DNA sleuths discount a human-Neanderthal link

New evidence suggests crossbreeding didn't occur, but it could offer insight into life during the Stone Age.

Robert C. Cowen
Special to The Christian Science Monitor

Move over human-DNA detectives. Here come the Neanderthal sleuths.

Paleontologists had suspected that the stocky, beetle-browed hominids interbred with modern humans. New DNA evidence shows they are innocent of that charge.

The DNA comes from a 29,000-year-old fossil found in a cave in the northern Caucasus. That's a time when scientists believe modern humans were replacing Europe's Neanderthal population.

The Neanderthal DNA is well enough preserved to compare key sequences with representative human DNA samples. The two DNA types are different enough to strongly suggest that we have no Neanderthal coding in our genes. The researchers estimate that our most recent common ancestor with the Neanderthals lived 151,000 to 352,000 years ago.

In addition to answering long-held questions about a Neanderthal-human link, the new evidence is also revitalizing the field of ancient-DNA research, scientists say. The result could be a fuller understanding of the Stone Age.

This research confirms the findings of the first Neanderthal DNA analysis reported four years ago. That DNA came from the original Neanderthal fossil discovered nearly 150 years ago in Germany's Neander Valley. It, too, showed no gene flow between Neanderthals and humans. But scientists wanted more evidence before accepting that conclusion.

Now they have it. Matthias Hoss at the Swiss Institute for Experimental Cancer Research in Epalinges says this confirmation is why the new study "is probably the more important of the two."

It provides what he calls "invaluable corroboration for the authenticity of Neanderthal DNA sequences." He says scientists may be "truly on our way to Neanderthal population genetics."

Igor Ovchinnikov and colleagues at the University of Glasgow in Scotland stress this point in today's issue of Nature. There are many pitfalls in getting reliable data from ancient DNA.

To begin with, DNA decays fairly quickly unless preserved in dry or oxygen-free conditions. Second, the techniques for extracting and analyzing ancient DNA samples are easily fooled by contaminating DNA that has leaked in from the environment. This has undermined some of the spectacular early claims of decoding DNA from insects buried in amber or from million-year-old plant remains.

Many such claims could not be independently confirmed. To avoid this trap, the original Neanderthal DNA project led by Svante Pääbo of the University of Munich in Germany had an outside laboratory check their work.

Likewise, the Glasgow team asked Stockholm University's Archaeological Research Laboratory to confirm its findings.

The Glasgow team notes that those findings do not absolutely rule out a remote possibility of some Neanderthal influence on human genetics.

Most of the DNA that carries genetic instructions is in the nucleus of the cell. A second type of DNA resides outside the nucleus in energy-producing units called mitochondria. That's the kind of DNA the Neanderthal researchers use. It's also the kind of DNA criminal detectives use.

Mitochondrial DNA is easier to get at than nuclear DNA. There's only one set of nuclear DNA per cell. Cells have as many as 1,000 mitochondrial DNA copies.

In his commentary, also published in Nature, Dr. Hoss notes that scientists cannot yet "exclude the possibility of a contribution of nuclear Neanderthal DNA genes" to the overall human gene pool.

Yet the lack of Neanderthal genes in the mitochondrial DNA - which is inherited along with the nuclear DNA - would be enough for a court of law to clear Neanderthals of the charge of crossbreeding. Moreover, "the excellent preservation of this specimen leads to the potential of analyzing the entire Neanderthal mitochondrial genome," the Glasgow team notes.

Source: http://www.csmonitor.com/durable/2000/03/30/fp2s2-csm.shtml