit, just because we have not found fossil great apes in Africa does not mean that they are not there. This is true. But there are many fossil sites in Africa dated to between 14 million and seven million years ago - some of which have yielded abundant remains of forest-dwelling animals - and not one contains great ape fossils. Although it is possible that Eurasian great apes, which bear strong resemblances to living great apes, evolved in parallel with as yet undiscovered African ancestors, this seems unlikely.

It would be helpful if we had a more complete fossil record from which to piece together the evolutionary history of our extended family. Ongoing fieldwork promises to fill some of the gaps in our knowledge. But until then, we must hypothesize based on what we know. The view expressed here is testable, as required of all scientific hypotheses, through the discovery of more fossils in new places.

Function, Phylogeny and Fossils: Miocene Hominoid Evolution and Adaptations. Edited by David R. Begun, Carol V. Ward and Michael D. Rose. Plenum Press, 1997.

The Primate Fossil Record. Edited by Walter Carl Hartwig. Cambridge University Press, 2002.

Rudabánya: A Late Miocene Subtropical Swamp Deposit with Evidence of the Origin of the African Apes and Humans. László Kordos and David R. Begun in Evolutionary Anthropology, Vol. 11, Issue 1, pages 45–57; 2002.

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