If you don't understand branching, you don't understand evolution. If you do understand branching, then questions like this are trivial to answer.
Evolution is NOT a linear chain, with one species replacing its ancestor completely (if it was, that would be a really STUPID theory, as there would logically only be one species today!). Instead, evolution is a constantly branching bush.
What causes a species to split into two branches? If a subpopulation is isolated from the rest of the species for enough generations, that subpopulation will evenutally lose the ability to interbreed with the parent species. The one species is now two species. (This process ... 'speciation' ... can be demonstrated quite easily in a simple lab experiment.) Once split like this, the two branches can never again interbreed, never again exchange genetic material (such as mutations). As long as neither goes extinct, both branches will continue to get more and more different over time.
With this in mind, the image of monkeys->apes->humans is the CARTOON version of evolution promoted by people who are anti-evolution ... using the tactic of presenting a ridiculous (and incorrect) version of the theory just so they can then call it "ridiculous." (This is what we call a "straw man" tactic ... if you can't refute your opponents position, then describe it incorrectly so that you have something you can refute.)
Humans did not evolve from what we now call monkeys and apes (the species we now see). Instead monkeys and apes are separate branches ... and what are now humans, chimps, bonobos, gorillas, orangutans, gibbons, etc. are all many branches from that ape branch. Monkeys and apes are not some sort of intermediate stage on our branch of evolution ... they are themselves the endpoints of their own branches. Each branch shares a common ancestor with the other branches ... but once the split has occurred, they (all existing monkey and ape species) have continued evolving just as we have.
So (for example), a chimp is not some sort of "unevolved human" ... it is a fully evolved chimp.
Branching, branching, branching. That's your answer. "
theymos answered Thursday September 6 2007, 5:41 pm: That is a common misconception, Humans *did not* evolve from apes. Apes share the most genetic code with us, which means that they also evolved from the same thing we did, and turned out to be the most similar to us. Whatever we evolved from is no longer around, it evolved into different kinds of apes and us, and by the very nature of evolution, those that didn't evolve died. [ theymos's advice column | Ask theymos A Question ]
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