Neanderthals Didn't Mate With Modern Humans, Study Says

Ker Than
for NGN

Neanderthals and anatomically modern humans likely did not interbreed, according to a new DNA study.

The research further suggests that small population numbers helped do in our closest relatives.

Researchers sequenced the complete mitochondrial genome—genetic information passed down from mothers—of a 38,000-year-old Neanderthal thighbone found in a cave in Croatia. (Get the basics on genetics.)

The new sequence contains 16,565 DNA bases, or "letters," representing 13 genes, making it the longest stretch of Neanderthal DNA ever examined.

Mitochondrial DNA (mtDNA) is easier to isolate from ancient bones than conventional or "nuclear" DNA—which is contained in cell nuclei—because there are many mitochondria per cell.

"Also, the mtDNA genome is much smaller than the nuclear genome," said study author Richard Green of the Max Planck Institute for Evolutionary Biology in Germany.

"That's what let us finish this genome well before we finish the nuclear genome," he said.

The new findings are detailed in the August 8 issue of the journal Cell.

A Small Population

The new analysis suggests the last common ancestor of modern humans (Homo sapiens) and Neanderthals lived between 800,000 and 520,000 years ago. This is consistent with previous work on shorter stretches of Neanderthal DNA.

Contrasted with modern humans, Neanderthals exhibited a greater number of letter substitutions due to mutations in their mitochondrial DNA, although they seem to have undergone fewer evolutionary changes overall.

The fact that so many mutations—some of which may have been harmful—persisted in the Neanderthal genome could indicate the species suffered from a limited gene pool. This might be because the Neanderthal population was smaller than that of Homo sapiens living in Europe at the time.

A small population size can "diminish the power of natural selection to remove slightly deleterious evolutionary changes," Green said.

The researchers estimate the Neanderthal population living in Europe 38,000 years ago never reached more than 10,000 at any one time.

Homo neanderthalis first appeared in Europe about 300,000 years ago but mysteriously vanished about 35,000 years ago, shortly after the arrival of modern humans—Homo sapiens—in Europe.

"If there were only a few, small bands of Neanderthals, barely hanging on, then any change to their way of life could have been enough to drive them to extinction," Green said.

"One obvious change would have been the introduction of another large hominid—modern humans."

Stepping Forward

Stephen Schuster, a molecular biologist at Pennsylvania State University, said the new study should silence a lot of theories about interbreeding between Neanderthals and modern humans.

The study shows that "at least for the maternal lineage, there are no traceable genetic markers that suggest admixture of Neanderthals and modern humans," he said.

Schuster added that the researchers were exceptionally careful to isolate the Neanderthal DNA.

"Many more precautions were taken to ensure that no contamination with human DNA has flawed the analysis," he said, noting that researchers sequenced each letter about 35 times to be sure of their work.

"This was the weak point of previous reports," said Schuster, who was not involved with the study.

Thomas Gilbert, an ancient DNA expert at the University of Copenhagen in Denmark who also was not involved in the study, called the research a "step forward" and a taste of what might come when the Neanderthal nuclear DNA is finished.

The team's argument that the Neanderthal population was small 38,000 years ago is speculative, Green said, but "it's better than what we could have said before."