Out of Africa

L.A. Moran lamoran at gpu.utcs.utoronto.ca
Wed Nov 13 12:58:58 EST 1991


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



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