I have a very limited knowledge of biology (whatever learned from high school and popularizations). But I don't understand how could humans evolve the ability to be able to swim and dive in water. For all I know, humans evolved only on land in Africa and the ability to swim should have been a very rare necessity. That should also hold true for our most immediate evolutionary ancestors. Even more mysterious is the fact that the human skin and outside cavities can stop water from entering the body even under a great water pressure. I could understand it should evolve to be resistant to rain, but how come most humans can dive in depths many meters deep? How could the skin and cavities evolve to stop so much water?