Forgive my ignorance and don’t be brutal but; what is the possibility that the missing link (the last common ancestor of chimpanzees and human beings) was actually not another station on the evolutionary line, but rather the result of mating between a chimpanzee and another creature (call it what you like; a chimpanzee, a mutant chimpanzee, or something else)?
Zero.
Well, that explains it.
Richard Wrangham thinks so, but primatologists are generally more apt to think that primates are really people.
So you’re saying “no homo”?
First of all, scientists don’t use the term “missing link” anymore.
But the answer to your question is: Possibly, but not likely.
Keep in mind that the common ancestor of humans and chip was neither a human nor a chimp. The Homo line itself is much younger than that common ancestor, too. The earliest ancestor that we call Homo lived about 2M years ago +/-. Our common ancestor with chimp lived about 6M years ago. +/-.
We do know that our ancestor interbred with different versions of Homo rather frequently, so it’s certainly possible that our common ancestor with chimps might best be described as some group of closely related species out of which arose chimps and humans. And there is evidence that the human/chimp line split for awhile, merged back together again later, and then split again for good.
But generally speaking, we usually assume that there was some ape species living about 6M years ago whose living descendants today are only chimps and humans and nothing else.
Just what does this statement mean? First of all, if the lines later merged, doesn’t that mean that they didn’t actually completely split the first time? Second, if A splits into B and C, then B and C merge into D, and then D splits into E and F, how do we justify identifying A with E and B with F?
Split reproductively. Most likely due to a split spatially (geographic isolation of the two groups for a time).
The method used to determine when chimps and human split gave results, under this particular analysis, that were best understood by assuming there was a long (maybe a few MY) period during which the lines did not interbreed, but a time after the initial split when they interbred again. IIRC, the first split would have been about 10MYA and the second about 5MYA. But this was one study years ago, and I don’t know if it has stood the test of time or not. I only throw that out there as a possibility along the lines of what the OP is thinking. It likely did occur for some animals somewhere.
First, watch this:
Second, the big problem is that most hybridized animals are sterile. This the reason you don't see mule stallions and mule mares giving birth to baby mules. It is exceedingly rare for a mule mare to be fertile under any conditions, but a mule-mule pairing is unheard of. I suspect the same phenomenon would prevent various hominids from producing fertile hybridized offspring.Great apes have 48 chromosomes, which is (among other reasons) why modern humans and chimps can’t breed. But we don’t know when we lost the two that differentiate us from them.
You’re the First Homo!
I don’t know if most are, but the important thing is that many aren’t.
Differences in chromosome number does not make interbreeding impossible. Horses and donkeys have different chromosome numbers. We did not lose any chromosomes. Two “ape” chromosomes fused onto an existing ones-- our chromosome #2 = two chimp chromosomes, combined (roughly speaking). We do know this happened before the Neanderthal split, since Neanderthals have the same Chromosome 2 that we have.
We also don’t know for sure that modern humans and chimps can’t interbreed. In fact, there are no two species of large mammals that are more closely related than humans and chimps are that cannot interbreed, and many that are less related that can. It would not be too surprising if a human/chimp hybrid were possible.
It doesn’t make producing offspring impossible, but it makes breeding impossible, since the resulting offspring are effectively sterile. Notwithstanding the fact that our “lost” chromosomes are really fused, we can’t breed with chimps because the offspring would end up with an odd number. So you might get a chimpanhuman, but it would not be able to produce any of its own offspring.
Good point on the Neanderthal thing, though.
What are some examples, if you don’t mind? Curious, not challenging you.
IIRC, there are even some cases of crossings with different chromosome number producing fertile offspring.
Missing link is a term that should not be used:
Well, mules are not always infertile, so there’s that. But yes, sometimes the hybrid offspring are fertile, although usually not.
There are scientific papers claiming genetic evidence that proto-chimps and proto-humans did exactly what John Mace suggests: splitting but then interbreeding at a time when the offspring where barely fertile, but nevertheless interbreeding enough to leave a signature of it in the human DNA. (I’ll let those with better Google-fu track down the paper(s).)
As for breeding with an (off-by-two) chromosome-number mismatch, it’s certainly common. The first generation offspring will have an odd number of chromosomes.
Yes, I was specifically thinking of those papers. I think we even discussed here in some thread or another. I wasn’t able to track them down either, but I’m pretty sure it was within the last 10 years.
Well, I do know how to Google for posts at SDMB. 
From 4 years ago: