Kiko asked in Science & MathematicsBiology · 1 decade ago

Why is there only one species of humans?

So, we can see new species being formed in a pretty short, recordable amount of time. Examples are seeing new bacteria species in labs, new bird species, etc. So, why only one species of human? Humans live in every habitat imaginable, with a wide range of required needs. Those humans have lived in those habitats long enough to have at least formed new species by now, so why haven't they? Why is it that after thousands of years, humans from the Sahara desert can still reproduce with humans from the arctic zones?

Update:

Ok, so I like the answer by griblin, except I could add another question to that.... Humans are supposed to have evolved in about 5 million years. Apparantly, that is only enough time to get a new species, much less see the whole changes we do from humans to other primates. Why can that change occur so quickly?

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  • 1 decade ago
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    There was a study recently preformed on E. coli, which watched them evolve into a different species:

    http://en.wikipedia.org/wiki/E._coli_long-term_evo...

    This experiment was started in 1988, so it has taken 20 years to get a defined new species in this study. Since bacteria reproduce roughly once per hour - this represents about 175,200 generations of bacteria.

    Humans reproduce (roughly) once every 20 years. So 175,200 human generations would take 3,504,000 years.

    This is a huge oversimplification, of course: humans have sexual reproduction, which provides additional mechanisms for evolution by gene recombination. And evolution rates are not fixed, but will vary from time-to-time and place-to-place due to different environmental selective pressures, etc.

    But basically, there has not been anywhere near long enough for humans to have speciated.

    (also, the ability of humans to travel across the world results in a mixing of the gene pool of H. sapiens, and therefore prevent speciation - which requires geographical isolation before reproductive isolation can occur).

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    Edit:

    > "Ok, so I like the answer by griblin, except I could add another question to that.... Humans are supposed to have evolved in about 5 million years. Apparantly, that is only enough time to get a new species, much less see the whole changes we do from humans to other primates. Why can that change occur so quickly?"

    As I mentioned - my answer is a huge oversimplification. The mechanisms of sexual reproduction and genetic recombination speed evolution up significantly (though to exactly what degree remains an open question), and we have little knowledge of the kinds of selective pressures our ancestors would hev been under (which are also required to model evolution rates).

    Basically, I was just trying to point out that humans have not really been around long enough to speciate. But, IMO, the biggie is the fact that humans can and do move around a lot - even back before we were a "global village", we still mingled our genes faster than significant divergence was taking place: this has led to the controversy between the "Out of Africa" and the "Multiregional" models of human evolution.

    That said - it is still apparent that *some* divergence was occurring (witness the different physiolgies and appearances of different races). If we had remained technologically undeveloped, it is quite possible that we would have speciated eventually, as the "gene mixing" might not have been quite quick enough to keep pace with the gene pool changes in isolated populations.

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    Edit (again):

    I've just remembered a relevant point:

    The rhesus factor (blood typing) is believed to have arisen in an isolated population in Western Europe during the last ice age.

    This mutation is *almost* a speciation event: without modern medicine, people with Rh+ and with Rh- would be (nearly) unable to interbreed, as immune complications kill babies of Rh- mothers who bear Rh+ children.

    So you can see how something like this *could* have resulted in two species of humans. We are just fortunate that it hasn't.

  • 1 decade ago

    Because we would have to call another species something else. "Human" is the name of a single species. From everything we've dug up, there used to be plenty of species that more closely resembled humans than anything else today, but all except for our own species became extinct some time ago. It's widely believed that our own species is responsible for the extinction of the others.

    "Why is it that after thousands of years, humans from the Sahara desert can still reproduce with humans from the arctic zones?"

    Because it has only been thousands of years... Even so, the differences between a person from the Sahara Desert and one from the Arctic are obvious. Just because they can still reproduce doesn't mean that the relative genetic drift hasn't been substantial, especially considering the short span of time from which such change occurred.

    It's not hard to see the obvious differences within our species (by looking at the different races), which happen to be geographically relevant...

  • Anonymous
    1 decade ago

    It sounds like you're wondering why we aren't as specialized as birds. Not all birds can mate yet all humans can. We just aren't that specialized because evolution just hasn't gone that way for us. There hasn't been a need for us to evolve that way. There are way more species of birds than there are of humans and they interact with way more creatures than humans do causing specialization. There are different species of humans. Just not as different as birds. If you came to this planet from another and studied humans, you'd see we have more than one species. We're more like dogs where any breed could mate with any breed in theory if not for the major size differences like the Chihuahua and Great Dane. We have Africans, Hispanics, Europeans, Indians, etc. So we're not just one species. If there was one species of human, we'd all be truly the exact same. There wouldn't be a such thing as an interracial relationship.

  • 1 decade ago

    We ate the other ones.

    Seriously, though: Speciation generally requires extended genetic isolation, which humans have not had. Even then, it is not guaranteed that the species will not be able to reproduce; polar bears and brown bears, for example, interbreed freely if given the chance.

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