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“Science,” writes paleoanthropologist Ian Tattersall (2001), curator
at the American Museum of Natural History, “is constantly subject to
rearrangement and change as our collective knowledge increases.” Nowhere
is this more true than in the field of genetics, which in the past two decades
has produced a series of mind-staggering revelations about human evolution;
knowledge which, it seems, is only just beginning to unfold.
The pace of upheavals in genetic knowledge began to seriously quicken about
15-20 years ago in both biochemical and informational technology. Biological
methods were much enhanced with the advent of recombinant DNA technology,
and through gene mapping techniques such as restriction fragment length
polymorphism (RFLP) analysis. By pinpointing mutation sites, researchers
can catalogue the sequence changes occurring within DNA loci. Using increasingly
sophisticated statistical grouping procedures such as PAUP or GEODIS to establish
the most parsimonious ancestral trees, geneticists are now able to map the
identified segments of polymorphisms (called haplotypes) within and across
human populations (for more detail and a DNA glossary see Schurr, this issue,
p.62).
The time element of how long genetic strains have been separated can also
be calculated by a “molecular clock” of counting numbers of mutations
from known, fixed points on the chain where they once were identical. Such
methods promised to resolve long-standing problems that could not be clarified
through the fragmentary record of fossil humans.
 Fifteen years
ago, when haplotype studies were much newer, three scientists (Cann, Stoneking,
and Wilson 1987) published evolutionary trees of the mitochrondrial DNA (mtDNA)
haplotype. mtDNA helps reproduce the cell’s energy-producing mitochondria
whose genome is inherited solely through the maternal line. These showed
the oldest extant human branches to be in Africa, and (based on the time/mutation
rate of the mtDNA haplotype) indicated a major migration out of Africa between
150,000 to 80,000 years ago. Based on the mtDNA evidence alone, this seemed
to account for the entire modern human genome, and thus the theory was
established (and took root in the popular press) that all modern humans descended
from a single African woman, nicknamed “mitochondrial Eve.”
Fig.1: Main trends of Templeton’s new model of the expansion
of human populations, with three “out-of-Africa” migrations starting
with Homo erectus about 1.7 myr ago (after Templeton 2002).
Most paleoanthropologists agree that hominids originated in Africa, then
first spread out of Africa to Eurasia and South Asia as Homo erectus about
1.7 to 1.8 myr ago. The “mitochrondrial Eve” theory based on Cann
et al.’s highly influential (1987) paper holds that all hominid lines
which had descended from these original migrating Homo erectus populations
of about 1.7 myr ago became extinct - including all the Asian and Eurasian
varieties of Homo erectus, and all archaic Homo species such as
Neanderthals who evolved outside of Africa. All were eventually replaced
after 150,000 years ago by the African descendants of mitochrondrial Eve.
This seemed to effectively refute the “lattice” or
“trellis” model of multiregional continuity. This view (with many
variations) holds that various descendants of Homo erectus throughout
the Old World (including Neanderthals) have intermixed their genomes throughout
the 1.7 million years since Homo erectus first left Africa. In support of
the “regional continuity” side of the human evolutionary debate,
geneticists including Alan R. Templeton (1992) of Washington University showed
flawed use of the statistical (PAUP) program in the original (Cann et al.
1987) mtDNA paper. Meanwhile a revised, more detailed “mitochrondrial
Eve” study was published (Vigilant et al. 1991), restating the total
replacement hypothesis after 150,000 BP.
Over the next few years, haplotype trees generated for Y-chromosomal DNA
(nuclear DNA found only in males) seemed to basically support the mtDNA-based,
recent out-of Africa replacement scenario. While some Y-chromosomal studies
showed longer time depths for the out-of-Africa movement (i.e. back to 200,000
years ago), such results based on male descent were still compatible with
the mtDNA full genetic replacement model as it applies to the fate of Homo
erectus, Neanderthals (see AR 2,4), and the entire hominid fossil record
outside of Africa before 100,000-200,000 years ago.
