Out of Africa

Ask friends how they enjoyed their African safari and you’ll hear something like, “Fantastic; I can’t wait to go back. You know, Africa seems to get into your blood.” After reading this delightful book, you will learn that not only is Africa already in our blood but it is also in our genes. Following a review of archeological, fossil and genetic evidence, Christopher Stringer and Robin McKie proclaim that Africa was the original homeland to our species, Homo sapiens. Stringer, director of the Human Origins Group at the Natural History Museum in London, and McKie, science editor of London’s Observer, join forces here as scientist and journalist to weave together a knowledgeable and carefully crafted rendition of the origins of the most egocentric species known--ourselves.

It’s no secret that paleoanthropology, the study of human origins, is among the most contentious scientific pursuits in the public eye, and the current debate on the origins of modern humanity is no happy exception. The passions that boil to the surface may be understandable when we consider that the controversy is about the origins of all living people; this is pretty heady stuff. To their credit, the authors reveal little of the intensity of the acrimonious debate, which often bitterly divides different camps on the issue. References are few to the fireworks that often accompany international scientific conferences on the origins of modern humans and only occasionally is a colleague singled out when his style is considered abrasive.

Stringer is one of the most fervent defenders of the “out of Africa model,” which advocates an African genesis for H. sapiens, roughly 200,000 years ago, and a subsequent diaspora replacing other archaic hominid populations. The opposing school, usually dubbed the “multi-regional model,” asserts that more archaic populations persisted and gave rise to modern human races in different geographical regions.

For the first time, in a book written for a lay audience, Stringer and McKie present a comprehensive and easily understandable account of the “out of Africa model.” Sadly, equal treatment is not afforded the “multi-regional model,” and the reader is often left craving more details of this alternative theory. This is especially true when little attention is granted the multi-regionalists’ view of the place of Neanderthals in the ancestry of modern humans.

The tenets of multiregionalism were initially forged, in large part, by Franz Weidenreich, whose exhaustive studies of the Peking Man (H. erectus) fossils distinguish him as one of the most accomplished scholars of hominid anatomy ever to lay calipers on a fossil skull. He believed that the anatomical differences, which characterize living races, could be traced back to H. erectus ancestors, hundreds of thousands of years ago.

Weidenreich’s notion of regional continuity found a zealous advocate in Carleton Coon, author of the 1962 book, “The Origin of Races.” His evaluation of the fossil evidence for human evolution led him to submit that H. sapiens arose five times, independently: Congoids and Capoids in Africa, Caucasoids in Europe, Mongoloids in Asia and Australoids in Australia. Racial undercurrents in Coon’s writing have always been a source of discomfort to anthropologists, particularly the implication that because African and Australian achieved modernity tens of thousands of years after white and Oriental races, they were less evolved.

Coon’s work nourished racist views, which unfortunately still lurk in many corners of human society. In some circles, if racial identity (notoriously impossible to define, by the way) has deep evolutionary roots, the differences we see between groups must have deep biological meaning. This might seem very obvious when we look at variation in eye color, nose shape, hair texture and especially skin pigmentation, but genetic studies have convincingly demonstrated only minuscule differences between human populations. Let this be a warning to those who exaggerate the meaning of superficial differences to imply different levels of superiority. In this vein, Stringer and McKie write, “what unites us is far more significant than what divides us.”

A drawback to the regional continuity model is the notion that independent hominid populations all evolved toward a single goal. There is no evolutionary trajectory that has as its goal modern humanity, no matter how special we think we are. There is no reason why all populations should have evolved toward one goal. After all, evolution is not a unilineal but a branching affair. If we had, in fact, been reproductively isolated, human populations should be even more distinct from one another and, in extreme cases, perhaps even branched into a different species. On the other hand, if genes freely flowed among humans, we should all look pretty much alike. Alan Thorne of the Australian National University and Milford Wolpoff of the University of Michigan, two of the most ardent champions of the “multi-regional model,” suggest that isolation was adequate to generate distinctive physical differences, yet gene flow was sufficient to keep them all evolving, collectively toward H. sapiens.

Gene flow is a crucial element in the “multi-regional model,” but Stringer and McKie question whether ancient human population densities were sufficient to facilitate continual gene flow among hominids as widely distant as those in South Africa and Indonesia. Additional impediments to gene flow might have included climatic disruptions and geographic features such as mountains, deserts and bodies of water. They further suggest that cultural differences, such as taboos against mating outside of one’s immediate group, may have been an even more important barrier to interbreeding.

