Thursday, March 11, 2010

more on ancestors in and out of africa

they went thataway

I've been researching the above thesis, if you can call a bit of web browsing research, and I'm less sceptical. The key seems to be the work done on mitochondrial DNA, mentioned in the program, but in a way that was perhaps unclear. From the research it appears that our most recent common female ancestor lived in Africa about 200,000 years ago. Actually, I'm feeling sceptical again. If the remains found in Israel and dating to say 100,000 BP represent 'out of Africa' specimens of Homo sapiens [sapiens?] which didn't survive, how can we be so sure that there weren't other crossings of the northern African desert barrier?

But we'll leave that very good question hanging for now and return to mitochondrial DNA. Mitochondrial DNA [mtDNA] is inherited from the mother in most multi-cellular organisms. The reason or mechanism for this isn't entirely clear but it obviously has to do with the fact that the female ovum contains a thousand times or more mtDNA than the male sperm cell. MtDNA is thought to have a different origin from nuclear DNA, possibly originating from bacteria engulfed by the ancestors of our eukaryotic cells.

When we talk of our 'mitochondrial Eve' [that's the most recent common female ancestor, of 200,000 BP], we're talking of the one for Homo sapiens [sapiens?]. Theoretically we should be able to trace a much older mitochondrial Eve [but then she wouldn't be called Eve, perhaps], being the most recent common female ancestor for the whole Homo genus, and then further back to the Pan genus and so on. We can trace things back in this way because of DNA markers called haplotypes. It would take me quite some study to get my head around haplotypes – I'm encountering them for the first time. They're the key to tracing both matrilineal and patrilineal descent through DNA.

I'm not sure if tracing matrilineal descent is easier than tracing patrilineal descent, but clearly it's a different process. There's no mitochondrial Adam, instead there's a Y chromosomal Adam, a most recent common male ancestor, much more recent than Eve. The most recent literature apparently claims Adam to have lived 60,000 to 90,000 years ago, in Africa – but they confuse things by then saying that this makes Adam some 60,000 to 80,000 years more recent than Eve. As far as I can work out he should be more than 100,000 years more recent. But hey, what's a few tens of thousand of years, in this line of research.

Now if our Adam lived in Africa 60,000 years ago, and our descendants came out of Africa 70,000 years ago... isn't there something wrong there? Okay, we can play with the dating to get our ancestors coming out of Africa after Adam's time, but does that mean that any escapees from Africa before Adam weren't our direct ancestors? Well it must mean that, but could they not be Homo sapiens? Were the people found in the Es Skhul cave not Homo sapiens?

Or maybe they were Homo sapiens without being Homo sapiens sapiens?

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