Message #309:
From: AzTeC SW Archaeology SIG
To:   "'Matthias Giessler'" 
Subject: Neanderthal DNA Tests
Date: Sat, 09 Aug 1997 00:34:11 -0700 


Arizona Republic, Page B5, Friday August 8, 1997

Tangled strands of time: Reviewing the results of Neanderthal DNA tests

By G.A. Clark

In an article contributed to the New York Times and printed in The Arizona
Republic ("The Cradle of Humanity," Opinions, July 29) British
paleoanthropologists Chris Stringer and Robin McKie report on the claims of
a team of molecular biologists, led by Svante Paabo of the University of
Munich, that they have sequenced a tiny fragment of mitochondrial DNA from
the upper arm bone of the original Neanderthal skeleton, recovered from a
limestone quarry in Germany in 1856.

While the research was apparently meticulously done (it is very difficult
to recover DNA from ancient bone), what it means in respect of modem human
origins, and the Neanderthals' relationship to ourselves, remains very much
open to question.

The analysis of mitochondrial DNA has become important in recent years for
inferring evolutionary relationships among populations and species because
mitochondria, unlike nuclear DNA, are inherited only from the mother and do
not recombine.  This means that they can be used as a kind of molecular
clock since, sequences remain the same from one generation to the next
except when mutations occur to change them.  Under the problematic
assumptions that mutations accumulate at a constant rate, that the rate can
be established and "calibrated' and that these mutations are adaptively
neutral (that is, they do not affect the reproductive potential of the
organisms involved), the number of accumulated mutations between two
mitochondrial DNA sequences is a measure of the degree of evolutionary
divergence between them.

After three months of painstaking work, the Paabo team isolated a sequence
of 379 base pairs from the control region in the DNA of mitochondria, the
cells' energy-producing organelles, which have their own tiny genome.  Then
they compared the Neanderthal sequence with 986 sequences from living
humans and found on average, about three times as many differences (26)
between the Neanderthal and modem human sequences as between those of pairs
of modem humans (8).  Using a chimp human divergence date of 4 million to 6
million years ago to "calibrate" the clock, they came up with a rate at
which the control region accumulated mutations over time.  This in turn
allowed them to determine that the sequence ancestral to both modem and
Neanderthal mitochondria began to diverge between 690,000 and 550,000 years
ago, compared to less than 200,000 years ago for the sequence ancestral to
modem humans.  In order words, they contend that the last common ancestor
of Neanderthals and modems is about three times older than the last common
ancestor of all people alive today.

While an astounding technical breakthrough, and an important result, the
work of the Paabo team is evaluated uncritically by Stringer and McKie in
their article, essentially because it tends to support their biases and
preconceptions about modern human origins.  Other interpretations are
possible.  It should be noted first that molecular clock models are full of
questionable assumptions.  Leaving aside differences of opinion about the
rate of base pair substitutions, how to calibrate a molecular clock, and
whether or not mitochondrial DNA mutations are neutral, the fact that the
Neanderthal sequence (and it should be kept in mind that there is - to date
- only one) differs from those of modern humans does not resolve the
question of whether or not moderns and Neanderthals were different species.
 This is the primary conclusion of the research according to Stringer and
Mickie.  Continuity advocates (people who think that Neanderthals
contributed genetically to people of western Eurasian ancestry) argue that
differences were only subspecific. In support of this notion, Harvard
geneticist Maryellen Ruvolo points out that the genetic variation between
the modern and the Neanderthal sequences is within the range of other
single species of primates, and that there is in fact no genetic yardstick
which might allow us to define a species.  A more convincing test of the
implications of the Paabo data would be to sequence mitochondrial DNA from
an unambiguously modern early European or better yet from alleged archaic
and modem human fossils from the Israeli cave sites of Skhul and Qafzeh
(supposedly modern) and Kebara and Tabun (supposedly Neanderthal).  If the
Israeli fossils all show differences with modern humans of the same order
of magnitude of those between Neanderthal and moderns, that would be
compelling evidence that all Upper Pleistocene hominids diverge from
moderns by about the amount (evidence for continuity), and that the
distinction between Neanderthals and moderns in the Levant at least is
utterly without foundation.

That researchers cannot distinguish a Neanderthal from a modern might seem
surprising to some readers, but there is absolutely no consensus on what
these terms mean.  Arizona State University researcher Catherine Willermet
re-examined the criteria used to classify and describe Neanderthal and
early modem human skulls by continuity and replacement advocates, and found
that only 11 percent of the variables were common to both groups of
workers.  In other words, continuity and replacement advocates are using
different sets of variables to assess differences and similarities, and are
defining differently those variables held in common.

Without even discussing the archaeological evidence, which unequivocally
supports continuity in adaptation between Neanderthals and moderns wherever
they are found, it should be obvious that a single DNA sequence from a
single, undated skeleton can be interpreted in many different ways.  If we
are to come up with a satisfactory explanation for our origins, that
explanation must reconcile pattern searches in archaeology and human
paleontology, as well as in molecular biology.  Those who would argue -
like Stringer and Mickie - that Neanderthals became extinct without issue
must show how it could have occurred without leaving traces of disjunction
in the archaeological record and in the fossils themselves.

G.A. Clark is a professor of anthropology at Arizona State University and,
with Catherine Willermet, the editor of 'Conceptual Issues in Modem Human
Origins Research' (Aldine de Gruytar), which will published this fall.