Human beings and nonhuman apes inhabit neighboring branches on the tree of life, but there are differences. Humans have less hair and perpetually enlarged mammary glands, and unlike other apes, we sweat a lot. In 1960, to explain these differences, marine biologist Sir Alister Hardy posited a water-dependent species that preceded human beings. Hardy pointed out that only aquatic mammals like walruses and hippopotamuses have naked skin and subcutaneous fat—human traits not shared by other apes.
Hardy’s “aquatic apes” were not giant beasts living like Aquaman; rather, the species that eventually became Homo sapiens waded in and out of water and learned to swim and dive. This exposure to water, according to the theory, led to the development of human traits like walking upright.
A version of this hypothesis has been around since Greeks theorized that all living things came from the sea, but it gained the most popularity in 1972, when Elaine Morgan, an award-winning Welsh television writer, advocated for it in her book The Descent of Woman. She continued to champion the hypothesis in later books, including The Aquatic Ape.
But the aquatic ape hypothesis never got much support from the scientific community. Eric Delson, a paleoanthropologist at Lehman College in New York, says the hypothesis is more an exercise in comparative anatomy than a theory supported by data.