Sunday, January 30, 2011

Out of Africa, but When?


I'm very uncomfortable with popular claims about the migration of modern humans into Asia and Europe. You often see the date as 50,000 years ago and most people seem to think that this was a sudden event associated with the destruction of ancient hominids (Neandertal, Homo erectus) who lived in Asia and Europe at this time.

The recent date was based largely on mitochondrial DNA sequences and it required a reliable time reference that just wasn't there. As other nuclear genes were analyzed the dates indicated much older migrations. Then there's the fossil evidence. I'm not able to judge that evidence but it didn't seem to me to be as neat and tidy as a sudden exodus at 50,000 years ago would require.

Fortunately we have John Hawks, a scientist who's area of expertise covers archeology AND population genetics. He thinks that the date for "Out of Africa" should be older and he can back up his skepticism with evidence. Read his latest posting at: Jebel Faya and early-stage reduction. Will the new date end up being 100,000 years ago—or maybe even 150,000 years ago?

Since John doesn't allow comments on his blog I thought I'd open up some discussion here. John, aside from the question of modern Homo sapiens, when did Neandertals leave Africa and when was the migration of Homo erectus? How secure are those dates?



P.S. As I was about to publish this post I did a quick check to see when the movie was released. It was 1985. This means that none of the students in my molecular evolution class were alive when it came out. I feel old.

[Image Credit: The map is from The Human Journey.]

13 comments :

  1. Thanks, Larry! I feel old, too, but imagine how Robert Redford must feel.

    The older divergences within Homo have been totally thrown open by genetics, and it's not clear how they will shake out. There were Homo erectus in the Caucasus and Java by 1.8 million years ago. The Denisova sequence does not appear to have diverged from us (or from Neandertals) nearly that long ago. But it looks like its close relatives mixed with the ancestors of people in New Guinea, which means its population must have lived across some of the area occupied by H. erectus. Maybe they replaced H. erectus sometime later.

    A rough cut of the genetic divergence between Africans and Neandertals (and Denisovans) puts it between 250,000 and 400,000 years ago. This comes from a population model that I'm not yet ready to accept, but I don't know that the answer will be very different from this. This is rather more recent than we would have guessed from the fossil record; people with Neandertal features show up in Europe before 600,000 years ago. We used to assume that these were the ancestors of Neandertals. So there's an inconsistency.

    It is complicated and I don't think anyone has wrapped their head around the whole problem yet. We have hominins that we didn't know existed, and the ones we thought we knew about don't seem to fit in the tree anymore.

    An overall lower mutation rate makes some (but not all) of these problems go away, which is why I'm thinking about it so hard. We can't just take the fossils at face value, because in many, many cases apparent similarities turn out to be parallelism.

    ReplyDelete
  2. As long as you don't remember the book coming out (in 1937), you can't be so old.

    ReplyDelete
  3. Wil Roebroeks (Leiden, field: old paleolithic) doubts that on typology only one can make any claim about the makers of these tools.

    As to the DNA: how many Andaman islanders, South India tribes, tribal people from Yunnan, Melanesians are represented in those studies?

    ReplyDelete
  4. I'd love to see the hypothetical migrations mapped against a time sequence of climate maps.

    From the little reading I have done it seems that the size and aridity of the Sahara desert waxes and wanes with the global climate. Sometimes it would be a real barrier, sometimes I guess there could be an African West Coast route open (and then perhaps across to Europe).

    ReplyDelete
  5. A bit back, Harry Harpending and colleagues wrote this about the Mitochondrial DNA:

    The present data are clearly inconsistent with the strong Garden of Eden hypothesis. If there was indeed a single large expansion from Africa around ioo,ooo years ago, we should see the signature of it in the mtDNA differences, but instead we see indications of multiple later expansions associated with modern technology instead of modern morphology

    The other thing that strikes me is that if you read David Maddison's papers on the genetics, they call into question the mitrochondrial DNA conclusions. Add to that the data from Svante Paabo and colleagues who found Neandertal sequences in the modern genome (that is the general gist) and your species divisions become a little bit more blurred.

    You have the quasi-moderm material at Herto, in North Africa, between 150 and 160 thousand years ago and that seems to be the earliest material that can be called in some sense modern. So it is very easy to see what John calls “an incursion into the Middle East.”

    It has been argued in some quarters that the archaic nature of the Near Eastern early modern sample reflects Neandertal hybridization. Work by Art Durband and I seemed to indicate, instead, that it it might be late archaic/early modern North African morphology. The discovery of the Herto remains seemed to reinforce this.

