Gene data place home of 'Eve' in Africa.

A study of random genetic changes in one type of human DNA supports and expands a controversial 1987 DNA investigation that placed the origin of anatomically modern humans in Africa approximately 200,000 years ago. Although both studies indicate that modern Homo sapiens spread from Africa to other regions and replaced the descendants of an earlier African migration by H. erectus, some anthropologists still contend that modern humans originated closer to 1 million years ago in several parts of the world, with some genetic input from Neandertals along the way (SN: 6/8/91, p.360).

In the new work, molecular biologist Linda Vigilant of Pennsylvania State University in University Park and her colleagues studied mitochondrial DNA -- located outside the cell nucleus and inherited only from the mother -- obtained from 121 native Africans, 20 Papua New Guineans, one native Australian, 15 Europeans, 24 Asians and eight Americans of African descent. In samples from each individual, the researchers identified the chemical arrangement and random changes, or mutations, at the most rapidly evolving segment of mitochondrial DNA. The investigators assume that such mutations arise at a relatively constant rate, so that populations displaying greater numbers of such alterations represent older genetic lineages.

Native Africans possess more mitochondrial mutations than the other groups, the team reports in the Sept. 27 SCIENCE. Within each human population, specific chemical sequences turn up and others do not, but U.S. blacks show close mitochondrial DNA links to native Africans, the scientists assert.

Estimates of mutation rates in mitochondrial DNA provoke much controversy. Some researchers doubt the existence of any such "mitochondrial clock." But Vigilant and her co-workers argue that their analysis, based on a mutation rate derived from a comparison of mitochondrial DNA from humans and chimpanzees, indicates that the ancestral mother or mothers of modern H. sapiens lived in Africa between 166,000 and 249,000 years ago.

They made this calculation with two key pieces of information: a new reckoning that the chemical sequence of the rapidly evolving mitochondrial DNA in humans differs from that in chimps by nearly 70 percent, and a previous estimate that the chimp and human mitochondrial lineages split between 4 million and 6 million years ago.

The researchers note that even if the chimp-human split occurred 9 million years ago, as some scientists argue, human mitochondrial origins would extend back only about 373,000 years.

Such age estimates remain preliminary, they add. Possible differences in the mitochondrial mutation rates of humans and chimps may throw off the timing of the proposed mitochondrial clock.

Still, they conclude that the new data provide the strongest support yet for assigning a relatively young evolutionary age to the anthropological "Eve" -- our common mitochondrial DNA ancestor in Africa.