Mars as Home

Depending on whom you talk to, terraforming could take anywhere from 50 years to 100 million years to complete. The surface might one day look like our own Earth. It could also resemble a massive metropolis with people unable to live outside of domes or other manmade structures for hundreds of years.

The lack of gravity may cause humans on Mars to evolve differently than on Earth. Mars has one-third the gravity of Earth and we don't know how that could effect people long term because it's never been studied.

Studies on long-term weightlessness, however, have been ongoing for the past 30 years. From these we know that heavy exercise is needed to maintain health, and that changes in blood flow and heart density could be very problematic.

For this reason, it's possible that settlers on Mars might become physically unable to return to Earth. And over a long enough period of time, it may be more accurate to identify Martian humans as a different species than Earth humans.