not all that sure...
I thought they were just humans that evolved over all those years... That movie started out great i thought but the ending honestly lost me... i few people i work with played cops in the movie and it looked really good when i watched some of it being filmed where i live.. but i was so not expecting that ending.
Alright this is how the Blue Guy explains it not word for word but how I remember it. He explains when the moon was destroyed the survivers were split into two, women and children stayed uptop and Men went down. And After all of these years The ones below have de-evolved re-evolved as the perfect hunters quick strong and just enough brain cells to breath and catch their prey. Uptop it was a system of learning and passing on and combining upon evolving in the small groups or cities. There were several cities and underground structers all over the earth. To make it easier watch the first adaptation of this movie. It's titled The Time Machine (1960, starred Rod Taylor, Alan Young and a whole host of other great old time actors deffinetly on the top of my favorite movies list of all time. This version of the movie is a litle closer to the Book which mabey you should read if you have any actual questions.
I don't think all the men went below ground. From what I understood the underground species developed so differently because the cerebral race had to give up physicallity to become so strong mentally and the hunting race gave up the cerebral to increase physical strength. Either way those that survived above ground had to worry about becoming dinner for those underground, with a few select women being chosen to breed with cerebral to keep them going.