If we go back far enough, there is some point at which our generation could no longer reproduce successfully with our ancestors. I guess I’m not interested in knowing if I could score successfully with my Great*15 grandmother so much as knowing where and how the ability breaks. Might ancestor A and B be able to reproduce and B and C able to reproduce, but not A and C? Is there a something very specific and identifiable about the genes of those who are successfully able to reproduce?
Well, all the way back to the start of our species, because if we can’t breed with it it’s not of our species. (by definition.) 
Okay, this isn’t strictly true, there’s the issue of whether the offspring are fertile. (e.g. horses and donkeys can reproduce with one another but produce infertile offspring, and are therefore different species.)
More seriously, IANABiologist, but I doubt it could be shown exactly other than, perhaps, experimentally. (I don’t think this is currently possible, but it’ll probably be possible in a few years via variants of cloning; I suspect this wouldn’t be a good idea, however) I don’t think there’s any sort of obvious way of going directly a genetic code to fertility. I do expect that there’d be no problem at least 30k years back, though.
There are existing species which have distinct groups A B and C in which A&B can breed, B&C can breed, but A&C cannot. (Ring species, such as some arctic birds, in which the habitat of B is between that of A and C, and B breeds with A or C only rarely.) They’re still considered one species, I believe, because exchange of genetic material occurs between A and C (via B).
First of all, remember that in many cases the ability to interbreed, or to produce fertile offspring, is not all-or-none. When individuals are genetically differt, various things may happen: there may be reduced fertility, although some young are produced; or fertility may be OK, but the hybrids are sterile; or other variations. Therefore it might not be possible to define a particular generation when an ancestral form became incompatible with its remote descendants.
However, one pretty major break between humans and chimps is that we have different numbers of chromosomes. This is the kind of difference that is likely to produce intersterility.
This came up in a question a few months ago on whether or not humans and chimps could interbreed. I am not going to do a search right now because it’s the middle of the afternoon, but will try to find the thread later.
Also remember that there are two ways that species can be isolated. Everyone so far has talked about post-zygotic isolating mechanisms, meaning that if breeding happens there will be no baby, or it will be sterile, or have reduced fitness.
But often there are pre-zygotic isolating mechanisms too. Members of two species could interbreed and produce fertile offspring, but never actually do. This could be because they are geographically isolated, or they have different mating seasons, or have different mating cues, or are chronologically isolated.
Meaning that even if humans and chimps could interbreed and produce fertile offspring, it is very unlikely that this will actually take place, since chimps and humans have very different mating signals. For instance, human females would be unattractive to male chimps since they wouldn’t smeel right or have giant swellings of their buttocks during estrus. And female chimps are hairy, smell bad, and don’t know how to kiss for human males.
Although we propably will never be able to tell for sure, it seems to me very likely that modern humans could succesfully interbreed with any fossil called “anatomically modern Homo sapiens”. The lack of outlier mitochondrial DNA suggests but does not prove that matings between amHs and Neandertal man almost never produced succesful offspring.
Anyway, that would put the date at the oldest succesful potential mate at around 100,000 years ago. Probably earlier matings would be succesful, but people older than that wouldn’t look or smell attractive, there would be something about them that would make modern humans reluctant to attempt matings with them
I hope you meant “… modern humans could succesfully interbreed with any individual called ‘anatomically modern Homo sapiens’”. It’s rather hard to interbreed with a fossil.
I am a male human and I can say that I prefer my females without giant swellings of their buttocks whether in estrus or not. And… females being hairy, smelling bad, and not knowing how to kiss has never stopped me before (well, at least in college)
Originally Posted by Kid Charlemagne
Weeelllll, in your case, all the way back to last Thursday 
BWAHAHAHAHAHA
Sorry, Kid, I’m in a mood.
Cartooniverse
Oooh, I’ll post that link for Colibri: Can Humans and Chimps Breed? Some of the responses in that thread were really superior (and the OP wasn’t so bad itself).
I’ve heard that there’s somewhat of a theory that homo sapiens interbred with neanderthals sometimes; that they didn’t go totally extinct (you can sort of see some very cave-man like characteristics on some people). I don’t knowhow far back neanderthals go though.
Like the heavy brow we see on the bus driver on Speed (among millions of other people, the bus driver just being a known person with the brow)? Would that be considered an early homo sapiens characteristic?
–Tim