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This question already has an answer here:

Humans (homo sapiens) have been on the earth for thousands of generations, and we have kept evolving throughout that time. Why don't we just keep evolving so that, let's say, we live for an average of 300 years? Or why don't we evolve so that we don't need to sleep?

This could also apply to many other species.

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marked as duplicate by Chris, L.B., Corvus, AliceD, fileunderwater May 11 '15 at 8:42

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ This question also has an element of "why isn't evolution all-powerful?". The Understanding Evolution project at UC Berkeley addresses this nicely. $\endgroup$ – Corvus May 11 '15 at 15:41
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Welcome to Biology.SE!

There are a many misunderstanding of what evolutionary processes are in your post and it is therefore quite hard to answer (hence the current two close votes I guess). You might want to read some introductory textbook on evolutionary biology or other online resources such as the "understanding evolution" project.

The most obvious misunderstanding is that evolution is NOT some kind of forever "improvement" process in a static environment. Evolution is something else! But I can't make a good overview of evolutionary processes in just one post.

[why don't we live on] average of 300 years?

You may want to read about this post concerning the evolution of aging but it would probably be slightly too advanced but you would probably still get something out of the answers.

Why don't we just keep evolving [..]?

Humans, definitely keep evolving. Nothing have stopped. We evolved as we have always and it won't stop until we get extinct! This post may for example help you to understand that evolution keep occurring today in humans.

When you say Humans (homo sapiens) have been on the earth for thousands of generations it sounds a bit like human have came to existence some time ago and have evolved since. It is a terribly misleading point of view, one of the reason is that one cannot date the existence of the first humans without making a very arbitrary decision about what a human is. A more correct way of looking at life on earth is that everything have come to existence about 3.5 billions years ago and everything have kept evolving since. Any single living thing on earth have been evolving for the exact same amount of time. Humans have evolved for 3.5 billions (even if we don't call earlier forms as humans) and unicellular algea have evolved for 3.5 billions years (even if we don't call earlier forms as algea).

Hope that helped!

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Natural selection selects for individuals who can leave the most progeny in the next generation. Traits such as longer lifespan do not typically affect fecundity/fertility. So males and females who have the largest families as quickly as possible are providing the raw material for natural selection.

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    $\begingroup$ "So males and females who have the largest families as quickly as possible are providing the raw material for natural selection" is a bit misleading. The raw material for natural selection to occur is genetic variance. The males and females having the greatest number of offspring are those contributing the most to the future generations would be more correct. "Traits such as longer lifespan do not typically affect fecundity/fertility" is a bit wrong. Typically traits such as longer lifespan affect fecundity. $\endgroup$ – Remi.b May 11 '15 at 5:03
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    $\begingroup$ For a better understanding of the evolution of aging, have a look at this post $\endgroup$ – Remi.b May 11 '15 at 5:03
  • $\begingroup$ Thank you for correcting these inaccuracies. While I am familiar with classical arguments regarding post-reproductive fitness, for example in social species who may teach their progeny things that aren't hard-wired, I was not aware that post-reproductive life-span was positively correlated with brood-size. Hmmm, maybe I am not using fecund correctly. $\endgroup$ – mdperry May 11 '15 at 11:17

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