>>40936102 (OP)
There are a lot of signs that our ancestors started to adapt to water. I do believe that at some point some primitive humans just split from us and adapted to sea life, becoming mermaids, just like some of the ancestors of hippos split and became dolphins. It certainly happened during this period all old texts talk about, when the whole planet was under water.
Obviously mermaids don't turn to mermaids as they age like on your picture though, they are born mermaids.
Maybe these creatures we call greys and mistake for extraterrestrials are these aquatic cousins of ours.