A better question is: "Why are humans one of the LEAST diverse species on the planet?" The diversity of a species can be charted, and I would like to see a chart plotting that diversity. Humans will be on the edge of that chart, but I assume there will be close 2nd. Then the question will be, why is there only ONE species X?
The genetic difference between two of the most distantly related humans is actually less than that of a typical ape family. This leads geneticists to estimate that our species went through a bottleneck about 70,000 years ago when there must have been less than 20,000 humans alive.
Furthermore, for specialization to occur usually requires isolation and a whole lot of time for mutation and differential selection to occur. The human race is branching off into different species of human to this day, just give it another 50 or 80 thousand years and there will be multiple species of human.
Humans dominated this planet inside 10 thousand years, in evolutionary time frames that is a blink of an eye.
"just give it another 50 or 80 thousand years and there will be multiple species of human."
That isn't as clear as you might think. At the moment, we've returned to the entire human species living in one gene pool, one that spans the entire globe. We'd have to lose the ability to cross the world in timeframes on the scale of a human lifetime, and even if you postulate a total civilization collapse and a total loss of all knowledge, we know that civilization could return to an Age of Exploration-level of capability in a mere few dozen generations even under the absolute worst case scenario that still has humans left to talk about at all. Speciation under such circumstances is certainly not automatic.
However, talking about human speciation without accounting for the extraordinarily high chance that it will end up being driven by human intelligence is probably a waste anyway. Again, on the tens-of-thousands-of-years time frame, even if our civilization completely and utterly collapses, another one could arise on that time frame quite comfortably. (Civilization #2 faces some resource issues, but with enough time they can still be overcome.)
You can look at history as well. Australian aboriginal homo sapiens were isolated for 40,000-50,000 years or so without any speciation.
You'd need an isolation event and a VERY long period of time; we have much more control over our environment than the aboriginals, which would probably serve to minimize differences in selection pressures between isolated populations to a far greater degree. 50,000-80,000 years doesn't come close to long enough, IMO.
If brown dwarfs and giant wandering planets are as prevalent as we think they are, then there's almost always a port of call for humanity only 2 LY away for large swathes of the galaxy. If we can figure out how to create "seeds" of our civilization that are reasonably compact, we can send them across that distance in only a couple of decades using laser sails and other propulsion schemes not limited by the rocket equation.
I find the theory of Neanderthal predation (NP theory) of homo sapiens, set forth by Danny Vendramini[1], to be persuasive. It explains the genetic bottleneck in an elegant and intuitive fashion, along with a host of other related anthropological questions. In fact, it kind of opened my eyes to how much of human behavior and morphology deserves an explanation and could possibly be explained by a central selection pressure.
One epiphany that played a role in confirming my fondness for the theory surrounds the so-called Neanderthal Late Flowering. I'd never heard of this period before reading Vendramini's book, but while reading, based on NP theory, I wondered to myself why such a thing had not happened, as it would seemed to have been predicted by the theory. Theories need to have predictive value to test them of course. As it turned out, the last few chapters discussed a Late Flowering, and I was a bit blown away.
Also, within a few months of reading the book, a genetic analysis study came out which showed we have a small percentage of Neanderthal DNA (but, importantly, no female DNA) another crucial prediction of NP theory.
Anyway, it was a fun read by a talented amateur anthropologist. I recommend it.
you must be kidding. Check biology basics about predation.
The NP smells like Hollywood - big fury half-human looking monsters preying upon us and raping our fine women. Until a few of us left. And we gathered our strength and pushed back and revenged.
It's hard for me to believe that bottleneck's recentness makes NO difference in the likelihood of the current observed amount of diversity (which is what "entirely contingent" means to me). I can buy "mostly", of course.
It makes me wonder if our relatively little diversity is related to the uncanny valley and our ancestors eliminated any variation that wasn't enough like themselves.
The human response to diversity seems to be "KILL KILL KILL" unless mental effort is employed to override this natural response. I've wondered for a long time if there are no other hominids, and so little diversity in our own species, because we killed them all. Genocide seems to be in our natural emotional structure.
But I wonder if this could be related back to the bottleneck. During a bottleneck, inbreeding-related genetic disorders become an issue. Maybe evolution selects for genetic "purging" behaviors during such bottlenecks as a means of ensuring fitness... basically during the bottleneck you'd become Nazis to escape the effects of inbreeding, and these impulses might stay afterwords.
So your theory is that, to avoid inbreeding, we developed a "KILL KILL KILL" response to differences and diversity. Somehow, I don't think that that makes sense, unless you're thinking of it as a way to get societies to reject people born as a result of inbreeding. Neanderthal predation is a much better explanation, at least in my opinion: a intrinsic fear of things that look very like us but different because, not so long ago, things very similar to us were trying to kill and rape[1] us.
[1] Ellyagg mentions a study that showed that humans have some male neanderthal DNA, but not female neanderthal DNA. This, to me, suggests rape. http://news.ycombinator.com/item?id=2688389
Another point is that humans change environment to adapt, while other species must change themselves to survive.
Hence we are not diverse genetically, but our technology evolves rapidly.
The genetic difference between two of the most distantly related humans is actually less than that of a typical ape family. This leads geneticists to estimate that our species went through a bottleneck about 70,000 years ago when there must have been less than 20,000 humans alive.
Furthermore, for specialization to occur usually requires isolation and a whole lot of time for mutation and differential selection to occur. The human race is branching off into different species of human to this day, just give it another 50 or 80 thousand years and there will be multiple species of human.
Humans dominated this planet inside 10 thousand years, in evolutionary time frames that is a blink of an eye.