Why don't humans have gills?
Roughly 375 million years ago, a funny-looking fish named Tiktaalik ventured ashore using novel adaptations: lobed fins to propel itself to "walk" on land and air sacs in its throat to breathe oxygen from the air. Tiktaalik , which also had gills , is the earliest known common ancestor of tetrapods, or four-legged Animals . Over hundreds of millions of years, tetrapods evolved into countless species, including Homo sapiens . So, if humans evolved from fish, why don't we have gills? Part of the answer is practical: Gills need to stay wet in order to work, which isn't ideal for animals that don't live underwater. Gills have a large surface area and thousands of tiny blood vessels , giving oxygen easy access to the bloodstream. As water rushes over the gills, oxygen diffuses in and carbon dioxide diffuses out, said Chris Organ , an evolutionary biologist at Montana State University. If land animals had gills, they would quickly dry up, making it an inefficien...