By NICHOLAS WADE
Did modern humans wipe out the Neanderthal people
who inhabited Europe until 28,000 years ago, or did the two populations
merge through interbreeding? New DNA evidence, extracted from the
ribs of a Neanderthal infant, one of the last of its kind, supports
the thesis that these hardy, beetle-browed people left little or no
genetic legacy in today's populations.
Even though Neanderthals perished long ago, the surprising
retrieval of intact DNA, the second such sample to be recovered, has
set biologists speculating that with further finds the genetics of
this extinct human species could become quite well understood.
The two DNA retrievals, both suggesting that Neanderthals
were a separate human species, were separated in time by a startlingly
contradictory finding made last June. After studying the remains of
a thick-set boy recovered from a cliff-side grave in Portugal, considered
a final hold-out of the Neanderthals, paleoanthropologists said that
the human child had strong Neanderthal features, and that this "refuted"
the idea that modern humans had exterminated the Neanderthals without
interbreeding.
Neanderthals and their forebears occupied modern Europe
from around 300,000 years ago. They were adapted to the cold conditions
of the ice age and had stocky bodies, thick bones and enormous strength.
Though their stone tools seem similar to those of modern humans who
started to enter Europe from Asia around 35,000 years ago, they ceased
to flourish and abruptly disappeared throughout their home range around
28,000 years ago, leaving no clues in the archaeological record as
to the reason for their extinction.
Neanderthal DNA was first isolated three years ago,
from the original bones first found in the Feldhofer Cave in the Neander
Valley near Düsseldorf in 1856.
The finding was startling because no human DNA of
such antiquity -- at least 30,000 years old -- had been recovered
and because it showed a pattern of DNA that was quite different from
that of modern humans.
Though the Feldhofer DNA was extracted with elaborate
precautions, the finding was greeted with some reservation because
it was a single result.
Confirmation has now come from a second Neanderthal.
The remains were recovered by a Russian expedition
from the Moscow Institute of Archaeology to the Mezmaiskaya Cave in
the Caucasus, to the northeast of the Black Sea. They belonged to
a Neanderthal infant less than 2 months old, too young for the sex
to be determined from the bones. The bones were dated by the carbon
isotope method to 29,000 years ago, making the infant among the last
generations of the Neanderthals.
A sample of the infant's ribs was made available by
the Russian researchers to Dr. William Goodwin of the Human Identification
Center at the University of Glasgow. Dr. Goodwin works on paternity
cases and plane-crash victim identification, and studies ancient DNA
as a sideline.
Dr. Goodwin and his Russian and Swedish colleagues
report in this week's issue of Nature that the DNA sequence from the
Mezmaiskaya Cave is 3.5 percent different from that of the Feldhofer
Cave Neanderthal, suggesting a considerable genetic diversity within
the Neanderthal population.
But the two Neanderthal DNA sequences are very different
from those of modern humans, Dr. Goodwin and his colleagues say.
Based on the rate at which DNA changes over time in
living organisms, Dr. Goodwin calculated that the two Neanderthals
last shared a common ancestor at least 150,000 years ago, a date that
matches the first fully Neanderthal remains, and that the Neanderthal
and modern human lineages split some 600,000 years ago.
Two paleoanthropologists who favor the Neanderthal-human
assimilation theory, Dr. Fred Smith of Northern Illinois University
and Dr. Erik Trinkaus of Washington University in St. Louis, said
they did not dispute the new DNA analysis but noted that it did not
completely rule out the possibility of some interbreeding. Dr. Smith
said the new DNA data was "incredibly important and significant" and
"certainly strengthens the fact that there is quite a gap between
Neanderthals and recent humans in terms of mitochondrial DNA."
Mitochondrial DNA, inherited from the egg cell alone
and thus through the maternal line, is far more plentiful and likely
to survive than the DNA of the nucleus; both Neanderthal samples were
of the mitochondrial type.
But Dr. Smith and Dr. Trinkaus, who are experts on
the Neanderthals, believe that there was some interbreeding on the
evidence of the Portuguese boy with Neanderthal affinities.
Other anthropologists think the boy was just a "chunky"
human lad who in any case lived far too many generations after the
last Neanderthal had died for any evident influences to be expected.
"It's got one feature that is arguably Neanderthal
-- the shortness of length between the knee and ankle -- and even
that is not striking," said Dr. Richard Klein, an archaeologist at
Stanford University.
Dr. Smith and Dr. Trinkaus say that even though Neanderthal
DNA differs from that of modern people, it might be more similar to
that of their human contemporaries, the Cro-Magnons.
Curiously, no DNA has yet been recovered from very
ancient Homo sapiens fossils.
Dr. Klein agreed that new efforts should be made to
retrieve Cro-Magnon DNA, though he said he expected it would prove
similar to that of modern humans, sewing up the case that the Neanderthals
were replaced.
The factors that allow DNA to be preserved for thousands
of years are not well understood. "Even with two bodies in the same
grave, the level of preservation can vary considerably," Dr. Goodwin
said.
He thinks that something about the limestone cave
may have favored the durability of the Caucasus Neanderthal DNA.
If the reasons for preservation were better understood,
DNA experts would know which precious museum specimens were worth
sampling and which to leave alone.