    I do, however, agree with John's take that we cannot take the fossils at face value. Milford Wolpoff once upon a time said “The data do not speak for themselves! I have sat in rooms with the data for hours. They never said a word.”

    ReplyDelete
  6. By the way, John, what material are you talking about that is 600 ky old that has Neandertal characteristics?

    ReplyDelete
  7. A bit back, Harry Harpending and colleagues wrote this about the
    Mitochondrial DNA:

    The present data are clearly inconsistent with the strong Garden of Eden hypothesis. If there was indeed a single large expansion from Africa around ioo,ooo years ago, we should see the signature of it in the mtDNA differences, but instead we see indications of multiple later expansions associated with modern technology instead of modern
    morphology.


    The other thing that strikes me is that if you read David Maddison's papers on the genetics, they call into question the mitrochondrial DNA conclusions. Add to that the data from Svante Paabo and colleagues who found Neandertal sequences in the modern genome (that is the general gist) and your species divisions become a little bit more blurred.

    You have the quasi-moderm material at Herto, in North Africa, between 150 and 160 thousand years ago and that seems to be the earliest material that can be called in some sense modern. So it is very easy to see what John calls “an incursion into the Middle East.”

    It has been argued in some quarters that the archaic nature of the Near Eastern early modern sample reflects Neandertal hybridization. Work by Art Durband and I seemed to indicate, instead, that it it might be late archaic/early modern North African morphology. The discovery of the Herto remains seemed to reinforce this.

    I do, however, agree with John's take that we cannot take the fossils at face value. Milford Wolpoff once upon a time said “The data do not speak for themselves! I have sat in rooms with the data for hours. They never said a word.”

    ReplyDelete
  8. John -

    The following sentence fragment(?) appears in your post:

    An early movement followed by long interactions in this limited area would explain so much of the population structure and morphological variation of MSA Africans wasn't represented in the people who peopled Eurasia.

    Is the proper completion

    An early movement followed by long interactions in this limited area would explain why so much of the population structure and morphological variation of MSA Africans wasn't represented in the people who peopled Eurasia.

    or

    An early movement followed by long interactions in this limited area would explain so much of the population structure and morphological variation of MSA Africans that wasn't represented in the people who peopled Eurasia.

    or something else?

    ReplyDelete
  9. Possibly related?

    http://www.nytimes.com/2011/01/28/science/28africa.html

    ReplyDelete
  10. Thanks for the post; this is a subject I like to keep tabs on.

    ReplyDelete
  11. Those tools in the picture look pretty crude. Even the archaic Homo sapiens stone tools from the Middle Stone Age look better than that.

    ReplyDelete
  12. "Sometimes it would be a real barrier, sometimes I guess there could be an African West Coast route open (and then perhaps across to Europe)."

    Yes, or what about traveling up the coast. Having recently seen for myself how close Africa is to Spain,and knowing how volatile climate change has been during the time period being discussed, it's surprising to never see this route suggested. It's as if it is out of the question.

    ReplyDelete
  13. The discussion on the topic may be drying up, but may this also have a effect on the mtDNA estimates?

    http://mbe.oxfordjournals.org/content/27/10/2406.long

    Abstract

    Accurate reconstruction of the divergence times among individuals is an essential step toward inferring population parameters from genetic data. However, our ability to reconstruct accurate genealogies is often thwarted by the evolutionary forces we hope to detect, most prominently natural selection. Here, I demonstrate that purifying selection acting at many linked sites can systematically bias current methods of genealogical reconstruction, and I present a new method that corrects for this bias by allowing a class of sites to have a time-dependent rate. The parameters influencing the time dependency can be estimated from the data, allowing for a general method to detect the presence of selected sites and correcting for their distortion of the apparent mutation rate. The method works well under a variety of scenarios, including gamma-distributed selection coefficients as well as entirely neutral evolution. I also compare the performance of the new method to relaxed clock models, and I demonstrate the method on a data set from the mitochondrion of the North Atlantic whale-“louse” Cyamus ovalis.

    And the punchline...

    "The new method implied a TMRCA of near 0.015 [for the whale-“louse”]. Given a neutral mutation rate of 2×10 − 8 and three generations per year (e.g., Kaliszewska et al. 2005), the HKY + Γ and relaxed clock models predict TMRCAs of approximately 183 and 187 KYA, whereas the purifying rate model predicts a TMRCA of 250 KYA."

    ReplyDelete