A new timeline for Neanderthal interbreeding with modern humans

There's something endlessly humbling about looking beyond a human lifespan. It's at least one of the reasons I find the Long Now Foundation so intriguing.

It should also be noted another paper also came out today with new ancient DNA from a modern human whose bones carbon dated to around 45kya. The data suggests the individual was only 80 generations from the Neanderthal interbreeding event suggesting it happened around 45-49 kya. The event may have happened over a few generations realistically but I think 7000 years is unlikely due to the rapid expansion of humans out of Africa (or to be more precise an extremely successful expansion of a population of modern humans whose signal is difficult to filter out) all sharing this same introgression signature.

A species isn't defined by if it can interbreed with other species or not. Horses and donkeys can produce offspring (although they are mule - and there's some discussion about how certain Neanderthal/Sapiens combinations could possibly also be mule, but that's not important). And Darwin's finches (https://en.wikipedia.org/wiki/Darwin%27s_finches) are defined as different species, but can technically interbreed.

By calling them "H. sapiens neanderthalensis", the authors are tipping their hand. They view Neanderthal's as a subspecies. There are those who disagree. The definition of what a species is seems to depend on who you talk to.

There are examples of animals, currently classified as different species, being able to interbreed and produce fertile offspring.

e.g. Polar bears (Ursus Maritimus) and Grizzlies (Ursus arctos horribilis) can breed to produce fertile Grolar bears[1].

TL;DR: Although Polar bears and Grizzlies appear to be very different, they diverged relatively recently and have remained distinct mainly through geographic separation. They don't interbreed much because they generally aren't in the same places during mating season. Although their physical traits have changed considerably, genetic differences haven't yet built up to the point where their offspring are infertile.

It's conceivable that the different subspecies of humans were isolated from each other in very different environments and evolved along very different lines. However, they weren't isolated from each other so long that genetic changes made it impossible for them to produce fertile offspring.

[1]https://slate.com/news-and-politics/2010/05/when-polar-bears...

It is more like a race.

There are people with Neanderthal heretige living today. It is even used as a slur.

The classic elementary school definition of species is something like “x and y are different species if they cannot produce fertile offspring” which is obviously already incomplete (it causes eg two males of one species to be different species, for example) but could be made a bit more precise.

But the actual definition is much more “a species is a group of reasonably closely related and similar organisms, capable of reproducing fertile offspring given sufficiently many individuals, and the grouping is useful for scientists to talk about”. Obviously that definition isn’t useful for a layperson to decide how to taxonomize things, but I think it’s a useful definition to have when reading about classifications of things into species.

Species is a human taxonomy which doesn't strictly relate well to the variety of organisms in the living world. There are lots of different definitions and ways to approach it depending on context.

The term of art here is "species concept", of which there are several, well covered in [1]. Interfertility is a feature of some, but not all.

[1] https://bio.libretexts.org/Courses/University_of_California_...

In general, in biology species are not defined by the impossibility of having viable hybrids between them, but by the fact that in natural conditions they do not normally interbreed, which allows their genes to drift away, becoming more and more different with the passing of time.

For a species to exist, interbreeding must be both possible and frequent, which ensures that the entire population of the species remains uniform genetically for as long as the species exists, even when its genes may become more and more different from those of other species.

The most frequent way for a new species to appear is when there is geographic isolation between two populations, which prevents interbreeding. Then, after many generations, the genetic differences may become so great that even when the populations are reunited interbreeding will fail, resulting in hybrids that are either non-viable, or sterile, like mules.

With Neanderthals and Denisovans, the geographic separation has started the process of development of new species and they have become clearly differentiated from a genetic point of view.

Nevertheless, not enough generations had passed so that the differences would become so great as to make interbreeding impossible. Even if we know that successful interbreeding has existed, we do not know which was the percentage of success for such interbreeding, i.e. how many of the hybrids were both viable and non-sterile.

So it is a rather arbitrary choice to decide whether to call Neanderthals and modern humans as different subspecies or different species. In any case, they were easy to distinguish.

There are many closely related species between which interbreeding is easily performed in captivity, e.g. in zoos, but it almost never happens in natural conditions, where wild animals may choose partners according to their preferences, so those species are easily distinguishable, due to their genetic differences that are preserved by the lack of interbreeding.

What does "neanderthal" mean in this context?

eg: at some point there were humans who, genetically were 100% neanderthal and other humans who were genetically 100% not.

The first pair that mated had offspring with a 50-50 mixture of more or less random neanderthal and "not-neanderthal " genes (although even that is a simplification as the two parents would have had many common human genetic features).

Over the course of seven thousand years of side by side interbreeding and co-habitation there would have been individuals with ranging from 0% neanderthal to 0% "not neanderthal".

A further complication is the relationship between "features" (visible physical characteristics) and "percentage neanderthal" .. an individual may have almost no neanderthal genetics whatsoever save for that one gene that strongly codes for a "neanderthal forehead".

If there was some unique "neanderthal feature" that was clearly recognisable as neanderthal and only expressed itself within people that had some specific "neanderthal gene" ..

Then no, there wouldn't be people walking about with that specifically "neanderthal feature" unless they also had that specific "neanderthal gene".

The problem is that genetic expression really isn't this cut and dried, nor is the boxing up of humans as "this" or "that".

Oh there is more to humanity than that. For example the history of europe with even yamnaya, corded-ware, EHG, ANF all looked different with different heights, skin tones, skull shapes, cultures, etc as well.

And this is from around 3000 BC. Modern european as you know it (admixture wise) would likely be fully formed at around 400 BC

The ~2% that is kept in some humans and I have them and am walking among you. The majority of dna has to do with non-visible changes.

what is the best explanation for this closeness? the only two species capable of engaging in organized warfare? humans and chimps.

how likely is it that an intelligent entity that precedes humanity came along and decided to chose chimpanzees to breed with?

anybody telling you they know what happened before the time we had written records are selling you something.

>Considering we only differ from chimps by about 4% in genome

from what I understand, while human males differ from male chimps by about 1% in genome*, that still means that human males are more closely related to male chimps than they are to human females.

* https://knowyourdna.com/human-and-chimpanzee-dna/

Yes. A very long time after Neanderthals were extinct though, so I don't see the relevance.