Complex sentence: Some humans have developed genetic mutations allowing them to deal with the aquatic environment; they have webbed feet, which enable them to swim at high speeds, and gills located behind the ears, allowing them to breathe underwater.
Split into simpler sentences: Some humans have developed genetic mutations which allow them to deal with the aquatic environment. They have webbed feet, which enable them to swim at high speeds, and gills located behind the ears, allowing them to breathe underwater.