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Re: local evolutions
Gautam Majumdar (gautam@majumdar.demon.co.uk)
Wed, 28 Aug 1996 15:59:33 +0100
In article <susansfDwu3GC.80J@netcom.com>, "Susan S. Chin"
<susansf@netcom.com> writes
>Bruce Scott TOK (bds@ipp-garching.mpg.de) wrote:
>: On Thu, 22 Aug 1996 09:35:48 -0500, "C. Marc Wagner @ UCS"
>: <mwagner@indiana.edu> wrote:
>
>: : I think that this theory has begun to fall out of favor in recent years
>: : as genetic evidence has come down on the side of the "Out of Africa"
>: : theory that several times in the past Hominids have migrated out of
>: : Africa. If I understand it correctly, the genetic evidence suggests
>: : that all living Homo sapiens sapiens are descended from a single female
>: : who lived around 800KYA. Much later than Homo erectus left Africa.
>
>: I haven't been following this story recently, but when and why did this
>: time scale become 800 kya and not 200 kya?
>
>Also, my understanding is that all living humans (H.s.s.) can trace our
>mitochondrial DNA to a common female ancestor popularly known as
>Mitochondrial Eve (mtEve for short), who lived in Africa about 200,000 yrs
>ago. The assumption here would be that this original segment of mtDNA
>arose only once and was "housed" in a single individual female, and did not
>independently evolve in differing populations and geographic regions.
>This is different from "we are all descended from a single female" in
>that it's only the mitochondrial DNA which we share with Eve, not
>necessarily nuclear DNA, which is much harder to trace due to genetic mixing.
>
>: Yousuf Khan (ykhan@achilles.net) wrote:
>
>: : However, what evidence is there that it did originate in Africa?
>: : All we can guess at is how long ago it happened, but that doesn't
>: : tell us where it happened: it could've happened in Africa, but
>: : not necessarily so.
>
>: According to the original story, the evidence was that there is more
>: genetic diversity among humans in Africa than ahywhere else, consistent
>: with the hypothesis that diversity expands with time among non-migrating
>: populations. Also consistent with this is that the least diversity was
>: observed in native people from the Americas.
>
>: : What exactly do you mean? How do you measure genetic diversity?
>
>: I don't remember the details, but it had to do with certain alelle
>: frequencies in the mitochondrial DNA. It _was_ however quantitative.
>: Look it up in Nature articles from about 1992-1993.
>
>Plus the mtEve Theory has been questioned by other researchers who tried
>to duplicate their results using a computer generated program, PAUP
>(Phylogenetic Analysis Using Parsimony), and came up with differing, and
>more ambiguous results.
>
>For further reading:
>
>"Mitochondrial Eve: Wounded but not dead yet," SCIENCE, August 14, 1992,
>v. 257, pp. 873-875.
>
>This is actually a very non-technical account of the mitochondrial Eve
>theory. Some of the papers on this topic get a bit too technical for
>some of us to fully understand (for me anyway...)
Out of Africa hypothesis, based on mitochondrial DNA (mtDNA) data
is older than the Mitochondrial Eve theory. Jones K S and Rouhani S
(How small was the bottleneck ?, Nature 1986, 319, 449-50) suggested
that the size of the group emigrating from Africa was 40-4000
individuals and the time of emigration was earlier than 50 KYBP.
Cann R L, Stoneling M and Wilson A C (Mitochondrial DNA and
human evolution, Nature 1987, 325, 31-6) proposed the Mitochondrial
Eve hypothesis (a single ancestral woman) on the basis of the data
from 141 individuals (all from the old world but not many of African
origin). They assumed a mutation rate of 2-4% per million years. The
time of the Eve was estimated at 143-295 KYBP. Further support was
provided in another paper in which 2 hypervariable sequences of
mitochondrial DNA from 189 individuals (121 of African origin) were
examined (Vigilant L, Stoneking M, Harpending H, Hawkes K, Wilson A
C, African populations and the evolution of human mitochondrial DNA,
Science 1991; 253: 1503). In both these papers authors claimed that
they used the maximum parsimony (smallest number of mutations
possible for construction of the genetic tree).
Two big problems were pointed out by Templeton A R (Human origins
and analysis of mitochondrial DNA sequences, Science 1992; 255:
737). Firstly the data was inadequate for such statistical analysis. Far
more importantly he showed that the computer program used for the
purpose was influenced by the data entry sequence. The number of
maximum parsimony trees were hundreds and were at least 2 steps
shorter than the proposed tree. These results completely nullified the
basis of the hypothesis of African Eve used by the proponents.
In reply (p 737-39 of the same issue) proponents, including at least one
of the original authors (Hedges S B, Kumar S, Tamura K, Stoneking
M) admitted the fault in their computer program and agreed that there
could be thousands of trees more parsimonious than that of theirs and
even of Templeton. They admitted the inadequacy of their data.
Wills C (Human origin, Nature 1992; 356: 389-90) used the published
data of several reports of human mtDNA to construct a starburst
diagram and showed that there is overdispersion of the mutations.
Thus there are Asian individuals who fit best in the African set,
Australian aborigine who fits best at the base of the tree, and so on.
Author questions the notion that there is a constant rate of neutral
mutation of mtDNA, an assumption on which the theory of African Eve
is based.
Goldman N and Barton N H (Genetics and geography, Nature 1992;
357: 440-41) on the otherhand emphasised that the parsimony
technique might have not been the best technique for such analysis. An
alternative techniques (most possible technique) might be more useful
in such situation. All the authors, both proponent and opponent of the
African Eve hypothesis had used the parsimony technique which
neither proves nor disproves the theory (most parsimonious trees are
too numerous for definite conclusion, essentially needing much more
data).
Maddox J (Migration out of Africa, Nature 1994; 372: 32) in a detailed
(authoritative ?) review pointed out that the amount of data available is
too small (a few hundred individuals only) for any reasonable
conclusion to be drawn. Secondly, the variations and concordances
may have arisen independently of migration. Thus, the population of
Papua-New Guinea have a deletion identical with some African
population but the population of the mainland Asia do not harbour this
deletion. Possibilities are that the migrants from Africa went by sea
route crossing the Indian Ocean, or after they reached Papua-New
Guinea by the land route, the population of the mainland Asia has been
replaced (wholesale) by fresh migration. However, the third and most
likely explanation is that this deletion arose independently in the African
and Papuan populations. Very similar mutations are likely to occur if it
involves a mutation hot spot (hypervariable sequence as used by
Vigilant, et al) and such mutations are useless for tracking population
movement since they may appear several times independently.
Maddox concluded that mtDNA alone (with only 17,000 base pair)
would not provide the answer to the question of early migration of the
Homo sapiens. Inclusion of somatic DNA data will be essential for this
purpose.
Gautam Majumdar gautam@majumdar.demon.co.uk
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