9. Why Some Mammals Moved Back Into Water
We know that marine animals moved out of the water and developed limbs to crawl the Earth. It was the sensible thing to do, as the land regions contained a large amount of untapped resources ideal for successful evolution. But why some of those animals—like the immediate ancestors of whales and seals—moved back into the water remains unknown.
For one thing, it is evolutionarily much more difficult for land animals to move into the sea than vice versa, as learning how to swim for a walking animal takes a lot more energy. It’s something that has perplexed scientists for a long time. Sea mammals developed the far more efficient method of navigating by tails instead of paddling much later in the course of their evolution, which makes one wonder: Why go through all that trouble in the first place? It remains one of the biggest mysteries of evolution facing modern science.