We first stood on our own two feet in Africa, not Asia
Stephen Jay Gould
In his typically grandiloquent way, the late Henry Fairfield Osborn, America’s leading vertebrate paleontologist and president of the American Museum of Natural History, contrasted the ambiguity of history’s most famous prophecies with the certainty of his own (in italics and exclamation points):
”It was seldom that the Oracle of Delphi gave an im- mediate response to the solicitous inquiries of those seeking Divine counsel . . . [and] the final reply of the gods was diplomatic, so that the ambiguity of Delphic utterances has become prover- bial. Not so with the American Museum quest in the arid Temple of Nature in Mongolia; almost at the very outset the invincible leader, Roy Chapman Andrews . . . met with the unequivocal response: Asia is the mother of the continents!”
These are Osborn’s open- ing words in his foreword (1926) to Andrews’s popular account of the Central Asiatic Expeditions mounted by the museum under Andrews’s command in the early 1920s–On the Trail of Ancient Man. These expeditions retain a firm place in the legends of American science and exploration. Andrews and his colleagues found many wonderful things, including the first dinosuar eggs and abundant bones of large mammals. But since frustration is anonymous, we don’t remember that Andrews failed in his major quest (expressed in the title of his book)–to find human ancestors in central Asia.
Andrews mounted the expeditions to test Osborn’s theory that Asia had been, in his boss’s words, ”a paleontologic Garden of Eden” for mammals–a center of origin shuttling its products
in two directions, toward Europe and the Americas. If Asia had been such a mother throughout the age of mammals, surely, Osborn reasoned, her fecundity had extended to the relatively recent origin of our own lineage as well. Andrews searched with failing hopes and mounting frustration, finally admitting temporary defeat with the stiff upper lip of the true explorer (who always insisted that adventure in the field could only record bad planning):
”That some of us will find primitive human bones in the near future is probable . . . I believe that success will come if we persist.”
Andrews failed for the most basic and ineluctable of all reasons–the human lineage arose in Africa. (By human lineage, I mean all creatures that evolved on the evolutionary branch leading to Homo sapiens after the split from our last common ancestor with any other living primate. This last common ancestor was the parent of both humans and chimps.) Certainty is unattainable in science, and I can construct a purely speculative scenario, not entirely contradicted by facts, that would still permit a hypothetical Asiatic origin. But I think we know about as well as we know anything in paleontology that our branch of the evolutionary tree grew in Africa–for reasons that I shall soon relate.
In the light of such good evidence for African sources, I’ve been continually amused and frustrated by the stubborn persistence of hopes for Asiatic origin in the popular mind. Few themes run so persistently through my correspondence with interested and well informed non-scientists as the question: Why not in Asia
To cite just two examples of clutching at false straws for Asiatic origins: When Don Johanson announced the discovery of Lucy (Australopithecus afarensis) in Ethiopia, many people reasoned (and the press often argued): Ethiopia is closer to Asia than other human fossil sites in Kenya, Tanzania, and South Africa. Lucy is older than bones from these other places. Doesn’t this indicate an Asian origin spreading into Africa from Lucy southward? At this sophistry, paleontologists could only beat their brows in frustration. Fossils are tiny needles in vast haystacks. Lucy is (figuratively speaking) only a stone’s throw from such sites as Koobi Fora in northern Kenya. Her status as slightly older and slightly north re- cords the luck of sampling, not the order of nature. The same false logic that reads hints of Asia in Lucy would lead us to conclude that people grow ten feet tall in Hawaii because Kareem Abdul-H Jabbar plays in L.A. and Phil Rizzuto once graced New York.
In a second and more recent example, the announcement last August that Amphipithecus, a 40-million-year-old Asian primate, might be a common ancestor of the monkey-ape-human line, brought forth a chorus of press commentary about renewed Asian links for humans. But such claims are 35 million years off, and light-years wrong conceptually. The human lineage arose by branching from a common ancestry with chimps some five to eight million years ago. The division of the much larger monkey- ape-human limb from the rest of the primates (tarsiers and, further back, lemurs) occurred long before. We know that apes had spread into both Africa and Asia at least ten million years before the human lineage arose. An early origin of apes in Asia cannot speak againt African genesis for the human lineage if potential ancestors among apes had spread into Africa millions of years before the chimp-human split.
The persistence of such dubious arguments for Asian origins leads me to pose a question in return: What is Asia’s allure? Why do so many people cling to a hope contradicted by good evidence? The reasons are many, ranging from simple hidebound unwillingness to abandon comfortable traditions, to subtle and largely unrecognized remnants of racist arguments that viewed the tough climates of native Caucasians as prods to the evolution of higher things, and the languid tropics as a self-indulgent home for indolence and degeneration.
