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You have probably heard this question before and in different formats. Usually, it is used as a "proof" to disprove the theory of evolution.

Evolution

I understand that the apes we descended from are not the same apes we see today. However, I can't understand why there aren't any human-ape creatures anymore.

In the world of today, wouldn't an intermediate species (between apes and humans) "win" over both because it has the "feature" of the two.

A creature that can both climb trees and walk on two legs would be more suitable to live in forests than the current forest inhabitants. (apes and humans)

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    $\begingroup$ Every species on the planet is "transitional" - this is because there is no ultimate or final species. Species branch out from one another, sometimes species go extinct, leaving gaps between the extant branches. $\endgroup$ – rg255 Jun 22 '16 at 16:08
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    $\begingroup$ Possible duplicate of Examples of extant animals in a submature morphologically unstable evolutionary state? $\endgroup$ – C_Z_ Jun 22 '16 at 16:11
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    $\begingroup$ "In the world of today, wouldn't an intermediate species (between apes and humans) "win" over both because it has the "feature" of the two." - ever heard the expression "Jack of all trades, master of none"? An intermediate might do ok in the adaptive landscape of either species flanked by it, but it won't be able to outcompete within them $\endgroup$ – rg255 Jun 22 '16 at 16:21
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    $\begingroup$ @Mhmd I strongly recommend reading through the introductory course Understanding Evolution from the University of California, Berkeley. It will address a lot of the misconceptions you have. $\endgroup$ – MattDMo Jun 22 '16 at 16:42
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    $\begingroup$ Related (but closed): Where are the evolutionary “inbetweeners”? $\endgroup$ – Remi.b Jun 22 '16 at 23:36
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Every species on the planet is "transitional" - this is because there is no ultimate or final species. Species branch out from one another, sometimes species go extinct, leaving gaps between the extant branches. But it also comes down to how you look at it; if you were comparing blue whales and humans, then chimpanzees (and many other species) would be "transitional", so some "transitional" species are extant, others are extinct. In fact, it is estimated that 1% of species that have ever existed are extant, the remaining 99% are extinct.

It's very hard to say why the "transitional" species of chimpanzees and humans went extinct. There's a lot of (sub-)species that would fall in that bracket, thus to answer that specifically would be too broad and speculative for this site.


Some concepts and key points

  • Speciation is a gradual process with species forming over time out of one in to two (or more).

  • Species is a hard concept to pin down because it applies binary/categorical classification to a very much continuous distribution

  • Evolution is a process of change, and in all likelihood it will be an unceasing process because at least one of the mechanisms will likely be affecting every species in some way. This means that every species is "transitional" because no species represents an end point.

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This is just going to be a quick answer, as it's too long for a comment (I'll leave it to others to fill in the gaps if they wish). The image you present in your question is based on a false premise. Humans did not evolve from apes. Humans and apes evolved from a common ancestor, which was neither human nor ape. So, the arrow on the left of the image, saying There are millions of these, is incorrect. Our common ancestor is now extinct, as are all of the so-called "transitional species", another term which is incorrect because, as rg255 points out, all species on Earth are "transitional", because we are all evolving (at varying rates, to be sure). In between our last common ancestor (LCA) and modern Homo sapiens there were many species that existed for various amounts of time, then died out for various reasons, in some cases because of direct competition from more recently-evolved and better-adapted species.

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