How can people's bodies transform because of a climate?

i was watching this show which i NEVER got to see the last episode of called "face-off" which was recently or not too long ago on the channel syfy, and one time the group of makeup aritists had to make their own futuristic people designs and one of them, had people with gills and fins, because he said one day we might be flooded or be killed by a tsuanmi and have to restart the population or something, and that wed all "adapt and grow gills." Now ive heard before in science that cavemen have evolved from monkeys(you know the homosapiens), and as climate got hotter, they shedded fur, and features became less noticable like high cheekbones like monkeys, and the face and all, and i wanted to know really at the science of it, how can people or anybody evolve or even transform just from a little or big change in a climate??? it just doesnt seem possible to me...haha and im 16 yrs old btw so im jsu wondering bt still this interestes me alot jus thinking about it...so any smart people who know answers plz answer my question and feel free to give me websites or articles with good information if u want ok thanks and god bless u all....