Comments on: One principal component to rule them all?

By: Razib Khan

Razib Khan — Fri, 23 Jul 2010 20:57:38 +0000

chill out guys. actually, i understood the context of bio’s statement. he was kind of making fun of those who misrepresent the science (focusing on short term desertification and projecting back because the audience is too stupid to check). but i only understood cuz i have seen enough of bio’s comments to get the gist. in the future bio you might want to be more explicit, as your fame obviously doesn’t precede you

(to be fair, the same sort of confusions happen to me when i don’t connect dots in my posts, but here on this domain i am a god among men, and am given more latitude by the powers that be, by which i mean me)

By: onur

onur — Fri, 23 Jul 2010 20:09:23 +0000

I was precisely right. I have indeed ” often read, it’s been a desert for rather a short time”.

My objection was to your this statement:

“If over the last 100,000-odd years it has often been well-watered, grazable land (as some writers seem to insist) why did it form such a barrier to intermingling of humans?”

Throughout the last 100,000 years (and much beyond) the Sahara was mostly desert, varying in its extent depending on time and climate. But there were also rare and short periods when savannah conditions prevailed (like the one explained in my first link).

By: bioIgnoramus

bioIgnoramus — Fri, 23 Jul 2010 19:32:12 +0000

“Because you are wrong”: no, I was precisely right. I have indeed ” often read, it’s been a desert for rather a short time”. You would seem to be as stupid as you are intemperate.

By: onur

onur — Fri, 23 Jul 2010 19:03:53 +0000

onur, one thing. remember that small population sizes mean that you need less migration in absolute terms to equilibrate gene frequencies.

Yes, but it also means less need for migration and eventually less migrations. It would be nice to compare the two models by investigating known past migrations via genetic tests on modern and also ancient populations.

By: Razib Khan

Razib Khan — Fri, 23 Jul 2010 18:52:46 +0000

onur, one thing. remember that small population sizes mean that you need less migration in absolute terms to equilibrate gene frequencies.

By: Jumblepudding

Jumblepudding — Fri, 23 Jul 2010 17:36:46 +0000

Sorry if previous statement came off as disgruntled.

By: onur

onur — Fri, 23 Jul 2010 17:29:00 +0000

A few thousand years is more than enough time for populations to mingle.

In an era of low high-distance mobility, low communication and technology, and small population sizes, that mingling would be very limited in extent and scope, which is exactly what we see in the genetics of the relevant regions.

By: Jumblepudding

Jumblepudding — Fri, 23 Jul 2010 15:57:27 +0000

I love how onur took a single word “often” and extrapolated it into a long winded, intelligence-insulting list of links. A few thousand years is more than enough time for populations to mingle.

By: onur

onur — Fri, 23 Jul 2010 15:31:15 +0000

“..within Africa south of the Sahara …” reminds me that I’ve long wondered why the Sahara should be a long-lasting, rather firm divider if, as I often read, it’s been a desert for rather a short time. If over the last 100,000-odd years it has often been well-watered, grazable land (as some writers seem to insist) why did it form such a barrier to intermingling of humans?

Because you are wrong. The Sahara wasn’t a well-watered, grazable land for most of the last 100,000 years. Such wet (relatively speaking) periods of the Sahara were rare and didn’t last long (a few thousand years at most). For the most of the the last 100,000 years (and much beyond), the Sahara was an impassable large desert as it is today.

The last wet period of the Sahara lasted from about 7,500-7,000 BC to about 3,500-3,000 BC:

http://en.wikipedia.org/wiki/Green_Sahara

For some introductory information about the climate history of the Sahara:

http://en.wikipedia.org/wiki/Sahara#Climate_history
http://en.wikipedia.org/wiki/Sahara_pump_theory

By: JL

JL — Fri, 23 Jul 2010 14:18:36 +0000

Supplementary data are available here: http://jorde-lab.genetics.utah.edu/?page_id=23

By: John Roth

John Roth — Fri, 23 Jul 2010 13:23:15 +0000

I think the answer is that the parts north of the Sahara were resettled from Eurasian stock, leaving the ancestral populations to the south.

By: Sandgroper

Sandgroper — Fri, 23 Jul 2010 12:02:02 +0000

Because ‘the gate’ into the Middle East was ‘open’ only for discreet and fairly narrow time windows.

By: Tweets that mention One principal component to rule them all? | Gene Expression | Discover Magazine -- Topsy.com

Fri, 23 Jul 2010 09:39:58 +0000

[...] This post was mentioned on Twitter by Ron Simon, Sains & Teknologi, World Amazing Things, Al Poe, Maggie and others. Maggie said: One principal component to rule them all? | Gene Expression: Despite the reality that I’ve cautioned against takin… http://bit.ly/bdX8qG [...]

By: bioIgnoramus

bioIgnoramus — Fri, 23 Jul 2010 09:37:51 +0000

Forgive me if I ask a simple-minded question. “..within Africa south of the Sahara …” reminds me that I’ve long wondered why the Sahara should be a long-lasting, rather firm divider if, as I often read, it’s been a desert for rather a short time. If over the last 100,000-odd years it has often been well-watered, grazable land (as some writers seem to insist) why did it form such a barrier to intermingling of humans?