Stringer and McKie assert: “The realization that humans are biologically highly homogeneous has one straightforward implication: that mankind has only recently evolved from one tight little group of ancestors.” They base their argument on a detailed study of a form of DNA, called mitochondrial DNA, which supports the view that this group lived in Africa. A higher frequency of mutations in African DNA was interpreted as a reflection of their longer evolutionary past. In the popular literature, since mitochondrial DNA is inherited only along the female line, this idea is better known as the “African Eve hypothesis,” suggesting we are all descended from a single female. The “African Eve Hypothesis” is a particularly difficult concept to grasp, but Stringer and McKie do a superb job of explanation.

Adding to Stringer’s and McKie’s argument, the fossil evidence seems also to favor an African locale for our origins. Fossil hominids from southern Ethiopia (Kibish), northern Tanzania (Laetoli) and southern Africa (Klasies River Mouth), attest to the presence of modern anatomy slightly in excess of 100,000 years ago. The presence of H. sapiens dating to 100,000 in Israel (Qafzeh and Tabun) support the view that we left Africa through the crossroads of the Middle East.

Neanderthals pose a challenge to the regional continuity viewpoint that all roads led to modern humans. The place of Neanderthals in the bigger picture of human evolution depends on the interpretation of who Neanderthals actually were. For the multi-regionalists, Neanderthals were simply a subspecies, H. sapiens neanderthalensis, not significantly different from ourselves and certainly capable of interbreeding with us. Stringer and McKie interpret the unique Neanderthal skeletal anatomy as a testament to their separate species status--H. neanderthalensis. They posit a classic speciation event (the creation of a new species) in which a group of hominids, probably belonging to H. heidelbergensis (or the recently announced species H. antecessor) became geographically and, hence, reproductively isolated in southwestern Europe. Largely in response to the significantly colder environment, due to Pleistocene glaciations, they developed their own diagnostic anatomy. Even to the novice, the Neanderthal skull is easily distinguished by its backward sweeping cheek bones; projecting mid-face; broad nasal opening; enlarged sinuses; elongated, somewhat flattened braincase; receding forehead; and prominent, curved brow ridges. The limb bones, with large joints, are stout and thick, giving the body a squat appearance, not unlike that of Eskimos who live in cold environments today. (Stunning vindication of the uniqueness of Neanderthals was recently announced when scientists successfully extracted DNA from a Neanderthal fossil and found that it was clearly distinct from the DNA of modern humans.)

The exact manner in which Neanderthals met their demise will always be open to conjecture, but evidence for a genocide is lacking. It is more likely that modern humans displaced Neanderthals into landscapes that were not only unfamiliar but lacked reliable food sources upon which Neanderthals had become accustomed. This process took some time and fossil evidence suggest that Neanderthals hung on in Europe until 30,000 years ago, some 10,000 years after modern humans arrived, perhaps from the Middle East.

Stringer and McKie provide an excellent review of the archeological evidence which suggests that the more elaborate tool kit and the enhanced mental abilities possessed by modern humans created a more complex society, which spread widely throughout Europe and exploited a wide range of natural resources and, most important, usurped Neanderthal territory. Unlike Neanderthals who remained in one place year-round, the more sophisticated modern humans roamed over larger areas, collecting various raw materials and exploiting seasonal food sources. Excavation of Neanderthal sites strongly suggests that they did not wander far from their base camps. In short, while Neanderthals possessed many human-like behaviors, such as extended childhood, large brains, fire making, probably burials, the construction of shelters and so on, their minds were limited and the increased complexity of modern human society ultimately drove them into relict areas, such as southern Spain, where they expired about 30,000 years ago.

“African Exodus” reminds us that there are no guarantees in evolution. Neanderthals, after several hundred thousand years as a distinct lineage, ultimately met extinction. The final chapter of this thoughtful book reminds us of our own precarious existence. For example, since the agricultural revolution, humans have become a species out of control, reproducing at frightening rates, becoming more and more susceptible to some of the world’s most deadly epidemics and at the same time depleting the Earth’s natural resources at an alarming pace. Stringer and McKie warn us that we are no longer in balance with nature. Genetically we are little changed from our hunter-gatherer past, but our technology has taken us to the surface of Mars. Perhaps the imbalance created by the disparity between our biological and technological evolution is the cause of many of humankind’s ills. Stringer and McKie warn us that it may be foolhardy to assume that technology will triumph over nature. After all, we are often uncomfortable with technology; witness the numbers of us intimidated by the prospect of programming the family VCR.

The authors write, “We triumphed in the end for a variety of reasons: social, cognitive, behavioral and technological.” Where do we go from here as a species? We are survivors; we are the lone existing branch on the human family tree; we deserve to be here just like any other species, but we have the frightening obligation of being in charge. While “African Exodus” holds no secrets for our future tenure on earth, it stimulates us all to ponder our humanity.