But haplotype studies have continued to improve in focus, and expand in scope.
Now Alan Templeton (2002), long a critic of the sweeping replacement scenario,
has synthesized the data from ten different haplotype trees, combining those
of the previously studied mtDNA and Y-chromosome DNA with eight new ones,
including haplotypes from two X-linked DNA regions, and 6 autosomal DNA regions.
His main conclusions (presented in the 7 March 2002 issue of Nature) are,
first, that there were at least two major expansions of humans out of Africa
after the initial spread by Homo erectus 1.7 myr ago (fig.1), and secondly,
that there has been interbreeding, a “ubiquity of genetic interchange
between human populations” throughout, not replacement. One of
the major migration periods out of Africa occurred between 840,000 and 420,000
years ago (when Homo erectus were evolving into early archaic
Homo types).
Rebecca L. Cann (2002), co-author of the original (1987) “mitochondrial
Eve” paper, allows (in a commentary in the same issue of Nature) that
there is “strong genetic support” for this earlier population expansion
from Africa. The next, beween 150,000 and 80,000 years ago, basically coincides
in time with Cann et al.’s (1987) “recent-out-of-Africa” model,
but with the important difference that all ten haploytes (including eight
bisexually inherited nuclear genes) show there was continued interbreeding,
not replacement, and “strongly reject the recent-out-of-Africa replacement
event.”
Cann (2002) argues, however, that Templeton treats recombinant gene sequences
(inherited by both parents) and non-recombinant sequences (mtDNA and
Y-chromosome) as if they provide the same types of inheritance and temporal
information. This may create conflicting results since recombinant gene sequences
can mutate much faster than non-recombining ones. Templeton does acknowledge
this, and only makes explicit inferences and interpretations for those scenarios
where he obtained statistically significant results (using a confidence level
of 95% or higher).
Templeton’s analyses (based on the GEODIS tree-grouping procedures)
further signals a likelihood that the earlier out-of-Africa expansion event
between .84 and .42 myr ago was also characterized by interbreeding, and
not replacement. If these views prevail, the field is open for erecting a
new kind of trellis (multiregional continuity and interbreeding) model -
and for reviewing the case for integrating some Homo erectus and
Neanderthals into the direct line leading to modern humans.
As Tattersall (2001) also said, “How can we make progress in science
if what we believe today cannot be shown tomorrow to be somehow wrong or
at least incomplete? Religious knowledge is in principle eternal, but scientific
knowledge is by its very nature provisional.” Cann (2002), who views
Templeton as “perhaps overambitious in the scale of his analysis,”
cautions that larger and better samples are needed to make such global statements
on long-term genetic trends. Just as the theory of mitochondrial Eve and
of recent total replacement by anatomically modern humans after 150,000 years
ago may today (after 15 years) be undergoing replacement, as it is shown
to be incomplete, so Templeton’s compelling genetic evolutionary picture
(as he might well agree) is only provisional, until the next synthesis of
more and better data comes along - probably quite soon.
References:
Cann, R.L., Stoneking, M., and Wilson, A.C., 1987. “Mitochrondrial DNA and
human evolution.” Nature 325 (1 Jan 1987).
Cann, R.L.,
2002. “Tangled genetic routes.” Nature 416 (7 Mar 2002).
Tattersall, I., 2001. Monkey in the Mirror: Essays on the Science of What
Makes us Human. Harcourt Press.
Templeton, A.R., 1992. “Human origins and
analysis of mtDNA sequences.” Science 255 (7 Feb 1992).
Templeton, A.R., 2002. “Out of Africa again and again.”
Nature 416 (7 Mar 2002).
Vigilant et al.
1991 Science 253:1503-07 (1991).
This article appears in the Recent Finds in Archaeology section of Vol.3 No.2 of Athena
Review.
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