March 29, 2000
DNA Tests Cast Doubt on Link Between Neanderthals and Modern Man
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.
|