Paabo seems to think it unlikely that any of these introgressed alleles had a a significant selective advantage in humans, but that’s unlikely. I’ll bet money on this.
To be fair, I should explain why that is a sucker bet. John Hawks and I discussed about a situation with just a few tens of matings over all time: we were making the point that even in that minimal scenario, alleles with large advantages (on the order of 5%) could jump over to modern humans. The Max Planck estimate of 2% Neanderthal admixture is far more favorable to introgression: with that much of a start, and with at least 50,000 years to grow in, any allele with a selective advantage > 0.2% is likely to be over 50% today. Many such Neanderthal alleles should be fixed in Eurasians—or in some Eurasian populations in the right environments—or even in Africans, if the allele conferred global advantages. of course we’d have trouble proving this in Africans: the Science study really shows how much more Neanderthal ancestry Eurasians have than Africans, not the absolute amount in either population.
Note that the Fisher-wave velocity goes as the square root of the selective advantage: a Neanderthal allele with an advantage of 0.2% might have spread as far as the European lactase persistence variant, which probably had a selective advantage > 10%. Today we find that allele from north India, to Iceland, to the southern fringe of the Sahara.
Introgressing advantageous alleles derived from Neanderthals are probably more likely to go to fixation than most new favorable mutations. We now suspect that the majority of alleles that give large advantages to heterozygotes give smaller advantages to homozygotes and thus never go to fixation, using a variant of Fisher’s geometric argument. In the long run they are replaced by alleles that work better in homozygotes and do go to fixation—but when we stole alleles from Neanderthals, we were mostly getting old tested ones, rather than flash-in-the-pan alleles like sickle-cell.
There had to be such advantageous alleles because Neanderthals had been in Europe and west Asia for hundreds of thousands of years—they were well-adapted to that different, non-African ecology.
When there is introgression between species, transmission of adaptive alleles seems to always happen. We know a lot about some cases: one good example is introgression in cattle. Taurine cattle were domesticated in the Middle East, Zebu cattle in India, from ancestral stocks that diverged about half a million years ago. Zebu genes have introgessed a lot into African taurine cattle, in part due to known advantages in heat/aridity tolerance and rinderpest resistance. Creeping zebuization has been going on the Middle East for thousands of years. If you go as far west as Egypt, cattle are about 25% Zebu in the nuclear genome, while you don’t see anyzebu mtDNA or Y-chromosomes. This kind of discordance between the introgression of mtDNA/Y chromosomes and nuclear genomes is more common than not: looks like the same thing happened to us. Plausible when you think about it. Neanderthal mtDNA may well have had a selective disadvantage: they may have been blatant heat-wasters, since they had crummy clothing. Small population size might also have resulted in somewhat bunged up mtDNA, since selection is less efficient then.
Obviously some Neanderthal alleles had a selective disadvantage in humans, for example those that determined their different body form. Many more must have been effectively neutral, with no noticeable advantage over the version in anatomically modern humans. But some must have been useful—and the more useful they were, the more common they are today.
There seems to be a pattern in which an invasive species shows up, hangs around in an unspectacular way for sometime while it’s picking up alleles from local sister species, and then spreads out irresistibly.
We generally call those cosmopolitan species weeds.
Were some of those introgessing Neanderthal genes adaptive? Had to be. Do they account for the cultural big bang somewhat later? It would make sense, but it’s not a lock. I’d call it likely.
Paabo seems to think it unlikely that any of these introgressed alleles had a a significant selective advantage in humans, but that’s unlikely. I’ll bet money on this.
To be fair, I should explain why that is a sucker bet. John Hawks and I discussed about a situation with just a few tens of matings over all time: we were making the point that even in that minimal scenario, alleles with large advantages (on the order of 5%) could jump over to modern humans. The Max Planck estimate of 2% Neanderthal admixture is far more favorable to introgression: with that much of a start, and with at least 50,000 years to grow in, any allele with a selective advantage > 0.2% is likely to be over 50% today. Many such Neanderthal alleles should be fixed in Eurasians—or in some Eurasian populations in the right environments—or even in Africans, if the allele conferred global advantages. of course we’d have trouble proving this in Africans: the Science study really shows how much more Neanderthal ancestry Eurasians have than Africans, not the absolute amount in either population.
Note that the Fisher-wave velocity goes as the square root of the selective advantage: a Neanderthal allele with an advantage of 0.2% might have spread as far as the European lactase persistence variant, which probably had a selective advantage > 10%. Today we find that allele from north India, to Iceland, to the southern fringe of the Sahara.
Introgressing advantageous alleles derived from Neanderthals are probably more likely to go to fixation than most new favorable mutations. We now suspect that the majority of alleles that give large advantages to heterozygotes give smaller advantages to homozygotes and thus never go to fixation, using a variant of Fisher’s geometric argument. In the long run they are replaced by alleles that work better in homozygotes and do go to fixation—but when we stole alleles from Neanderthals, we were mostly getting old tested ones, rather than flash-in-the-pan alleles like sickle-cell.
There had to be such advantageous alleles because Neanderthals had been in Europe and west Asia for hundreds of thousands of years—they were well-adapted to that different, non-African ecology.
When there is introgression between species, transmission of adaptive alleles seems to always happen. We know a lot about some cases: one good example is introgression in cattle. Taurine cattle were domesticated in the Middle East, Zebu cattle in India, from ancestral stocks that diverged about half a million years ago. Zebu genes have introgessed a lot into African taurine cattle, in part due to known advantages in heat/aridity tolerance and rinderpest resistance. Creeping zebuization has been going on the Middle East for thousands of years. If you go as far west as Egypt, cattle are about 25% Zebu in the nuclear genome, while you don’t see anyzebu mtDNA or Y-chromosomes. This kind of discordance between the introgression of mtDNA/Y chromosomes and nuclear genomes is more common than not: looks like the same thing happened to us. Plausible when you think about it. Neanderthal mtDNA may well have had a selective disadvantage: they may have been blatant heat-wasters, since they had crummy clothing. Small population size might also have resulted in somewhat bunged up mtDNA, since selection is less efficient then.
Obviously some Neanderthal alleles had a selective disadvantage in humans, for example those that determined their different body form. Many more must have been effectively neutral, with no noticeable advantage over the version in anatomically modern humans. But some must have been useful—and the more useful they were, the more common they are today.
There seems to be a pattern in which an invasive species shows up, hangs around in an unspectacular way for sometime while it’s picking up alleles from local sister species, and then spreads out irresistibly. We generally call those cosmopolitan species weeds.
Were some of those introgessing Neanderthal genes adaptive? Had to be. Do they account for the cultural big bang somewhat later? It would make sense, but it’s not a lock. I’d call it likely.