This last sentence raises a delicate point, so please don’t misunderstand me. I accuse no reader of conscious or even covert racism (though we must recognize that some active originators of the Asiatic hypothesis, the patrician Osborn prominent among them, were motivated by the racist notion that Africa could only be a repository for once-noble stocks that had migrated from Asia and degenerated under the tropical sun). The racist traditions of our cultures run so deep that vestiges remain, in distant and unrecognized form, even among those who’ve struggled hard to overcome all prejudice.
(I may, perhaps, underscore this point by telling a story against myself. My friend and fellow paleontologist Bjorn Kurten wrote a fine novel, Dance of the Tiger, about the contact of Neanderthal and Cro-Magnon people in Europe some 35,000 years ago. He depicted the ”primitive,” indigenous Neanderthal as white and the ”advanced,” invading Cro-Magnon as dark. This jolted me because, quite subconsciously, I’d always pictured Neanderthals as dark and Cro-Magnon as light. Yet I realized that Kurten’s conjecture is much the more reasonable–since cold- adapted people tend to be light and Neanderthals were a European race of Homo sapiens, while Cro-Magnon people may have invaded from warmer climates. I then had to ask myself why I had unthinkingly shunned the more reasonable hypothesis. The answer can only be vestigial racism–I had, much to my em- barrassment, equated primitive with dark and advanced with white.)
The best evidence, of course, for African origin is direct–the abundant fossils of all earliest hominids (members of our lineage) in Africa, and none anywhere else. We speak not of dubious scraps, but of copious evidence for a substantial bush of related species. The australopithecines, our only hominid fossils from the earliest records some four million years ago to the origin of the genus Homo perhaps less than two million years ago, include three or more species, all exclusively African. The first member of our genus, Homo habilis, arose in Africa and gave rise there (or so the fossils relate) to Homo erectus. Only then, perhaps little more than a million years ago, did humans migrate out of Africa into Europe and Asia.
No one can deny this direct evidence. But supporters of Asia’s antiquity point to the scrappiness of our fossil record. After all, we have no hominid fossils from about four million years ago, right back to the common ancestry of chimps and humans some five to eight million years ago (an estimate based on genetic differences between modern chimps and people). Perhaps the chimp-human split occurred in Asia, and our ancestors migrated to Africa, where australopithecines then evolved. Perhaps australopithe cines also lived in Asia and we just haven’t found the fossils yet. These arguments, though strained, aren’t irretrievably condemned by their special pleading (though Asian sediments of appropriate age have been searched with negative results), for the fossil record is poor and, as the paleontological cliche goes, absence of evidence isn’t evidence of absence.
Asian hopes hang on, because many people assume that the argument for Africa rests upon fossils, and they understand that fossils don’t supply a definitive answer. To this extent, Asian hopes are based on an interesting misunderstanding of evolutionary arguments–and this error becomes a worthy subject for my series of DISCOVER essays about methods of doing science.
In the science of phylogeny (the study of evolutionary lineages and genealogical connections among organisms), we don’t usually proceed by the classical methods of experiment or direct obser- vation, for times are too long and events too vast. We try instead to collate several independent sources of inferential evidence– embryology, anatomy, vestigial organs, geographical distribu- tion, and fossil record, for example. Since each source could, in principle, yield a different answer (or no good evidence at all), the convergence of several independent lines upon the same resolution yields as much confidence as direct experiment produces in other branches of science. The whole, in such cases of independent confirmation, is far greater than the sum of parts.
The case for human origins in Africa is a classic example of independent confirmation from distinct sources. Our confidence derives from this confluence, not from bare bones. The major source of separate confirmation is so simple and obvious that we tend to pass it by as dull fact, rather than incisive evidence.
Many readers know that Darwin correctly predicted an African origin in his Descent of Man, published in 1871. I’ve asked several people over the years what they thought Darwin’s evidence might have been. They’ve all replied: he must have had some African fossils, ancestral ape or derived human. But the first genuine fossil of a human species older than our own was found ten years after Darwin’s death. Darwin’s argument rested entirely upon living organisms, and invoked a convergence between evidence of anatomy and geographical distribution: Anatomical evidence proves that gorillas and chimps, not orangs, gibbons, or any other primates, are our closest relatives among living forms. Gorillas and chimps live only in Africa, and we have no evidence that they ever inhabited any other region. Humans, therefore, probably evolved in the area always tenanted by their closest cousins. Darwin wrote in 1871:
”In each great region of the world the living mammals are closely related to the extinct species of the same region. It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man’s nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere.”
Darwin made his prediction tentatively, an appropriate stance since he lacked the independent confirmation that fossils have since supplied. With the convergence now provided between time (fossils) and space (geography), African origin has been strongly affirmed. I can imagine only two kinds of evidence that could break the link and renew hopes for Asia, and one source has been effectively disproved by another convergence of independent data.
