The Aquatic Ape Theory states that approximately 6-7 million years ago, our ape ancestors were trapped in a semi-aquatic environment. This forced them to search for food in the water and become very comfortable in the water; over time altering their bodies to be more effective in the water. The theory says that a founder population of apes got isolated on an island-like setting when East Africa flooded. So if a population got isolated, they would need to take to the sea for survival. It took whales 50-60 million years to go from fully terrestrial animals to fully adapted aquatic animals. Skeptics of Aquatic Ape Theory insist that it would take 50-60 million years to see changes in physiology of a transition to becoming aquatic. The one thing that they do not understand is that AAT states that a small population of apes became isolated. Evolution acts much faster on small populations, so some changes could have easily taken place in a million years. That is not to say that we became mermaids with a whale-like tail, just that we started the transition to becoming aquatic. Some of those traits can still be seen today.
There are many facts that make this theory very plausible. We know that 6-7 million years ago East Africa flooded, creating many islands. This flooding lasted for around a million years and is the same part of Africa where famous fossils like “Lucy” have been found. We know that mutations in small groups of populations can greatly speed up the evolutionary process. This means that if a population got split off from the founding population it could evolve more rapidly. We know that as a result of becoming aquatic, terrestrial animals generally will lose their hair, become more streamlined, and retain blubber. Aquatic mammals all have blubber to help insulate and create buoyancy. For instance, whales were once terrestrial, but after millions of years they lost their legs became more streamlined, lost their hair and replaced it with blubber. Humans are the only terrestrial mammals that can consciously hold their breath. Not even chimps or other primates can do this. Having blubber or fat under your skin makes you more buoyant in water. These are some of the facts that AAT is based on. 
- http://aquaticapehypothesis.com/aat.htm [↩]