One of the somewhat surprising things we have learned over the last decade is that massive admixture and homogenization has occurred between distinct human lineages over the last 10,000 years. By this, I mean that we’re not talking simply about continuous gene-flow between neighboring populations, but massive expansions of small groups and assimilation of very different groups from the expanding groups. As a stylized fact, it looks like “Early European Farmers” we as distinct from Mesolithic hunter-gatherers as modern Northern Europeans are from Han Chinese (pairwise Fst ~0.10). The fusion of these two groups later merged in much of Europe with migrants from the east, the western edge of the forest-steppe.
The empirical pattern seems to be that cultural innovations (e.g., agriculture) trigger demographic revolutions, which homogenize and admix vast regions. This is a story of demographic history. Phylogeography.
But there is another aspect, natural selection. Humans are not exempt from this. Selection operates upon genetic variation, which is preexistent (“standing variation”), or, comes from new mutations (de novo).
It seems plausible that cultural innovation has resulted in a great deal of selection over the last 10,000 years. So where did the raw material come from? One argument that has been playing out is between those who argue that it’s from variation within human populations that is ancestral and shared, and new variation. This is where admixture comes into play.
A new preprint on bioRxiv uses the 1000 Genomes data in the New World to suggest that admixture resulted in the introduction of a lot of adaptive alleles into populations of mostly European and Native background from African ancestry. Basically, it seems likely that the American tropics were colonized by African tropical diseases, which entailed adaptations which were already existent within African populations. Admixture-enabled selection for rapid adaptive evolution in the Americas:
Background: Admixture occurs when previously isolated populations come together and exchange genetic material. We hypothesized that admixture can enable rapid adaptive evolution in human populations by introducing novel genetic variants (haplotypes) at intermediate frequencies, and we tested this hypothesis via the analysis of whole genome sequences sampled from admixed Latin American populations in Colombia, Mexico, Peru, and Puerto Rico. Results: Our screen for admixture-enabled selection relies on the identification of loci that contain more or less ancestry from a given source population than would be expected given the genome-wide ancestry frequencies. We employed a combined evidence approach to evaluate levels of ancestry enrichment at (1) single loci across multiple populations and (2) multiple loci that function together to encode polygenic traits. We found cross-population signals of African ancestry enrichment at the major histocompatibility locus on chromosome 6, consistent with admixture-enabled selection for enhanced adaptive immune response. Several of the human leukocyte antigen genes at this locus (HLA-A, HLA-DRB51 and HLA-DRB5) showed independent evidence of positive selection prior to admixture, based on extended haplotype homozygosity in African populations. A number of traits related to inflammation, blood metabolites, and both the innate and adaptive immune system showed evidence of admixture-enabled polygenic selection in Latin American populations. Conclusions: The results reported here, considered together with the ubiquity of admixture in human evolution, suggest that admixture serves as a fundamental mechanism that drives rapid adaptive evolution in human populations.
The period after 1492 is easy for us to think about. But what ancient DNA has shown us is that it’s not as uncommon a phase as we might have thought.
“As a stylized fact, it looks like “Early European Farmers” we as distinct from Mesolithic hunter-gatherers as modern Northern Europeans are from Han Chinese (pairwise Fst ~0.10).”
But that’s only because of Basal Eurasian, isn’t it? The basic components came from the same root minus/plus Basal Eurasian. So its like two branches from the same root plus an additional ingredient – which phenotypical deviation has to be still investigated, since we have no source population for Basal Eurasians at hand.
It migth be primarily about the lack of Neandertal admixture after all, isn’t it?
So the difference is that the distance between European Mesolithics and Neolithics is primarily about admixture or the lack of it, whereas the difference between Northern Europeans and Chinese is a longer path of separate evolution in relative isolation from each other.
About the title:
“Humans are basically invasive weeds”
How does the title correlate with the content? I mainly hear that from people which say “humans are bad for the planet”, with a very negative connotation and view on our species as a whole.
But aren’t all species, isn’t all life the same in this respect? If there is an opportunity for expansion, any life form will take it and its just that we have knowledge about our human spread and look at it differently. The species in America before humans came were not at all autochthonous for millions of years, a lot spread like humans did, some even just a short time before. Even when life moved from the sea to land, it was an “invasion”.
Australia and many islands were changed with the arrival of humans, but when a landbridge would have come up, somewhere in the future, similar things would have happened with or without humans and the spread of old world lifeforms.
So where is the difference between “invasive weed” humans and other forms of life? I see none. Humans just use different and so far highly successful strategies because of their intelligence, which can overcome natural borders other species can’t. They take with them other forms of life and spread it. But e.g. species of elephants changed the habitat whereever they lived too. Any adaption of two species, like the Dodo and local island plants, had to be established as well after an arrival.
