Looks like someone was napping too much in their science class
Seriously, what kind of question is that? Your great grand father is not a fish. But if you go back millions of generations, all of our ancestors were not humans.
Cmon dude... Adam is made from Dust... God transformed the Silicon to Carbon based compounds and given it life. Than realized it was too much work to break down Silicon atoms to carbon.. so for next creation he ruptured Adam's rib, took it out of his chest and used it like a seed to create Eve...
They both had same DNA.... next they had unprotected sex to create offspring who inturn performed incest to populate the world.
Than the world got corrupt (including babies and infants), except for one 500 year old man and his family (Noah)... God wanted to do a "alt Cltl Del".. but he did not want to create creatures by nuclear fission (Silicon to carbon transformation) , so he ordered Noah to build the arc and shove the millions of species inside it. Everyone else died because of flood (since that was the most convinient way for God to kill everyone).. and incest followed by more incest.. and now u and me are here.
Everything makes perfect sense.