"It is very exciting that the geneticists now have internal dating procedures, but really I think the dates are very loose indeed," Dr. Geneticists believe that the world outside Africa was populated by the migration of a very small number of people who left east Africa about 50,000 years ago.These modern humans, with their more advanced and inventive culture, are thought to have displaced the archaic hominids like the Neanderthals, which had emigrated from Africa many thousands of years earlier.
But it now seems that the world filled up early and the first inhabitants were quite resistant to displacement by later arrivals. Richards's estimates, reported in the current issue of The American Journal of Human Genetics, are based on analysis of mitochondrial DNA, a genetic element that occurs in both men and women but that is transmitted only through the mother; thus, they reflect only the movement of women. Underhill of Stanford University and colleagues reported the first analysis of the European population in terms of the Y chromosome.
"The idea that clicks were lost from all languages other than Khoisan," Dr.
Traill said, "is stimulating, but I don't know what to make of it." Of the three disciplines that bear on human origins historical linguistics, population genetics and archaeology only archaeology has a rock-solid method of dating, based on radiocarbon and other kinds of radioactive decay.
"So all the behaviors of the Upper Paleolithic have an African pedigree," Dr. The behaviors were gradually assembled as a package and exported, "which is why it appears suddenly in Europe 40,000 years ago," she said. Klein said in an interview that he doubted some of the early dates proposed by Dr. Brooks, and that even if the dates were correct, modern behaviors conferred such an advantage that they should appear in a broad pattern, not just at the handful of places cited by his critics.
To understand what happened in the past, it is necessary to look for patterns and ignore the "noise," he said.