Modern Humans, Neanderthals May Have Interbred
Fossils point to a melding, not replacement, of species
MONDAY, Oct. 30, 2006 (HealthDay News) -- There may be a little Neanderthal in all of us.
That's the conclusion of anthropologists who have re-examined 30,000-year-old fossilized bones from a Romanian cave -- bones that languished in a drawer since the 1950s.
According to the researchers, these early Homo sapien bones show anatomical features that could only have arisen if the adult female in question had Neanderthal ancestors as part of her lineage.
The findings may answer nagging questions: Did modern humans and Neanderthals interbreed on a significant scale? And were the Neanderthals exterminated about 28,000 years ago -- as some anthropologists contend -- or did they gradually assimilate into the gene pool of people living today?
"From my perspective, the replacement vs. continuity debate that raged through the 1990s is now dead," said the study's American co-author, Erik Trinkaus, a professor of anthropology at Washington University in St. Louis.
Trinkaus comes down firmly on the side of the assimilation theory.
"To me, what happened is that the Neanderthals were [genetically] absorbed into and overwhelmed by modern humans coming into Europe from Africa, and they disappeared through this absorption," Trinkaus said.
His team published its findings in this week's issue of the Proceedings of the National Academies of Sciences.
Neanderthals first appeared in Europe and parts of western Asia about 230,000 years ago, evolving from the Homo erectus strain that moved into Europe from Africa about one million years ago. Neanderthals dominated Europe until the arrival of modern Homo sapiens from Africa about 40,000 years ago. Then they began to fade out. The last fossil traces of the Neanderthals were found in Spain and are about 28,000 years old.
For much of the 20th century, anthropologists (abetted by the popular media) cast this battle between the two groups as the elimination of "brutish" Neanderthals by the more highly evolved modern humans.
But Jeffrey Laitman, a specialist in early human craniofacial anatomy at Mount Sinai School of Medicine in New York City, said today's scientists don't give that scenario much credence.
"There's not this shining moment where the Neanderthals all disappeared because we ran out of the forest and clubbed them to death," said Laitman, who was not involved in the new study.
Still, debate has raged as to whether the Neanderthals were a separate species who simply lost their competitive edge with modern humans and died off, or whether they gradually mixed their genetic heritage with those of the invaders.
According to Trinkaus, a collection of bones discovered in the Pestera Muierii cave in Romania in 1952 holds the answer.
The bones, most derived from an adult female, consist of a cranium, a shoulder blade, a leg bone and other fragments. Because they were found lying on the cave floor's surface, the fossils were originally dismissed as being modern and remained unexamined for five decades.
But then Trinkaus' Romanian co-authors decided to radiocarbon-date the fossils. They found that the woman actually died about 30,000 years ago, during the Upper Paleolithic era, when Neanderthals and modern humans were thought to co-exist.
Examining the bones, Trinkaus discovered certain features that he believes are Neanderthal elements incorporated into this early Homo sapien.
Features at the back of the woman's skull and in her lower jaw, especially, "are found in high frequency in Neanderthals" but are absent in bones from older groups of Homo sapiens from Africa, he said.
There's also the intriguing find that the woman had a relatively narrow shoulder blade, or scapula. Modern humans have relatively wide scapula -- useful for throwing spears and other developed technologies. But the woman's scapula is narrower and "more similar to what we see in Neanderthals," who are not thought to have used these more advanced technologies at the time, Trinkaus said.
The bottom line? The Pestera Muierii bones are "basically modern human fossils with these characteristics that are very easy to derive from Neanderthals through some kind of interbreeding, but are very difficult to derive -- if not impossible -- from what we know of the anatomies of early modern humans out of Africa," Trinkaus said.
He pointed out that genetic sublimation of one group into another happens all the time, even across mammals considered to be from wholly different species. For example, the North American black duck is being gradually subsumed and eliminated by interbreeding with the European mallard, Trinkaus said. As a result, the genetic code of mallards in Europe now contains significant DNA from the disappearing black duck. Similar blendings are also occurring between wolves and coyotes, and between domestic cats and wildcats, he said.
A process very much like this probably occurred over time between Neanderthals and modern humans, Trinkaus concluded.
But not everyone is convinced. Laitman, director of Mount Sinai's department of anatomy, called the study "extremely interesting," but added that it "does not provide the magic bullet that pierces the mystery of what happened to the Neanderthals."
He said that people on the other side of the argument -- who contend that the Neanderthals maintained their unique genetic code up until the end -- still point to certain "derived traits" in the fossil record. "Indeed," he said, "some of the very last surviving Neanderthals have some of the most pronounced of these traits," countering the notion of a more gradual blending with modern humans.
Scientists at Pennsylvania State University and elsewhere are also working on reconstructing the Neanderthals' genetic code, using bits of DNA extracted from fossilized bone. Preliminary results of that work appear to refute the intermixing theory, tilting toward replacement instead.
But Trinkaus called the replacement theory "out of date." He believes there's now solid evidence that Neanderthals and humans met and co-mingled both socially and sexually.
They may not even have been all that different.
"When these two populations met, they saw each other as human beings," Trinkaus said. "They blended socially as well as biologically. To me, that tells us a lot about Neanderthals. And if we think that Neanderthals were a lot more primitive than modern humans, then maybe modern humans were a lot more primitive, too."
Find out more on human evolution at Minnesota State University.