Evolution is a trip. On the one hand, it’s a seemingly simple mechanism—those best fitted to their environment have more babies, while less fit individuals don’t reproduce as much, and their genes filter out of the system. But on the other hand (or paw or claw or talon), it has given rise to an astounding array of organisms. Some animals fly with feathered wings, others with membranes stretched between fingers. Some run on two legs, others four.

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