First, we might revise our views of genealogy and find that Asian orangs (or some other primate), not African chimps and gorillas, are our closest relatives. I see virtually no chance that such a revision could occur. The anatomical evidence has spoken clearly for our closest cousinship with chimps and gorillas; Darwin presented a long catalogue of similarities, and a century’s work has only increased the compendium. More impressively, an entirely ndependent source of data supports the chimp-gorilla link: genetics and biochemistry. Darwin and the classical anatomists built their genealogies before we had a science of genetics. Yet the independent evidence of DNA similarities affirms our link with chimps and gorillas and our much greater distance from orangs. In fact, our closeness to chimps is nothing short of astonishing– even disconcerting for lingering hopes of uniqueness and anatomical beauty. The average human protein is more than 99 per cent identical to its counterpart in chimps.
Second, we might find that our modern geographic distributions don’t reflect history. Perhaps the immediate ancestors of chimps and gorillas lived in Asia, produced the split between chimps and humans there, and then became extinct, leaving no fossil traces yet discovered. This scenar- io is by no means impossible, but it strains credulity because it requires two separate events bereft of evidence. It demands that at least two distinct lineages (chimp ancestors and human ancestors) independently migrated from Asia to Africa, leaving all evidence for their patrimony in Africa and none at their supposed source. I could believe such a story for one lineage (given the spottiness of our fossil record), but not for two. Yet this, I think, is the only straw now available to supporters of an Asian homeland.
Darwin’s correct and elegantly simple argument, advanced before we had found a single fossil for extinct species in our lineage, holds a special appeal for me because it embodies the most fundamental principle of all evolutionary reason–the cardinal importance of genealogy as the basis for order among organisms. Darwin wrote in the Descent of Man: ”Classifications may, of course, be based on any character whatever, as on size, color, or the element inhabited; but naturalists have long felt a profound conviction that there is a natural system. This system, it is now generally admitted, must be, as far as possible, genealogical in arrangement.”
To make his inference about our African home, Darwin simply posed the primal evolutionary question: Who is our closest relative?–and then worked from there.
This principle of genealogy-before-all as a source of meaningfulconnection among organisms remains at the root of biology. Organisms are tied by descent, and the temporal order of their branching defines the matrix of all evolutionary reasoning. A modern debate about details of genealogy will illustrate the fundamental character of this knowledge.
What is the order of branching among these three closest cousins: chimps, gorillas, and humans? Gorillas look much like overgrown chimpanzees; many differences in form between the two species might be interpreted as consequences of larger size among gorillas. As mammals grow, their shapes alter in predictable ways, a subject called allometry in the language of biologists. The larger brow ridges, sagittal crest (the ridge for muscular attachments along the top of the skull), and relatively more massive jaws of male gorillas could be interpreted as simple allometric responses to increasing size. For these and other reasons, biologists have generally assumed that chimps and gorillas must form a genealogical pair (a sister group, in our jargon), joining up with a sepa- rate human lineage further back, as in Figure 1 (below).
Methods of direct genetic analysis are sweeping systematics (thescience of classification) and beginning to provide confident answers to long-standing genealogical puzzles. The sample of nu- cleotides in DNA is so huge, and composed of so many independent items, that overall similarity in sequence can only reflect a history of common descent.
Charles Sibley and Jon Ahl quist of Yale have pioneered a methodof biochemical systematics that compares the entire DNA program of two species for single copies of their genes. (They elimi- nate so-called multiple-copy DNA from their analyses. Some genes are duplicated thousands of times in the DNA program of an organism. These repeated elements could give misleading measures of similarity by counting a single resemblance over and over again.) Their technique of DNA-hy- bridization (combining the DNA of two species to measure genealogical closeness by tightness of bonding) ranks among the most promising and revealing of new techniques.
The dust has far from settled on data still in the making, but Sibley and Ahlquist are getting consistent results for a surprising, indeed quite astonishing, conclusion. Genetic distances seem to be saying that the closest sister group among chimps, gorillas, and humans unites us with chimpanzees, and doesn’t bind the two great apes together. In other words, we share a common an- cestor with chimps, and gorillas diverged longer ago, as in Figure 2
We cannot yet confirm this outstandingly counterintuitive scheme, for two major reasons. First, the difference in genetic distance be- tween chimp-human and chimp-gorilla isn’t great, and the statistical jury is still out on the significance of the smaller measured difference between chimp and human. Second, Sibley and Ahlquist’s technique is subject to greater error for recent events (yielding quite small genetic distances, as between chimps and humans) than for scales of tens of millions of years, where it has been so successful (especially in resolving the phylogeny of bird families). Still, we must treat their unconventional hypothesis as a likely contender to traditional views.
I’ve tried to express my excitement about this revision to several friends and have, three times, received this response from non- scientists: What’s all the fuss? Who cares about such niceties in branching order? Who really gives a damn whether gorillas branched first, or humans branched first, since we all agree that chimps, humans, and gorillas share a common ancestry? My re- sponse can only be: nothing matters more (in evolutionary terms), for our science is fundamentally about order of branching and its deep and manifold consequences.
Consider just one reason why you must care about branching orderif you take any interest in evolution. The biochemical estimate for human-great ape divergence is five to eight million years ago, with many scientists pushing the more recent date. The earliest australopithecines (Lucy and her ilk) are some three and a half million years old. Lucy also walked upright, as well as you or
I, according to most experts. The anatomical transition to upright posture, involving a radical reorganization of legs and pelvis, was the most profound change in our evolution from ape-like ancestors–far more complex than the later enlargement of our brains.
The extent of this transformation in locomotion poses a deep problem for our usual assumptions about slow evolutionary change. The fossils are pushing hard upon the probable date of splitting, leaving very little time for this major alteration. The most extensive anatomical change in human evolution must be compressed into a mere million years or so. The most common exit from this problem holds that we’ve overestimated the extent of change by imagining a common ancestor too much like four- footed chimps and gorillas. Maybe the common ancestor of hu mans and great apes was already well on the road to upright posture, while the shared four-footed adaptations of chimps and gorillas– particularly their unique habit of walking on their knuckls–evolved later as a special trait of the chimp-gorilla lineage af- ter its separation from humans. The anatomical transformation to upright posture would then not be so great after all, and the problem disappears.
This argument does save our preference for stately evolution, but it requires one key condi- tion–the truth of the conventional branching tree now challenged by Sibley and Ahl quist. In the conventional tree, traits uniquely shared by chimps and gorillas need not be present in the common ancestor of hu mans. But in the Sibley-Ahlquist revision, our ancestor must have possessed whatever chimps and gorillas hold in common by descent–for common ancestor A (Figure 2) must have walked on its knuckles (as gorillas still do), and so did common ancestor B (as chimps still do). I need hardly add that B is our ancestor as well. If Sibley and Ahlquist are right, then our common ancestor was probably as four-footed as chimps and gorillas–and also walked on its knuckles. The case for accelerated evolution between a basically four-footed ancestor and the oldest upright australopithecine gains strength from the Sibley-Ahlquist revision, while an opposite argument for slower change from an ancestor well on the bipedal road requires the traditional tree, now seriously in doubt.
(The only way to save slow evolution on the Sibley-Ahlquist treeis to argue that neither ancestor A nor B walked on its knuckles, and that chimps and gorillas evolved this odd habit independently. We may suspect such convergence for one or two traits, but chimps and gorillas probably share too much of a locomotory pattern for independent evolution of all its features.)
Knowledge of genealogy, or branching order on the evolutionary tree, therefore provides our most fundamental source for inferences about ancestors and evolutionary pathways. We’ve seen how direct evidence of fossils and inferences based on the genealogical relationships of modern organisms combine to support an African origin for the human lineage. Yet, in singing the praises of genealogy, I fear I may have demoted the central importance of fossils. You may be thinking: If
we can infer properties of ancestors by knowing the branching order among descendants–as in our conclusion that the common ancestor of chimps and hu mans walked on its knuckles–then why do we need fossils at all? Therefore, in conclusion, three defenses by a profes- sional paleontologist for his beloved fossils:
1. Even if we can make good inferences about ancestors from the branching order of their descendants, nothing but fossil evidence can affirm (or reject) these hypotheses directly.
2. The reconstructed ”ancestor” inferred from branching order isn’t a living, breathing creature, but only a disembodied, minimal set of traits. We might claim with some confidence that the common ancestor of chimps and humans walked on its knuckles and grew five toes, but these logical methods cannot reconstruct the rich detail of a real, working organism. Genealogical order alone will teach us little about the peculiarities of our most ancient grandparent–its size, its particular adaptations in feeding, escape, and socialization, its hopes and fears (imperfectly re- corded in its ecological context). We will need fossils to transform a bare, logical list to a biological object.
3. Fossils are tangible–and beautiful. I’ve just returned from a week in the field with Richard Leakey, near the western shore of Lake Turkana in Kenya. We pros- pected an awesome, bare landscape, entirely devoid of water and almost bereft of vegetation. Bones lie scattered on the sur- face as ancient strata erode and crumble under the desert sun. (I couldn’t see a bone to save my skin, but found fossil snails, noticed by no one else, at almost every site. We’re all locked into the ”search images” of our own specializations; I’m a student of land snails.) We stood on the very site, in the very context of desiccation that may have driven our ancestors out of Africa and onward into history. Could anyone in his right mind give this up for the logic of branching diagrams? Genealogical order may provide a rigorous framework, but as
I held the precious skulls of our ancestors in the vault of Kenya’s National Museum, I could only tremble and say that Solomon in all his glory was not arrayed like one of these.
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