Africa The Leaky Crucible

If hominids first arose in Africa, as we believe, the circumstances surrounding their dispersal a couple of million years ago are important to ascertain. Africa had been the crucible of human origins but for some reason it began to leak around two million years ago. We have suggested that the leak first started when expanding dry country in the area of the proto-Sahara Desert began to push hominids before it, to the north and out of Africa.15

Until recently it was most reasonable to make sense of Homo erectuss evolution with the following hypothesis: Homo habilis (or Homo ergaster) gave rise to Homo erectus about 1.5 million years ago in Africa, and the species then dispersed to Eurasia. However, with the dating of three homi-nid sites in different regions of Eurasia (Mojokerto/Sangiran in Java, Dmanisi in Georgia, and Longgupo in China), all significantly earlier than 1.5 million years ago, it became clear that hominids existed outside Africa before this time. Until the fossil cranial remains were discovered at Dmanisi, the identity of this hominid was uncertain. Now it is clear that it was anatomically very similar to African late Homo habilis and earliest Homo erectus. The initial deductions of Tobias and von Koenigswald about a close African-Eurasian link had proved correct.

With these new data, one of us (R. L. C.) hypothesized that Homo ergaster (or Homo erectus ergaster as used here) dispersed from Africa and evolved in Asia, becoming a species that we can formally designate with a subspecies name as Homo erectus erectus.16 An equally persuasive case can be made for the evolution of Homo erectus erectus in Africa from the same species.17 We believe that we can accommodate both hypotheses and the observations on which they are based within a new interpretation of homi-nid evolution.

First of all, if the same evolutionary transition occurred in Africa and Asia—same species transition at the same time—they must be linked. The populations of Homo erectus ergaster in Africa 1.9 million years ago must have been in genetic contact with populations of Homo erectus ergaster in Asia. This sounds remarkably like Weidenreich's 1947 idea of hominid evolution and its modern version, multiregional evolution—a widely spread-out species with populations connected by gene flow evolving on a wide front. Despite a basic similarity, new genetic data and interpretations argue against the specifics of such a model.

A massive amount of molecular data from living humans shows that the evolutionary branches of biomolecules that we can compare among human populations are short. These particular molecules evolve rapidly and they converge to a common ancestral molecular configuration not older than about two hundred thousand years.18 Molecular geneticists have tied this biochemical origin of the typical human genome to fossil evidence documenting Homo sapiens in Africa at about the same time. Many paleo-anthropologists have agreed that these data confirm that Homo sapiens appeared at this time in Africa and then spread out into Eurasia. We, however, believe that the molecular data have been widely misinterpreted, both by most molecular geneticists and most paleoanthropologists. The missing perspective comes from an older branch of genetics, one that came to prominence in the first half of the twentieth century—population genetics.

The conception of human expansion out of Africa held by many molecular geneticists, whose primary data we must remember come from laboratory test tubes and electrophoresis gels, is that of an abstract group of hominids surrounding an ancient African female named, for marketing purposes, "Eve," of course. Paleoanthropologists, whose primary data are merely isolated bits of fossil bone and chipped stone, have also conceived of the epic expansion of hominids into Eurasia as something like a single band of scruffy, spear-toting hominids walking into Asia Minor, peering over the horizon into the promised land of Eurasia. With as little data to go on as we apparently have, it is perhaps understandable that such B-grade cinematographic short takes are tolerated. One can almost hear the leader of the band exhorting the group to go forth and multiply.

Population genetics is a field of study that uses mathematical methods and field studies of modern species. Its conclusions can escape the notice of paleoanthropologists, whose research interests focus on the past, and molecular biologists, whose research interests focus on the laboratory. However, calculations in population genetics show that a single band of hominids, and certainly not a single woman nicknamed "Eve," could not have come across from Africa to "replace" all the hominid populations. But neither did regional populations of hominids in Eurasia simply absorb the newcomers from Africa into their gene pool. The residents were replaced as genetic species. It just happened over time and with many populations—a story with perhaps a less dramatic storyline but one that is ultimately more compelling because it is more likely what actually occurred.

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