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There are two species belonging to the Pan genus (Pan troglodytes and Pan paniscus). That's just two.

I mean, humans have changed radically since we split from chimpanzees, we have Australopithecus afarensis, africanus, anamensis, Paranthropus robustus and boisei, Homo erectus, habillis and so on.

A chimpanzee fossil that is 2 millions years old should be pretty different from a modern one, why aren't they considered another species?

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    $\begingroup$ The rest you listed that are not Homo sapiens are evolutionary ancestors of humans. Not humans. Close to humans? Sure, some of them. But not humans. $\endgroup$
    – MCM
    Sep 21 '15 at 19:08
  • $\begingroup$ @MCM Yeah, but my question stills stands even if they are not called humans. $\endgroup$
    – DisplayName
    Sep 21 '15 at 19:29
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    $\begingroup$ Could you re-phrase your question? Are you asking why we don't find/identify more fossils of Pan ancestors? $\endgroup$
    – MCM
    Sep 21 '15 at 21:36
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    $\begingroup$ There really is only one species "human" (Homo sapiens). I guess you want to know why "our" branch brought up a lot of species, whereas the "chimpanzees" branch remained nearly "untouched" since the last common ancestor? I'd say this is due to an greater expansion of our ancestors across a wide variety of different terrains. $\endgroup$
    – AstronAUT
    Sep 22 '15 at 16:59
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    $\begingroup$ The first chimpanzee fossils were found in 2005 nature.com/nature/journal/v437/n7055/full/nature04008.html and date back to ~500K years ago... the have been very few fossil finds for the chimp lineage, so unlike with Lucy which gets us back to about 3.3 million years ago for humans, we have not uncovered the same artifacts for Chimpanzees. It could be that the range of the Chimpanzees was not conducive to the fossilization and preservation of bones that the human lineage was afforded. $\endgroup$
    – AMR
    Sep 26 '15 at 2:51
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For fossils the answer is simple chimps live in forests and forest are really bad at preserving fossils, we lived in savannah which include some floodplains which are really good at preserving fossils. Thus we find a lot more human fossils than chimp fossils.

Otherwise Chimps exhibit far more genetic diversity than humans. humans are just a weird branch of chimp.consider the genetic diversity below. If we were not so morphologically biased we sould easily conclude all hominids as just one species based on genetics.Chimps actually have quite a wide morphological variation its just we as humans are far better at noticing human variation. enter image description here

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  • $\begingroup$ Can you please give the reference for this graph? $\endgroup$
    – Remi.b
    Mar 9 '18 at 5:54
  • $\begingroup$ Its from this royal society publication rstb.royalsocietypublishing.org/content/366/1567/1080 a more detailed and updated version is available here ucsdnews.ucsd.edu/archive/graphics/images/… $\endgroup$
    – John
    Mar 9 '18 at 14:08
  • $\begingroup$ Cool +1 I am surprised there is really so little genetic diversity in humans despite our gigantic population size. I suppose we only very recently went through population expansion and still have the genetic diversity of a small population. But chimpanzees typically have rather small populations... $\endgroup$
    – Remi.b
    Mar 9 '18 at 16:15
  • $\begingroup$ It could also be that humans keep killing of entire bloodlines and tribes, technology makes conquest easier. This is just an idea however. $\endgroup$
    – John
    Mar 9 '18 at 21:13
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That's just two [species of chimpanzees]

C'mon!

There is a huge behavioral gap between the two species. I get your point that the human lineage had several, not just two, forms. Yet these two chimpanzee species are so brutally different that they make most of our "human diversity" seem so regularly the same.

About your question - it seems obvious to me the point brought by @Pete: That we, humans, came from the forests to the open land, which brought the seas and continents to us. Many different habitats would lead to wider variation.

A chimpanzee fossil that is 2 millions years old should be pretty different from a modern one

Why should it? The African Forests changed drastically in the last 2 million years? If not, then why should chimpanzees have changed? Take a look at the horseshoe crabs (Xiphosura, Limulidae): they've been that way for hundreds of millions of years! The punctuated equilibrium seems to be the norm, more than the exception, doesn't it? (I remember the horse evolution through the Mesozoic, apparently linear, but in a closer look there ain't some teeth in the line? Aren't there plateaus - long periods of stability - among the high precipices of sudden genetic/form/function change?

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    $\begingroup$ I think this is the first answer that looks like an answer, but it still needs to more clearly deconstruct the question. 1-The difference is greater than the species count suggests; we're missing chimp data. 2-Humans are more similar than you think, but they have had more dynamic geographical selection pressures to amplify their differences. 3-It's common for much less change to occur than what we see in both these examples. $\endgroup$
    – James
    Nov 30 '16 at 9:30
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Chimpanzees are relatively isolated and confined in so far as their environment, it's all much the same... forest. They haven't been subject to the environmental factors humans have... arctic, tropics, deserts, mountains etc. Therefore there hasn't been the need to change as much. Animals adapt to their environments, chimpanzees only have one, humans have hundreds.

If you have a look at the humans living in the same distribution area as compared to chimpanzees then you'll see that they don't have much variation either for that area.

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  • $\begingroup$ Chimps haven't been subjected to tropics and mountains? $\endgroup$
    – AliceD ♦
    Sep 22 '15 at 23:14
  • $\begingroup$ They certainly have $\endgroup$
    – Pete
    Sep 23 '15 at 2:28
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    $\begingroup$ @AliceD I think Pete is trying to say that humans cover a much more varied ecological environment due to our greater expansion. This is a good point. $\endgroup$
    – Alex
    Oct 3 '15 at 15:03
  • $\begingroup$ Chimpanzees have only been subjected to a specific type of mountain and tropical environment. Humans have inhabited every type of mountain range and tropical environment there is... Hence our greater adaptive variations. $\endgroup$
    – Pete
    Oct 5 '15 at 16:12
  • $\begingroup$ @Alex This is a good comment that Pete has made, but it's not an answer. $\endgroup$
    – James
    Nov 30 '16 at 9:24

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