I understand that humans cannot successfully interbreed with gorillas and chimps, because we have different numbers of chromosomes. As such, this defines us as separate species(?)
But if we all came from one species, how and why did the different number of chromosomes happen? Was it at this exact point that we became homo, rather than (ape-o?)
Also: would it not be possible for a human and a chimp to have a sterile baby, like donkeys and horses have mules and hinnies?
(Unlike the dog-birth lady, I have no inclination to try this).
Humans have 46 chromosomes (23 pairs), compared to 48 in apes. Our chromosome pair #2 appears to represent a fusion of two smaller ape chromosomes.
This phenomenon is quite frequent in evolution. Although differences in chromosome number (karyotype) can cause interspecific sterility, this is not necessarily always the case. In fact, even within the same species there can be variation in chromosome number, including shrews and boars.
We have had a number of threads on the possibility of a human/chimp hybrid (sterile or otherwise). Although this is probably unlikely, it cannot be ruled out. There has not been any definitive research on the subject, at least not that anyone will admit to, for obvious reasons.
As Colibri indicated, differing number of chromosomes are not, in all cases, a barrier to cross-species fertilization. Horses and Donkeys have different chromosome numbers. Also keep in mind that the classification system we use (Genus, species, etc.) is to a large extent an artificial construct and is not always a good guide to cross-species fertility either.
As for a humanzee (human/chimp hybrid), who knows? My own personal guess is that it could be done, but might need some help from the lab, such as IVF. There certainly have been hybrids between some species that split evolutionarily longer ago than humans and chimps did.
Perhaps someone one day will try to hybridize chimps and gorillas, as a way to indicate if a human/chimp hybrid is possible. If they could do it, chances are we could, too. But again, that’s just a guess.
Note: there is no “exact point” in evolution. Think about it. One human ancestor got shorted in the gene department. That ancestor had descendents. Where did they come from? His/her mate presumably had 48. They interbred successfully, etc. Maybe all their kids had 48 too, but some grandkids got 46, etc. No doubt interbreeding between individuals in that group continued for many generations.
Good point. It’s also unclear if the fusing of the two chromosomes was a critical or necessary event in making us human. AFAIK, we don’t know if this is a unique trait to H. sapiens or if it happened in one of the first ancestors in the human line (ie, long before the genus Homo arrived).
It sure seems like it should be significant, but we really don’t know.
Paleoanthropologist use the appearance of stone tools in the fossil record to mark the emergence of the genus Homo, sometime around 2.5M years ago. But this is by necessity a somewhat arbitrary decision. It’s hard enough to classify exant species, and much more so for extinct ones. In fact, even today you will find some scientists who feel that chimps and humans shouild be in the same genus. Probably the most famous is Jared Diamond (of Guns, Germs and Steel fame). He titled one of his books on that idea: The Third Chimpanzee.
BTW, your “ape-o” genus was most likely the genus we call Australopithicus. Still upright walkers, but very ape-like in facial appearance and cranial size.
An interesting consequence of neo-Darwinian evolutionary theory is that species classifications can be usefully made only in a temporal cross-section. That is to say, we could collect all of the creatures alive today (or, say, in the second millenium A.D.) and make meaningful distinctions between species; but if we collected all of the creatures that ever lived, there wouldn’t be a principled way to divide them up without referring to some particular late period as a ‘baseline’.