New U-M study suggests that's the way it was.
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ANN ARBOR --- Two million years ago somewhere in Africa, a small group of individuals became separated from other australopithecines. This population bottleneck led to a series of sudden, interrelated changes -- in body size, brain size, skeletal proportions, and behavior -- that jump-started the evolution of our species.
That is the conclusion of a new University of Michigan study published in the current (January 2000) issue of Molecular Biology and Evolution that analyzes a broad range of genetic, fossil, and archeological evidence to decipher the most likely scenario for the start of human evolution.
The analysis, by researchers at the U-M Department of Anthropology, is the first to examine the full spectrum of paleontological, archeological, and genetic evidence available, each reflecting a different part of the puzzle of human origins. By estimating the ranges of error in the different types of evidence, the researchers were able to narrow down the common, overlapping areas of agreement to construct an explanation that disproves some high-profile recent theories and supports one of the oldest modern versions of the origin of homo sapiens.
"All the available evidence supports an 'Out of Africa' theory, that humans first evolved in Africa about two million years ago, then spread to other regions of the world," says John Hawks, first author of the paper and now an assistant professor of anthropology at the University of Utah. "This original population lived before humans colonized regions outside of Africa. In fact, it was the act of becoming human that made these colonizations possible."
Examining the anatomical evidence, the authors, including U-M anthropologist Milford Wolpoff, conclude that a "genetic revolution" took place in a small group isolated from other australopithecines. "The earliest H. sapiens remains differ significantly from australopithecines in both size and anatomical details," notes Wolpoff. "Insofar as we can tell, these changes were sudden and not gradual."
A second reason for suspecting that a population bottleneck led to a rapid genetic reorganization that started the process of human evolution comes from archeological evidence of a series of behavioral changes suggestive of a new adaptive pattern of hunting, gathering and scavenging. "Body size is a key element in these behavioral changes," the authors note, "because of the locomotor changes that large body size denotes, and the increased metabolic resources it requires." These behavioral changes are far more massive and sudden than any earlier changes known for hominids, they point out.
According to the researchers, the available genetic data do not disprove a simple model of exponential population growth following a bottleneck two million years ago and extending through the Pleistocene Epoch, when ice covered much of North America and Europe. But they are incompatible with a more recent population-size bottleneck.
"Many details of subsequent human evolution over the period of the ice ages remain unclear, but one certain finding from both anthropological and genetic data is that there was no later time when the size of the human species became small again," says Hawks. "So the 'Eve theory' of modern human origins, which states that modern human populations very recently arose as a new African species that replaced all other indigenous peoples such as Neanderthals, can be put to rest."
Co-authors of the study with Hawks and Wolpoff are Keith Hunley, U-M Department of Anthropology, and Sang-Hee Lee, Department of Biosystems Science, Graduate University for Advanced Studies, Hayama, Kanagawa, Japan.
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