edit: I'm also always intrigued by statements like this: "Interestingly, they suggest that 6%-7% of the genomes of West Africans is archaic in origin". I know it's over-simplifying for the lay-person, but 6-7% is the high end of what people claim is the genetic difference between us and chimpanzees, and we can't even reproduce with chimpanzees and don't even have the same number of chromosomes. So there's a lot more than that we have in common with some archaic species, and a lot less that we'd expect to have with closer relatives.
But when you look closely enough, there aren't really any such exact groupings in nature. Conception only involves the DNA of two individuals, not some larger group. And we know that species today have common ancestors, and it can't be that there was a precise second at which they became distinct, every split must have happened gradually.
(Like us, animals also have their own heuristics about other animals, including what sort of things they will mate with, but it's imperfect. Denim jeans cannot reproduce with poodles, as it turns out.)
Apple trees mate readily with pear trees. Most citrus fruits are blends of other citrus fruits. Peppermint is a cross breed. 
And then there's ring species , whose members can each mate with similar-enough-dna members and produce offspring.
They also use string similarity algorithms to determine morphological order, so we don't strictly know the order at the most precise level.
Of course, this is all very controversial, some people think we're always changing gradually and some people think we always change rapidly. I think they're both wrong--sometimes organisms change rapidly, and sometimes they change rapidly, depending on environmental factors.
disclaimer: I'm no expert, I've just looked at it from an algorithmic view and read some taxonomical texts as I had the same question as you.
It's my vague impression that you get species that can't interbreed after extended geographic/habitat isolation. But short of that, it's a tangled mess, with complex fertility patterns.
Some examples of hybrids without human influence (well that's arguable):
The grand daddy of "hybrids" (twelve species):
Also you can go beyond hybrids:
As an amateur I'd hypothesize if the species diverges and then relocates, but is subject to similar environmental pressure, the morphological changes won't be great enough to prevent the organisms from interfacing and then reproducing.
We've determined this through string similarity algorithms, so really I'd say we can't be completely certain here either.
As to the 6% thing that's in the the same sense that you got 50% of your genes from your mother. Most of the genes there will be identical to the ones your father has on a codon by codon basis but their origin is your mother. And by looking at the 1% of genes where humans vary from each other we can tell which genes came from this ghost population.
You can't. In practice, extinct branches of humans are classified as separate species, and animals are classified as a bunch of species because biologists get credit for discovering new species. In contrast, living branches of humans (e.g. bushmen vs. pygmies vs. everyone else) ar classified as a single species even if they have evolved largely in isolation for hundreds of thousands of years and rarely interbreed, because classifying them separately could give ammo to racists and worse.
>I'm also always intrigued by statements like this: "Interestingly, they suggest that 6%-7% of the genomes of West Africans is archaic in origin". I know it's over-simplifying for the lay-person, but 6-7% is the high end of what people claim is the genetic difference between us and chimpanzees, and we can't even reproduce with chimpanzees and don't even have the same number of chromosomes. So there's a lot more than that we have in common with some archaic species, and a lot less that we'd expect to have with closer relatives.
It usually means 6-7% of genes that vary between humans.
They were classified that way, until the presence of Neanderthal & Denisovan DNA was discovered in modern humans.
> In contrast, living branches of humans (e.g. bushmen vs. pygmies vs. everyone else) ar classified as a single species even if they have evolved largely in isolation for hundreds of thousands of years and rarely interbreed
They are classified as a single species with the rest of humans because they can interbreed with other humans, not for political reasons.
We couldn't draw the same conclusion about extinct branches until recently because we don't have living groups of them that we can observe interbreeding. Now that we see the genetic evidence of that the classification has changed.
This is not the standard for, e.g. birds, however. Consider the mallard. It's happy to breed (with fertile offspring) with just about any duck. However the ornithology community does not say "great, they're all ducks, maybe with separate subspecies". Instead they complain about "genetic pollution" wiping out species of ducks through genetic drift.
Myself, I think it would be perfectly reasonable to call these subspecies and keep the Biological Species Concept, but in actual usage the Biological Species concept is dead.
A horse and a donkey can breed just fine, but a mule cannot.
This makes sense because we know that species eventually differentiate and whether or not they can recombine is a good indication of how far apart that separation is.
But nowadays all that stuff is thrown out of the window. I don't really understand why it is no longer considered a useful distinction.
That's almost always true, but isn't an absolute. There are documented cases of fertile mare mules: https://en.wikipedia.org/wiki/Mule#Fertility
I've seen no evidence to support the proposition that the children of San and Pygmies with outside human populations are sterile.
