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Why haven't we evolved the ability to fly? I mean, it is advantageous when you take into account how many predators and parasites exist on the land, and there may be less or no dangers from birds up there as perhaps no bird preys on humans or harms them as land-based predators do (correct me if I'm wrong here). So why did evolution not favor human flight?

Note: I want an explanation of why flight didn't evolve in us, not of why a possibly advantageous trait in general doesn't.

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  • $\begingroup$ Re: your note - it's very important that you be able to answer that part of your question from the linked duplicate. The answer is exactly the same. $\endgroup$ – Bryan Krause Apr 28 '18 at 2:10
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The answer is because it favored something else.

Probably the most important issue is the square/cubed law. As a general rule, lift from wings scales by surface area, while mass scales by volume. The bigger you get, the harder it is to fly. We don't see large flyers, and the ones that we do see (like Quetzalcoatlus) are tremendously specialized to fly, meaning they aren't specialized to do anything else. Flight always sounds cool, but it isn't free. You pay a huge price to have a bodyshape that is capable of flying. Bones are less dense, limbs are less useful, etc.

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I think there are several points to be considered.

First, as @Cort Ammon explains, the square/cube law puts fairly strict weight limits on powered flying creatures. The heaviest flying birds today weigh about 40 lbs/18 kg, and have evolved many specialized adaptations to allow flight, such as feathers & hollow bones. Most larger flying birds actually spend most of their time soaring, rather than using powered flight. These would most likely have evolved from smaller birds.

If we look at bats, which are at least fellow mammals, the weight limits seem to be even stricter: the largest bat weighs a mere 2.6 lb/1.2 kg: https://en.wikipedia.org/wiki/Giant_golden-crowned_flying_fox

So a flying "human" wouldn't be very human-like.

The second factor is time. Flying birds have evolved for about 150 million years, bats for at least 50 million. Depending on just where you draw the line, humans have been around for maybe 2 million. This does not seem like enough time for a creature like H. habilis to develop the adaptations necessary for flight. Even if we push the evolutionary branch point back to early primates, what we'd be likely to get would be more like a cross between a lemur or tarsier and a bat than anything recognizably human.

Finally, there is the matter of evolutionary pressures. Once the primate ancestors of humans came down from the trees, they faced a set of evolutionary pressures that were incompatible with the evolution of flight. Evolution goes by small steps, not great jumps, and once you have evolved into a pre-human form, you run up against the "can't get there from here" problem. That is, there's no sequence of small steps, each of which confers some evolutionary advantage, that changes a pre-human into a flying creature.

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