Originally Posted by Cheehwawa
Do you think that, over thousands of years, humans who live in the mouth of an ocean and spend all of their time in the water catching food and such could develop "gills" AND retain a minimal variant of a modern lung to breathe above water, too?
Not over thousands of years - the number of genetic mutations required would be enormous (unless you go by some other means of "evolution"). However there are characteristics that humans already have, that could develop because those that have them best would be more successful at breeding. What you need is high lung capacity, a strong Mammalian diving reflex - Wikipedia, the free encyclopedia
, and various other physical adaptations that the most successful freedivers have, such as larger spleen, ability to relax and slow heartbeat, that would enable them to spend longer periods underwater. You've also got to think about how they would see - presently human vision underwater is too poor to hunt unless you have an air space in front of the eyes. It takes 200 diopters of correction to enable human eyes to see underwater.
Edit: if you look here, http://en.wikipedia.org/wiki/Aquatic_ape
, there is a theory that our human ancestors did once live in the kind of environment you describe.