Out of Africa: bogus methods and mistaken inferences

arlin at ac.dal.ca arlin at ac.dal.ca
Sat Nov 16 16:20:06 EST 1991


In article <robison.690235642 at ribo>, robison at ribo.harvard.edu (Keith Robison) writes:
> 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 

[. . . stuff deleted]
>  
> 	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. 
> 
> Keith Robison

With regard to 1 & 2: 

What's all this business about counting branches?  Ever seen it before?
Ever seen anyone do a "hypergeometric test"?  What is it, anyway?  Don't
molecular systematists usually do a "bootstrap" to test the significance
of a particular branching order, instead of inventing their own method?

Why didn't Vigilant, et al (1991) just do a bootstrap?  (hint: they wanted
to support their conclusion, not weaken it).  And even if they could find
a way to get a significant rooting to give an African clade on one side of
the tree, does this really lead one to *conclude* an African origin?


With regard to  #3:

Chimps and gorillas are *not* "ancestors" of humans-- they are RELATIVES.
Far from being another one of my characteristically vacuous semantic 
arguments, this distinction is very important to evolutionary inference.
The facts that chimps and gorillas are i) the closest living relatives of
humans; and ii) limited to Africa, would only be relevant to this discussion
if we had no paleontological evidence on the ANCESTORS of humans.  Human
ancestors that are NOT chimp/gorilla ancestors include australopithecines 
and the _homo_ species _habilis_ and _erectus_.  Pre-sapiens _homo_ fossils
have been found in Indonesia (ever heard of "Java man"?), China (ever heard
of "Peking man"?), Vietnam, Greece, France and Africa.  Since not all of 
these places are in Africa, we may rightfully say that the pre-human 
hominids, including the most likely candidate for a human ancestor 
(homo erectus), lived in Africa, Asia and Europe. 

Keith Robison may be correct if he is suggesting that the great apes
(chimps, gorillas, humans) originated in Africa.  However, since _homo_ 
ancestors were distributed in Africa, Asia and Europe after their divergence 
from _pan_ and _gorilla_ ancestors, the purported African origin of the 
great apes is irrelevant to the question of the purported African origin
of _homo sapiens_.

Arlin Stoltzfus

Department of biochemistry
Dalhousie University
Halifax, Nova Scotia 



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