A 'leaky' model

By Razib Khan | January 28, 2011 1:22 am

John Farrell pointed me to this Anne Gibbons’ piece, A New View Of the Birth of Homo sapiens. Here’s some interesting passages:

The new picture most resembles so-called assimilation models, which got relatively little attention over the years. “This means so much,” says Fred Smith of Illinois State University in Normal, who proposed such a model. “I just thought ‘Hallelujah! No matter what anybody else says, I was as close to correct as anybody.’ ”

But the genomic data don’t prove the classic multiregionalism model correct either. They suggest only a small amount of interbreeding, presumably at the margins where invading moderns met archaic groups that were the worldwide descendants of H. erectus, the human ancestor that left Africa 1.8 million years ago. “I have lately taken to talking about the best model as replacement with hybridization, … [or] ‘leaky replacement,’ ” says paleogeneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, lead author of the two nuclear genome studies.

I suppose ‘assimilation’ sounds too generic, but ‘leaky replacement’ seems more fitting for a building ‘super’. But it isn’t as if paleoanthropology has a Don Draper, a rogue with a way with words.

Here’s the infographic that went along with it:

One issue I’m wondering: what’s the best mammalian analog for humans? By humans I mean our lineage since the emergence of H. erectus (or whatever they’re calling it now) and its expansion into Eurasia. Off the top of my head, I think the gray wolf is the best candidate:

  • onur

    The word “assimilation” brings to mind a model in which a relatively small group of African modern humans assimilate many many non-African archaics into the modern human identity and culture after moving out of Africa and thus today’s non-Africans (including North Africans) are overwhelmingly descended from assimilated archaics. :)

    In other words, elite dominance. :)

  • Bruno

    onur’s view is chilly

  • http://faculty.washington.edu/tennej Jacob

    Or the lion (Panthera leo), which was about as widespread as the wolf before going extinct in the Americas. Since we know more about the genetics and evolution of humans than any other mammal, the question really should be put in reverse: for which species is the human a good molecular ecology model?

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    Since we know more about the genetics and evolution of humans than any other mammal, the question really should be put in reverse: for which species is the human a good molecular ecology model?

    this is a correct point on the merits. the reason though i suggest looking at other animals is that human evolution seems way overloaded with egos, biases, and politics. the utility isn’t due to the science, but because of the cultural context of the science.

  • onur

    onur’s view is chilly

    It was just a joke featuring a Palaeolithic elite dominance.

  • http://abugblog.blogspot.com Blackbird

    I agree, to look into the phylogeography of non-human systems can help us look into human data with a fresh mind. “Replacement with introgression” could be the new model name. Introgression has been shown to happen in many systems. One that comes to mind is the brown hare system. In several areas in Europe there are mtDNA haplotypes of mountain hare origin – an arctic boreal species – in brown hare populations, despite quite distinct nuclear DNA. In northern Spain this is also found, where the mountain hare does not currently occur, and hasn’t lived there since the glaciations. This introgression is hypothesized to have happened during transition periods glacial to interglacial when ecological transitions made easier for the species to come into contact.
    This introgression was quite important, and gives the impression that the mountain hares were assimilated by the expanding brown hares, but this excerpt from the paper reminds us that the actual hybridisation events do not need to be many to leave a permanent imprint in the expanding species:
    “Currat & Excoffier (2004) have simulated such scenarios of competitive replacement and concluded
    that the traces of hybridization events at the contact zone, even if rare, could be amplified by the
    demographic expansion dynamics of the invading species. This eventually leads to high frequencies of alleles from the outcompeted species at the front of expansion of the invading species.”

    In your example, the wolf also hybridizes with coyotes, which enhances similarities with the human system. Razib, where is the wolf map coming from? It appears to include the Egyptian jackal of recent fame as it is actually a lineage of gray wolf (or a hybrid lineage? Unfortunately, the authors only looked at mtDNA: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0016385

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    got it from wikipedia.

    how about “replace, don’t repeal?” or “assimilate the job-killing eurasian hominins?” :-)

  • http://www.riverellan.blogspot.com Tom Bri

    Interesting map. They do have the wolf ranges in N America well to the north of the current southern boundary. Bit outdated, perhaps.

  • http://www.rishon-rishon.com David Boxenhorn

    “Out of Africa with benefits”?

  • Rafe

    A priori it makes since that wolves would provide a good analogy to pleistocen human population dynamics as they occupy similar niches. That was what my thesis was going to be about before I decided to focus on Gymnastics coaching and Parkour.

    The model seems especially appropriate now because of new findings in both fields. In addition to the evidence of introgression in modern humans from Neanderthals and Denisovans. It now look’s like there are two newly found distinct lineages of wolves in the himalayas and india respectively http://rspb.royalsocietypublishing.org/content/271/Suppl_3/S1.abstract
    With separation from the main gray wolf clade on the order of 400,000 years and 800,000 years on the MTDNA line. This is not dissimilar to estimated distance between AMH and neanderthals and AMH and denisovans. The paper intimates that hybridizing might be happening right now between himalayans and gray wolves which would fascinating to study in general and as an analog for the dynamics of the AMH and archaic hybrdization events. It would be very interesting to see the autosomal DNA from these areas.

    Furthermore there is evidence of an ongoing lineage separation on the british columbian coast between marine hunting wolves and interior wolves focused on ungulates http://www.raincoast.org/files/publications/papers/Munoz_et_al_2009_JBiogeog.pdf

    Combine that with the dynamics of Canis Lycaon, Canis Lupus and Canis Latrans in eastern north america and there is evidence of some very interesting dynamics in the wolf lineage.

    I think that this is good possible model for archaic human population dynamics in the Pleistocene. With a relatively stable adaptive tool set and niche archaics were able to spread out across a huge range similar to that of the wolf. With a degree of behavioral plasticity and low population density it would not be to difficult for lineages to separate and follow slightly different adaptive lines but achieving full reproductive isolation would be uncommon. Just as some circumstances would push populations apart other circumstances could just as easily reverse the speciation process. At any given time there might have been quite a few sub species following fairly separate adaptive paths before coalescing back into a mainline population or going extinct when circumstances changed.

  • pconroy


    In the case of hares thought, there is the Irish Hare, which recent research suggests, should be a separate species, and not a subspecies of Mountain Hare. It seems that it survived the Last Glacial Maximum (LGM) in an ice free tundra zone in the extreme South of Ireland, and has been genetically isolated from other European hares for 30,000-60,000 years!!



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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com


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