`Y guy' steps into human-evolution debate.

Mitochondrial Eve, meet Y-chromosome Adam. Call him Y guy--he's a younger man, after all. The scientists who tracked down Y guy see him as a potentially key figure in the debate over the location and timing of humanity's origins. Yet other investigators view Y guy as a statistical apparition generated by dubious evolutionary assumptions.

Y guy is a genetic reconstruction of the common ancestor of males today, according to a report in the November NATURE GENETICS. He resided in eastern Africa and first trekked into Asia between 35,000 and 89,000 years ago, say the researchers. In contrast, mitochondrial Eve--the hypothetical common female ancestor of all people today--lived in Africa and migrated into Asia around 143,000 years ago, other researchers have concluded from genetic analyses.

The Y and mitochondrial chromosomes apparently dispersed throughout the human population at different rates, suggest geneticist Peter A. Underhill of Stanford University and his colleagues, who published the new DNA dossier on Y guy. Nonetheless, Underhill's team says, the genetic data behind both Eve and Y guy support the theory that modern humans originated relatively recently in Africa and then spread elsewhere, replacing groups such as the Neandertals.

The researchers used 167 chemical markers to probe alterations of nucleotide sequences in the Y chromosomes in modern men. DNA samples came from 1,062 men from throughout the world. Underhill and his coworkers used a statistical program to identify men with the same sequences. They then constructed a tree of branching evolutionary relationships for men from the different parts of the world.

Men from eastern Africa fell into a genetic group at the root of the Y chromosome tree. Not only did their DNA contain a distinctive pattern, but it exhibited the greatest number of mutations. Underhill's model assumes that such mutations accumulate randomly at a relatively consistent rate over time--like a molecular clock--allowing for their calculation of Y guy's age range.

The Y chromosome segments in the new analysis exhibit much less variability than DNA regions that have been studied in other chromosomes. Low genetic variability may reflect natural selection, in this case, the spread of advantageous Y chromosome mutations after people initially migrated out of Africa, the researchers suggest. That scenario would interfere with the molecular clock, making it impossible to retrieve a reliable mutation rate from the Y chromosome, they acknowledge.

Uncertainties exist in the genetic data, but the new report takes "a quantum step forward" in the study of prehistory, comment archaeologist Colin Renfrew of the McDonald Institute for Archaeological Research in Cambridge, England, and his colleagues, in the same issue of NATURE.

"This is a beautiful piece of work," adds anthropologist Henry Harpending of the University of Utah in Salt Lake City. The Y chromosome data support several other DNA studies indicating that modern humans arose from a small number of Africans who lived from 100,000 to 200,000 years ago, Harpending says. He suspects that the Y chromosome mutation rate is slower than that assumed by Underhill's team, meaning that Y guy lived closer to the time of mitochondrial Eve.

However, some critics say that the new study shares much deeper flaws with other genetic analyses of human evolution (SN: 2/6/99, p. 88).

"We don't know what selection and population structure are doing to the Y chromosome," says geneticist Rosalind M. Harding of John Radcliffe Hospital in Oxford, England. "I wouldn't make any evolutionary conclusions from [Underhill's] data."

For instance, greater Y chromosome diversity in African men may have arisen because more people inhabited that continent than anywhere else during the Stone Age, not because the African population is older, Harding says.

Moreover, men may have occasionally moved from one region to another after leaving Africa and spread advantageous Y chromosome mutations, thus fostering the low genetic variability observed in the new study, Harding adds.

If the critics are right, Y guy could be history, not prehistory.