Re: Human Evolution Questions

On Sat, 27 Dec 2008 09:16:58 -0800, rnorman wrote:

On Dec 27, 9:18 am, martin <usen...@xxxxxxxxxxx> wrote:
Hi Guys

I'm not quite sure where this is going to go, so bear with me a bit.

I was reading a few months ago about humans breaking out of Africa and
only succeeding once around 60,000 years ago (give or take). That
sounds like quite a long time but would seem to be only around 3,000
generations, maybe up to 5,000 depending on breaks etc.

Then I got to thinking about how different humans look, for example
native Africans, Europeans, native Australians, extreme N. Americans,
mainland Americans.

Humans seem to be generally conservative about their mating habits and
up to 5,000 generations doesn't seem to be enough for these changes to
take place. Changing from black skin tones to white skin tones seems
like a big change to me in that time scale if the only driving factor
is Vit D generation - unless there was some kind of sexual preference
as well which would speed it up. Does anyone have any recommended
reading for me? What are the driving forces and timescales needed to
achieve them.

Also, how did humans get to Australia?

I was laid up in bed for a couple of days over christmas with man-flu
and it's strange the things I was puzzling about.

First, I hope you are well recovered. Flu of whatever variety can be
quite vicious.

People have been wandering around the countryside for as long as there
have been people. However a lot of evidence does suggest that one
particular wave out of Africa on the time scale you mention replaced any
possible previous migrants. A good source is Wikipedia at
http://en.wikipedia.org/wiki/Recent_single-origin_hypothesis

I think it is pretty well established that sexual preferences are
important in establishing the visible differences between subsets of
modern humans, including differences in skin color. That the
differences may have some biological significance (as in skin color and
Vitamin D) and that the sexual selection may have some biological basis
pales (any ironic reference to darkness of skin here is intentional) in
comparison to social and cultural forces in sexual selection and group
membership. As a result, it seems silly to me to even worry about
biological bases when cultural forces are so powerful. Given strong
cultural preferences to "mate with one of your own kind", an awful lot
of change can happen in just a few hundred generations. Of course,
hot-blooded adolescents often couple without much regard to those social
conventions and that will slow the rate of change. Still, if there is
any social stigma to looking "other" in the structure of the society and
the distribution of benefits, that will be a terribly potent force for
change.

The Australian Department of Immigration and Citizenship has a nice web
site about immigration to there
http://www.immi.gov.au/

Oh, you mean the original inhabitants! The general story is that "At
the time of the migration, 50,000 years ago, Australia and New Guinea
were joined by a land bridge and the region was also only separated from
the main Eurasian land mass by narrow straits such as Wallace's Line in
Indonesia. The land bridge was submerged about 8,000 years ago."
http://www.physorg.com/news97857326.html
There is some question about how to bridge that Wallace Line strait but
there is also some speculation that the early human migrants were also
capable boat people (at least enough who traveled the oceanic routes
were).

I believe Jared Diamond discussed migrations to Australia in "Guns,
Germs, and Steel" but I don't have my copy handy with me.

I would expect you will receive abundant information from people here.

I also vote for the boats. But I disagree as to sexual preferences having
much to do with skin color. Skin color correlates exceptionally well with
incident solar radiation (I will look up the reference article in
Scientific American if you want). If sexual preference were a main factor
this should not be true - the classic examples of sexual selection
(peacocks tails) are those in which there is no correlation to a
selective explanation.

As to the concern about insufficient generations, I thought (but can't
find the reference) that tens of generations are all that is required for
fixation of a favorable trait, depending on effective population size.
And certainly lactose tolerance was fixed in some northern European and
sub-saharan African populations on a time scale of thousands rather than
tens of thousands of years.

Yours,

Bill Morse

.

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