Pleistocene Epoch

1.8 million to 12,000 years ago

recent skull

Modern humans evolved in Africa about 100,000 years ago.


Modern humans became lighter and more gracile. (Gibbons, 1997). The sphenoid bone shortened, meaning that the face did not project quite as far. The skull became higher and rounder in contrast to the low, long skulls of Homo erectus and Neanderthals (Lieberman, 1998). How did modern humans evolve? There are two different models.

--Replacement Model:
In the replacement model, it is thought that somewhere between 1 million and 100,000 years ago, modern humans left Africa, migrated throughout the continents, and completely replaced all other hominids (such as Homo erectus, Homo neanderthalis) so that they alone were ancestral to all modern human groups. Proponents of this model vary on the timing, often due to discipline; paleontologists favor older dates, geneticists favor younger dates. One strong piece of evidence that is frequently observed is that the greatest genetic difference in human populations occur between African populations. The evidence for some of the oldest branches of humanity exist in Africa suggest that humanity evolved in Africa (Zischler, 1995; Waddle, 1994; Vigilant, 1991; Wood, 1997). Advanced tools (referred to as Acheulan; they included items such as hand axes) first appear in Africa and are known outside Africa as of 500,000 years ago (Lewin, 1987).

--Regional Continuity Model:
Although modern humans did leave Africa over 100,000 years ago, they interbred with the hominid populations which were already present on other continents (Homo erectus in Asia, Homo neanderthalis in Europe) to produce many of the regional differences in modern humans. The discovery of apparently ancient DNA sequences in Asian and Australian populations which are not found anywhere else in the world supports the model of regional continuity (Harding, 1997; Holden 2001). Some anthropologists conclude that a number of fossil finds of Homo erectus and Homo neanderthalis have physical traits which correspond to the modern humans from those regions from much later dates (Wolpoff, 1993; Thorne, 1992; Thorne, 1981) .

In comparing the microsatellite DNA in the human genome, the most divergent patterns are observed in some African populations, suggesting an African origin of humanity (Goldstein, 1995). Comparisons can be made between mitochondrial genomes (which are primarily passed from women to their children) and Y-chromosome sequences (which are passed from men to their sons). These comparisons tentatively indicate that women have actually been more mobile than men (in that they did not necessarily stay in the areas in which they were raised) and thus were greater agents of gene dispersal (Pennisi, 2001).
Modern humans arrived in Australia by 40-60,000 years ago (there are sites in East Timor which are slightly younger at 35,000 years old). The Clovis culture of big game hunters is known in the New World as of 12,000 years ago. In recent years, there have been a number of pre-Clovis sites found in both North America (ranging from 12-19,000 years old) and even in Chile (12,500 years old). The ancestry of the first Americans may be more complex than what was first thought—some skeletal traits of fossils and genetic analyses indicate that native Americans are more similar to Europeans than Asians (Marshall, 2001; Pena, 1999; Gibbons, 2001; Holden, 1996; Morell, 1998).
Molecular analyses point to three groups of aboriginal Americans: Amer-Ind, Na-Dene, and Eskimo. The Na-Dene and Eskimo might not represent separate migrations but rather early offshoots of the Amer-Ind group which were isolated in northern regions by changing climates (Bonatto, 1997).