Given that, the question is what would the motivation possibly be to call them a separate species from the rest of humanity?
Those percentages aren't measuring the same thing. Chimp to human is a total genome comparison. Human to Hominid ancestor X is based on identified humane genes. The latter is a tiny subset of the former.
It's just a level of difference in which somebody goes, yip let's draw a line here and from anything going forwards is a new species.
Heck, it certainly isn't defined by having interchangeable blood as we all know that don't work with humans.
Female Tiger + Male Lion = Liger.
Male Tiger + Female Lion = Tigon.
EDIT: Wikipedia: "Notably, ligers typically grow larger than either parent species, unlike tigons"
Does the adjective 'Franco-German' reveal an insidious pro-French bias in English?
I talk about sexism pretty regularly on HN. Sometimes it doesn't go well and sometimes it does. You absolutely can discuss sexism here. This is just not a good approach to doing so.
Hoever, just as with Liger and Tigon, the two donkey/mule variants have different names:
Male Donkey (jack) + Female Horse (mare) = Mule
Female Donkey (jenny) + Male Horse (stallion) = Hinny
Hinnies are rarer, but do exist (https://en.wikipedia.org/wiki/Hinny)
As others here have noted, it's generally or historically been deemed to be a pairing expected to produce a fertile offspring that can interbreed with others of the same group.
Though, really, DNA is just two RNA. We are made up of the same code that makes bacteria and even viruses. So it's bound to be messy and highly hackable.
I can't readily find a really good article about the (controversial) theory that humans descended from a pig-chimp mating, but you can Google it.
In a recent episode of the Grand Tour locals from Colombia were proud to fuck their donkey so I didn't want to dismiss this hypothesis on forehand.
We do have to discuss and reason about living creatures with other humans though, so it's helpful to name and categorize, just know that the names and categories are a human language construct, nothing more.
This was a definition of species for a long time. In recent years some have advocated for changing it a lot, to the point that they should just start calling different dog breeds their own species.
Or to go the other way, are dogs and wolves different species?
Is it homo neanderthalensis and homo sapiens or homo sapiens neanderthalensis and homo sapiens sapiens?
In my opinion, Denisovans and Neanderthals are both obviously types of Homo Sapiens and the argument they aren't is at this point mostly promoted by those desperately clinging to the now disproven out of africa hypothesis.
What? When was it disproven? There's evidence of interbreeding outside of Africa with other species, in some sense of that word, which also originated in Africa, right? Maybe I missed something.
This is in contrast to a "Multiregional origin of modern humans", which still has "expanding from Africa" first, but far before modern man developed, followed by evolution and development everywhere without modern features coming from Africa "all at once". This too is clearly false and misleading. There were large migrations from Africa with large genetic distances from the native hominid populations, and the resulting mixture appears to be much closer to African than the native hominids.
The modern synthesis is multiple waves of expansion out of Africa and significant gene flow making the tree look more like a river delta: lots of forking but also lots of merging, though not to the point of an undifferentiated sea either. The exact details are constantly being reevaluated as more genetic data is acquired. This is significantly different from the strong Out-of-Africa hypothesis, but claims there have weakened to include essentially this picture. The difference between the two point of views is a matter of scale at this point: how strong are the waves out of Africa, how much was displacement vs interbreeding, how much do genes flow in patterns besides out-from-Africa, how much is one giant expansion a reasonable approximation, etc.
>We still think that most – anywhere between about 92% and 98.5% – of the ancestry in people not living in Africa today does indeed derive from the out-of-Africa expansion.
>Interestingly, they suggest that 6%-7% of the genomes of West Africans is archaic in origin. But this archaic ancestry wasn’t Neanderthal or Denisovan.
How are these numbers reconciled with other statements like 99% of the human genome is shared with Bonobos 
Some papers will use different definitions, and it can be confusing.
Between species (e.g. humans and chimps), it's most often considering "non-synonymous mutations," i.e. how many genetic differences there are in protein-coding genes (these are relevant to structure and function of the protein)
For distant relatives (e.g. humans and bananas), it's similar, looking specially at those genes which we can identify across many species (particularly those related to cell upkeep, DNA replication, structure, etc.).
For within-species, there are a few ways of doing it; you could model how much of the genome you expect came from each source (as this study did) (see admixture analysis, coalescence), and you could look for overall differences on the genome (of all existing variation in the species, how much is consistently different between populations) (see F-statistics for example)
99% of any hominid genome - whether Neanderthal, Denisovan, or the "archaic" ancestor refererenced - is also shared with bonobos. The branching point with bonobos is at a much earlier stage of evolution, so all descendant branches have roughly the same affinity to them.
