1.5The Origin and Evolution of Humans
Two models have been proposed to explain how Homo sapiens came into existence. One model is the "Multiregional Hypothesis." According to this hypothesis, primitive humans (called "Homo erectus") emerged in Africa 6 million years ago and radiated through the Middle East to Eurasia 1.8 million years ago, with Java man evolving into the indigenous Australians (aborigines), Peking man becoming the East Asians, and Neanderthal man becoming Cro-Magnon man, and ultimately, the modern Europeans. The other model is the "Out-of-Africa Theory." According to this theory, the ancestors of all modern humans on the earth were in Africa 150,000–200,000 years ago, and spread from there to the rest of the world (Fig. 1-5). The latter model is now widely believed to be the correct one based on DNA analysis.
Mapping the Genome of Neanderthal Man
By studying our genome (see Chapter 3), it is possible to find out whether Neanderthals were the ancestors of the present modern humans. Morphological observations of bones have led to debate about whether Neanderthals, the fossil humans who became extinct about 30,000 years ago, are the direct ancestors of modern humans, or whether they are a separate hominid species that branched off from the phylogenetic tree of humans. Some researchers believe that if we put a suit on a Neanderthal man and took him downtown, nobody would notice him. Others believe that he would be clearly distinguishable because of his large size, thick bones, and prominent layer of hair.
Mitochondrial DNA was extracted for the first time in 1997 from the first fossil (bone) discovered in the Neander Valley in Germany. DNA can usually be preserved for only a maximum of about 50,000 years. Dinosaurs became extinct 64,000,000 years ago, and therefore, their DNA is difficult to collect unless it is exceptionally well preserved. Thousands of mitochondria are present in one cell, and this number is overwhelmingly larger than the mere 2 copies of nuclear DNA that are present in one cell. Consequently, the DNA that has been extracted from ancient organisms is almost all mitochondrial DNA. In the first analysis of this DNA, differences between humans and Neanderthals were found in 26 out of 379 consecutive bases. Differences among even the most dissimilar humans occur at no more than 8 sites; therefore, 26 sites is a very large number. By comparison, differences between humans and chimpanzees have been found at 50 sites in the same location. It has therefore been concluded that modern humans and Neanderthals diverged about 500,000 to 650,000 years ago and that Neanderthals are not our direct ancestors.
The gene sequence is thus an important record that contains the results of evolution. It is possible to infer the species of an extinct animal in a museum from its hair, to determine where a person came from by analyzing his bones excavated from an archaeological site, and to understand the process of the evolution of pathogenic microorganisms.