Out of Africa

Keith Robison robison at ribo.harvard.edu
Fri Nov 15 15:07:22 EST 1991


lamoran at gpu.utcs.utoronto.ca (L.A. Moran) writes:


>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?
>
>...
>
>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


	I believe that the molecular evidence for an African origin of humans
is rather certain.

1. mtDNA sequence analysis (see ref. below) generates a tree in which one must
   climb eight nodes from the root before finding a branch which contains
   non-African sequences on both sides of the branch (the tree was rooted
   with a chimpanzee sequence).

2. One can climb several more nodes from the point described above before
   reaching a branch with large numbers of non-African sequences on both
   sides of the branch.

3. The three closest ancestors of humans, pygmy chimps, common chimps, and 
   gorillas, all live in Africa (pygmy and common chimps diverged relatively
   soon after the chimp--human--gorilla split).


	Based on these facts, if one assumes a non-African origin of humans
one must also assume wide-spread migrations of humans into Africa and a 
similar influx of chimps and gorillas.  An African origin of the great apes
is by far a simpler explanation. 


Vigilant et al.  1991.  Science 253:1503-1507.



Extra opportunity to flame:

	Based on the molecular evidence, it is clear that the genus Pan should
be re-defined as including the species:

	Pan troglodytes		(common chimp)
	Pan paniscus		(pygmy chimp)
	Pan gorilla		(common gorilla)
	Pan sapiens		(modern human)



Keith Robison
Graduate Student
Harvard University
Program in Biochemistry, Molecular, Cellular, and Developmental Biology

robison at nucleus.harvard.edu



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