I thought that the recent Scientific American article by Cavalli-Sforza (1)
was very sloppy and it grossly overstated the case for the Out-of-Africa
hypothesis of the origin of modern humans. I have no reason to reject this
hypothesis but I was prompted to re-examine the original data to see how
strongly it supports the idea that modern humans arose in Africa and spread
out from there replacing Neanderthal (and others). This re-examination has
led me to conclude that many scientists, particularly geneticists and
molecular biologists, are guilty of unwarrented assumptions concerning
human evolution. I think that we should be more cautious accepting the
Out-of-Africa hypothesis.
In order to begin some discussion on this topic I will quote from a recent
paper (2);
"An African origin for Homo sapiens sapiens is supported by an
abundance of archaeological, linguistic, and genetic evidence,
including the use of the chimpanzee to root the human genealogical
tree."
I don't believe that this statement is correct for the resons that are listed
below. Can anyone enlighten me?
Archaeological evidence: It is my understanding that anthropologists and
archaeologists are deeply split on this issue. Although most will agree
that the genus Homo arose in Africa (that's where the earliest fossils
are) they do not agree that MODERN man originated in Africa and spread
out from there replacing the indigenous populations. There seems to be
good evidence that Homo has been present in Eurasia and Africa for over
one million years and that the transformation to modern Homo sapiens
sapiens occurred simultaneously in many parts of the world. My readings
indicate that some anthropologists claim that the earliest examples of
Homo sapiens sapiens can be found in South Africa and these date to about
125 thousand years ago. These claims are disputed on several grounds.
Is my understanding correct?
Linguistic evidence: I am not aware of ANY linguistic evidence that supports
the Out-of-Africa theory. As far as I can tell the linguistic data
suggests that many Eurasians share a common linguistic background and
that it may be possible to incorporate some other languages into a larger
superfamily of languages. As far as I am aware it has not been possible
to conclude that ALL modern languages are descended from a single ancestral
language and it certainly has not been possible to deduce where these
original (mythical) ancestors lived. The correlation between language
and genetics that is being pushed by Cavelli-Sforza (5) seems to me to be
mostly trivial. It is the exceptions that are interesting. I note that
Cavelli-Sforza believes that it is the acquisition of language that
provided modern humans with the advantage necessary to exterminate other
humans such as Neanderthal (5). Does anyone want to defend the idea that a
study of modern languages supports an African origin of Homo sapiens
sapiens?
Genetic evidence: A lot of genetic "evidence" seem to me to be highly suspect.
It relies more heavily on imagination and unproven assumptions than on
real science. An exception is Wilson's original work on mitochondrial
DNA polymorphisms and Cavelli-Sforza's data on nuclear genes. However,
this data can be interpreted in two ways (IMHO). The data indicates that
the earliest split in the evolution of modern humans gave rise to two
main groups whose descendents now live in Africa on the one hand and
Eurasia on the other. What is the genetic evidence that the ancestors of
these two groups lived in Africa as opposed to Asia (or even Antarctica,
for that matter)? Isn't the data just as consistent with an Asian origin
of modern man and subsequent migration of the first sub-population to
Africa rather than vice versa?
Cavelli-Sforza (1) says,
"Our first result supports a conclusion that has emerged
from studies of human physical anad cultural remains: an
African origin of our species. We found that the genetic
distances between Africans and non-Africans exceed those
found in other intercontinental comparisons. This result
is exactly what one would expect if the African separation
was the first and oldest in the human family tree."
As far as I can see the results may be consistant with the Out-of-Africa
hypothesis but they are also consistant with an Out-of-Eurasia hypothesis.
In a more scientific paper (6) Cavelli-Sforza's group doesn't even
mention the Out-of-Africa hypothesis but instead they point out that
Europeans are a genetic mixture of African and Asian populations
and that the people who replaced Neanderthal in Europe probably
originated in West Asia. These West Asians "...may have already been
a mixture between Africans and East Asians, because West Asia is
geographically intermediate between Africa and East Asia." Is this
an argument for two independent origins of modern man (Africa and
East Asia)? Or is it leading up to a defense of the idea that modern
man may have evolved from widespread indigenous populations?
In the original paper (3) Wilson's group makes a special point of the
fact that the Eurasian group of mitochondrial DNA sequences also
includes some populations that now live in Africa. They conclude that
Africa is the "..likely source of the human mitochondrial gene pool..."
because this assumption "...minimize(s) the number of intercontinental
migrations needed to account for the geographical distribution of
mtDNA types." I don't believe that the number of required migrations
is any different if one assumes a Eurasian origin so that the logic of
this statement is questionable.
Incidently, Cavalli-Sforza describes Wilson's results as follows;
"...the Wilson group derived a tree of descent that showed
more differentiation in Africa than anywhere else. That
finding indicated that human mitochondrial DNA had been
evolving for the longest time in Africa - that is, it can
be traced to a single African woman."
Strictly speaking what Wilson's group showed was that there was more
"differentiation" (ie. sequence divergence) among Black Americans than
among other identifiable human populations. They assumed that all mtDNA
sequences present in Black Americans could be traced to an African
origin in spite of the fact that several of these individual sequences
clustered with European and/or Asian mtDNA types. The mitochondrial
DNA lineages do not make a lot of sense - there are too many unexplained
anomolies. See also ref (4).
Rooting the tree: One could also use orangutans to root the human geneological
tree but this would not be considered evidence that humans originated
in Indonesia! I assume that the authors meant to say that the closest
living ancestors of humans are chimps (and/or gorillas) and that these
species also live in Africa. But this is evidence in favor of the African
origin of the GENUS Homo and not of the SPECIES Homo sapiens sapiens.
Am I confused about this? (Does anyone know if the fossil evidence of
chimp and gorilla evolution confirms that these species are confined to
Africa?)
(1) Cavalli-Sforza, L.L. (1991) Genes, peoples and languages.
Sci. Am. 265, 104-110.
(2) Rapacz, J. et al. (1991) Identification of the ancestral haplotype for
apolipoprotein B suggests an African origin of Homo sapiens sapiens
and traces their subsequent migration to Europe and the Pacific.
Proc. Natl. Acad. Sci. (USA) 88, 1403-1406.
(3) Cann. R.L., Stoneking, M. and Wilson, A.C. (1987) Mitochondrial DNA
and human evolution. Nat. 325, 31-36.
(4) Vigilant, L. et al. (1989) Mitochondrial DNA sequences in single human
hairs from a southern African population.
Proc. Natl. Acad. Sci. (USA) 86, 9350-9354.
(5) Cavelli-Sforza, L.L. et al. (1988) Reconstruction of human evolution:
bringing together genetic, archaeological, and linguistic data.
Proc. Natl. Acad. Sci. (USA) 85, 6002-6006.
(6) Bowcock, A.M. et al. (1991) Drift, admixture, and selection in human
evolution: a study with DNA polymorphisms.
Proc. Natl. Acad. Sci. (USA) 88, 839-843.
Laurence A. Moran (Larry)
Dept. of Biochemistry
University of Toronto