Humans bring a new force into the biological game of a region, shuffle some things and make a new order possible, but so far, they didn’t change the rules, they are no real game changers.
Actually I think a lot of the ecological doomsday preachers highly overrate current human impact and capabilities. As if humans really control or ruin anything as completely as natural disasters in the Earth’ past did. We are still a rather small player in comparison and rather have to look out for our own good and what we can do for this planet to prevent a real complete meltdown to happen.
The adaptiveness of African tropical genes in the tropical regions of America was long noted. African slaves introduced diseases which killed a lot of the native population and made it even necessary in some places, to rely even more on the import of more African slaves to keep up a stable economy.
If the same pattern would have been present in e.g. Australia, a lot of places on Aussie-land would have been much worse to colonise by European settlers than they were. Because it was not the tropical climate as such which was so hostile to Europeans, but the diseases which existed among a lot of old world tropical populations – which lived for a long time there and co-evolved with diseases and parasites with which they could deal with (more or less, never fully), but newcomers couldn’t (even less). If you transfer a population, you usually take diseases with them. What was an advantage for Europeans settlers in Northern America, became a disadvantage in tropical America because of the introduction of Subsaharan workforce and associated diseases.
This can be observed to this day, after Malaria was wiped out in the 1950s:
“It is inevitable that we are going to see cases in the United States because [malaria] is such a major global problem and people are constantly moving from endemic areas in to the United States,” he says. The World Health Organization estimates that there are more than 200 million cases of malaria occur globally each year, the vast majority in Africa.
The new study found that the majority of those hospitalized were black men age 25 to 44.”
https://www.npr.org/sections/goatsandsoda/2017/04/24/525405956/malaria-wiped-out-in-u-s-but-still-plagues-u-s-hospitals?t=1569842880434
India is the perfect example with Indo-Aryans and IVC-people (presumably Dravidians) adapting faster to the tropical parts of South Asia by admixture with AHG populations living there. The adaption would have been much slower and more painful without.
I think I read about how advantages for lighter and darker skin pigmentation are in competition in South Asia, with both having – obviously – advantages and disadvantages, its about what’s more important in a given context.
But how about other traits in India? Any studies done on the issue which traits introduced by which ancestral component being favoured or disfavoured in South Asia as a whole and specific regional/social contexts?
How does the title correlate with the content? I mainly hear that from people which say “humans are bad for the planet”, with a very negative connotation and view on our species as a whole.
But aren’t all species, isn’t all life the same in this respect? If there is an opportunity for expansion, any life form will take it
Yes, indeed. But only some species succeed so well in expanding that they become a big new part of the area. Only those are the ones that get to be called “invasive”.
And, yes, there is usually a hidden moral judgment: the “invasive” has changed things for the worse.
@Roger: I think that’s largely due to the fact most people still see a regional ecosystem as if it was stable and enduring for millions of years, when in fact it was not.
All species were at some point invasive and changed the ecosystem they entered. You might say it sometimes changed for the worse, but that’s a matter of perspective.
If you look at the same invasive species 100.000 years later, it will appear like all others, perfectly adapted and in some sort of superficial “harmony”.
If we humans have a problem with invasive species, a lot of times its because they change a habitat to the worse for us, from our current perspective and because the new stability hasn’t evolved yet and more instable, chaotic and destructive results might be recognisable. But if one thing is for sure, than that this happened over and over again throughout the history of life on this planet.
Nature is not stable, caring or a save thing to start with. People in modern urban centres oftentimes forget about that, especially the recent anti-human ecofanatics.
I completely agree that too many “people still see a regional ecosystem as if it was stable and enduring for millions of years, when in fact it was not.” It is a major problem with much discourse about “the environment”.
In biology, “invasive” has a somewhat narrow meaning. To be cynical, it means “succeeding in a new ecosystem and causing changes I don’t like.” A species that is new to an ecosystem is simply called an exotic. Lots of exotics don’t survive and most of the rest just settle in and don’t cause much in the way of noticeable change.
It is generally considered progressive to see ecosystems as good the way they are, and thus they should be protected from immigration of non-native species (at least those that would cause unwelcome changes). It is generally considered conservative to see societies as good the way they are, and thus they should be protected from immigration of non-native people (at least those who would cause unwelcome changes).
How does the title correlate with the content? I mainly hear that from people which say “humans are bad for the planet”, with a very negative connotation and view on our species as a whole.
i’m speaking to the plant geneticists in the audience. no moral judgment.
What about those of us who are not plant geneticists? We get nothing?
I have heard a theory of invasive species. It starts with a claim that invasive species don’t explode immediately on arrival, but sit quietly for a while. It is hypothesized that this waiting period is while they pick up a little admixture to adapt to the new environment.
Is this what you are referring to?
Is this theory specific to plants? How well-supported is it? I thought that it was believed by default because there is no competing theory. The age of genetics should make it easy to test.