Correct me if I'm wrong on any of this...

Slight mutations/copy errors are made every time we reproduce and all of us modern humans alive today are still of the same species because we are interbreeding as a whole and pretty much pulling from the same gene pool. BUT, I imagine that gene pool we are pulling from today would be quite different from the gene pool of modern humans a very long time ago. So with that said, here is my question...

Could we, humans born in the past century, interbreed with earlier "modern humans"? This is kind of a "what if" scenario since early homo sapiens are no longer around. But if we could time travel like in our favorite sci-fi movies how far back in history would we have to go before we come across homo sapiens that we could not interbreed with? And if there is a point would we technically no longer be of the species homo sapiens? Or are all homo sapiens throughout history (even "Homo sapiens neanderthalensis") still genetically similar enough to theoretically interbreed?

Joseph,

It is likely (in my less than considered opinion!) that the 'junk' in the gene pool for all humans alive today would not prevent us producing viable progeny with our earliest Homo sapiens ancestors (who probably 'arrived on the scene' about 180 - 200kbp). We term them 'anotomically modern humans' and the genetic disparity between the 'oldest' populations of humans in existence (variety within peoples in Africa) and those of 'newer' populations (say Terre del Fuegans) is, in terms relative to other closely related species, very, very small. Thus, I would say that we would be able to breed with our distant H. sapiens 'ancestors'.

Neanderthals are generally classed as not being a subspecies of H. sapiens and are classed as Homo neanderthalensis. This being as it may, there does appear to be some evidence that there was an amount of interbreeding between AMH (Cro-Magnon Man, at this time) and Neanderthals in Portugal / Southern Spain around 38-35kbp. Whether AMH would have been able to successfully breed with early Neanderthals is not known, as they did not have the chance. This is one of the major problems with defining species in terms of being 'biologically compatible' replicating units i.e. if one took another species concept and applied it to this problem, you may well end up with a completely different answer. Have a look at the species concept essay if you don't know about species concepts. This same reasoning works for moving back in time to other hominin species...

Short answer, almost certianly yes with H. sapiens  back to about 200kbp and probable with H. neanderthalensis up to about 40kbp, for before then, with 'other' species, you're just into very murky speculation.

EDIT: Darn, I spent too long writing and someone beat me to it! Oh well, here's my reply anyway:

I suspect you would have to go quite a long way back to find other humans with which you could not interbreed...evidence had shown at least limited interbreeding between humans and neanderthals (nobody seems to have quite decided yet whether or not H. neanderthalensis should be a species or subspecies of H. sapiens yet - both are currently in usage), and we are separated evolutionarily by around 300,000 years. I think we probably would be able to interbreed with the earliest H. sapiens of around 200,000 years ago.

It is therefore possible that we could also have interbred with some of the other archaic Homo species (e.g. heidelbergensis and rhodesiensis) that left Africa before us, as they were closely related to H. neanderthalensis, and the amount of time separating us was not too much greater in some cases (although remains of the debated H. antecessor found in England date from between 700,000 and 950,000 years ago, which is considerably longer).

Whether or not we could interbreed with earlier (and possibly ancestral) species such as H. erectus or H. ergaster is much more debatable, because of the greater time (and space) separation - H. ergaster went extinct around 1.4 million years ago, and H. erectus left Africa almost 2 million years ago. However, evidence suggests that a relict population of H. erectus was still hanging around in Asia until about 50,000 years ago. There is as yet no evidence of gene exchange between H. erectus and H. sapiens, and the closeness of our relationship to them is open to debate - it is more likely that the African H. ergaster was a closer relative (and possibly ancestor) to us than the Asian H. erectus.

That said, other species that have much longer separations can still interbreed - most species of Canis can still interbreed to produce fertile offspring, and they split some 3 to 4 million years ago. The same is true for some other groups. Whether or not reproduction is still possible is all dependent on the arrangement of the chromosomes and whether it is conserved throughout evolution. To some extent...hybridisation (although usually infertile) is even possible between some species with different numbers of chromosomes (e.g. mules, which have 63 chromosomes, are the sterile offspring of a horse (which has 64 chromosomes) and a donkey (which has 62). Distance is another factor - canids have been able to remain
interfetile for so long because they have remained in physical contact
just enough over the millenia to allow some exchange of genes.

Humans have a different number of chromosomes to chimpanzees, which acts as a barrier to reproduction (in theory - apart from one Russian named Ivanov in the 1920s, nobody has made serious attempts to create a hybrid, for obvious reasons!), but as long as we had chromosomes arranged in a similar manner to our Homo relatives, with the same number of pairs, reproduction would in theory (and I stress in theory) be quite possible. However, not only were some species of Homo separated by a great deal of time, they were also separated by several continents in some cases, so the opportunities to exchange genes and maintain interfertility between species would be limited.

Thus far H. neanderthalensis is the only other human species that we have been able to successfully extract DNA from, so for now the question will have to remain very theoretical, I'm afraid!

Last edited by Rachel Jennings (19th Jul 2011 22:59:48)