Scientists have squeezed enough DNA out of a Neandertal fossil to compare it with the only other existing Neandertal DNA sample, as well as with the genetic makeup of people today.
Even with these new data, however, debate continues over the role of Neandertals in the evolution of modern humans.
The two ancient sequences of mitochondrial DNA—inherited from the mother—contain similarities that they do not share with comparable gene sequences in modern humans, report geneticist William Goodwin of the University of Glasgow in Scotland and his coworkers. Further analysis yielded no support for a Neandertal contribution to the modern human mitochondrial DNA pool, the researchers contend.
Proponents of an out-of-Africa theory welcome the new study’s conclusions. These scientists argue that modern Homo sapiens originated in Africa around 100,000 years ago and then spread throughout the world, replacing Neandertals in the process.
Supporters of multiregional evolution regard the DNA evidence as inconclusive. They hold that humans originated 2 million years ago in two or more parts of the world and later occasionally interbred with Neandertals.
Goodwin’s team focused on the fragmentary skeleton of a 29,000-year-old Neandertal infant unearthed at Mezmaiskaya Cave in the northern Caucasus Mountains, near the Black Sea. The researchers took two small samples from one of the child’s ribs and analyzed them in separate laboratories.
One lab isolated and made many copies of a large piece of a particularly variable part of mitochondrial DNA. The other lab also identified part of that segment, raising confidence in its validity.
The researchers compared the ancient sequence with the other Neandertal DNA sample, which was taken from a German fossil dating to at least 30,000 years ago (SN: 7/19/97, p. 37), and modern human mitochondrial DNA. Variation between the Neandertal sequences equals that seen in random pairs of either modern Europeans or modern Asians, and it exceeds that in modern Africans, the scientists report in the March 30 Nature.
The Neandertal specimens exhibit enough similarity to be placed in a genetic category distinct from, but still related to, modern human mitochondrial DNA, Goodwin and his colleagues assert.
Both Neandertal sequences differ to the same extent from mitochondrial DNA of people living in various parts of the world today, the scientists add. They say that this contradicts the implication of multiregional evolution that modern Europeans, theoretically descended in part from Neandertals, should show the closest genetic likeness to the Neandertals.
“Limited [interbreeding] may have occurred between Neandertals and modern humans, but that appears less likely with these new genetic data,” remarks anthropologist Christopher B. Stringer of the Natural History Museum in London.
Other controversial data suggest that human numbers declined sharply prior to 50,000 years ago, reducing genetic diversity before the population rebounded. The new study shows that Neandertals had too much genetic variation to have taken part in that stage of human evolution, Stringer says.
Multiregional-evolution advocate Milford H. Wolpoff, an anthropologist at the University of Michigan in Ann Arbor, disagrees. Natural selection appears to have reduced mitochondrial-DNA variation after the time of Neandertals, Wolpoff argues (SN: 2/6/99, p. 88: http://www.sciencenews.org/sn_arc99/2_6_99/bob1.htm). He predicts that any mitochondrial-DNA lines from around 30,000 years ago, including those of Neandertals, would differ equally from all regional mitochondrial-DNA lines today, as Goodwin’s team found.
Isolation of mitochondrial DNA from Stone Age H. sapiens would help clarify the meaning of the Neandertal discoveries, says anthropologist John H. Relethford of the State University of New York in Oneonta.