How can a homo sapien in Europe share 99% of their genome with a Bonobo, but only 92% with another homo sapien in Africa
Both numbers are ridiculous and meaningless so don't pay them any mind.
I agree that they have little meaning for the purpose of individual and group identity. It's been pretty disheartening, for example, to see some people extrapolate from the discovery of Neanderthal DNA in non-African populations to the current economic disparities between non-African and African societies. Ironicallly, that is a reversal of the previous false stereotypes associated with Neanderthal influence in humans. I guess once it was proven, it had to be turned into a "good" thing.
But the percentages are meaningful for building a picture of ancient human evolution and migration.
Her destiny ( teased throughout the series ) was to sacrifice herself to lead humanity to salvation ( earth ). It's similar to Christ in a way - self-sacrifice and the second coming to lead the faithful to the kingdom of god.
I think from day one of her existence, she was a kind of power on the same level as Baltar and Six's angelic alter egos. Watching it the first time when it originally aired, I had thought she was one of the Five (even though part of me thought that was too obvious, and a cop out)... and then the fifth member was revealed and it wasn't her.
Clearly the writers had given her a role beyond merely human or Cylon, something part of the grand scheme of God that Messenger/Head Baltar and Six were part of.
Is it that human origins are rooted in chimp-pig hybridization? Probably not, but it is high on my “just might be crazy enough to be right” list. The odd similarity-dissimilarities among human, pig, and primate are just too interesting to be dismissed out of hand.
Perhaps they dumbed down the complexity for a bunch of freshmen, but I remember wondering then why some combination of the two wasn't possible.
It certainly looks like a mix of the two canonical models matches the data that is being revealed through genetics.
That said, classical multiregionalism is very, very dead and has been for awhile. Essentially what you're asking about has started to come up lately as a sort of in-Africa multiregionalism. This is a nightmare to model mathematically and so people just couldn't until recently when new fossils made it pretty hard to explain things any other way.
It hasn't fully emerged yet and many people don't really deal with it. Take for instance, this paper. One of the fundamental assumptions is that there was a singular set (or other time limited) of introgression events between archaic and human populations. That's a reasonable assumption in older models (and dramatically simplifies things), but it's possibly violated in an African multiregionalism model.
They claim it's not a problem because of symmetry, a point I'll admit I don't fully understand their explanation for in the supplementary material.
edit: On a related note, my siblings' DNA test results say that they're something like 4% Native American, yet we have very reliable documentation of pure British genealogy back on all lines almost all the way back to the 1500s. Very unlikely to actual have a modern link. I'm sure the companies are likely overplaying the similarity more than anthropologists would, but am I to conclude that I have a closer link to Native Americans than other random samples from Europe?
It's whatever opinion/theory the person speaking disagrees with.
It's more likely that at some point in the last 15,000 years someone in your lineage had a child with someone with some Siberian background way far back.
Also, the classical meanings of terminology like "negroid" or "caucasoid" doesn't really map to genetics. My personal experience is that they're almost exclusively used by people who aren't familiar with modern understandings of human evolution. There are a lot of cranks talking about it, so it's often best to avoid archaic terminology that might get you mistakenly grouped with them.
In practice, this means that two individuals are said to belong to a different (resp. identical) species if everyone in the community agrees that they do. That's what it means to be a "social construct", not that the differences per se aren't real.
Justifying other statements on the lines of "X is a social construct" similarly, where X ranges across a variety of more-or-less controversial concepts, is left as an exercise to the reader.
After all, #0000ff is #0000ff no matter what you call it or through whose eyes it reflect photons. ;)
Blue is a social construct because what "blue" means is taught to us by the culture around us, and different cultures classify colors differently[3,4]. If one culture considers blue and green to be the same color, and another considers them distinct, that's not merely a disagreement over taxonomy, but concept.
It's a cool thought experiment, but also very cool to think about how much we lucked out not just to be on Earth, but to be on Earth at this point in time. Emerge a bit earlier and suddenly industrialization becomes exponentially harder.
Who's this "our"? Is the author implicitly excluding Africans from her audience (or whoever she's talking about), or do I have the timeline wrong and the aforementioned interbreeding ancestors went back to Africa, and then the "Out of Africa" expansion happened?
But nevertheless, I become 2 years old every time I encounter comments like this. Thanks for the good laugh.