Expert: Dana Krempels, Ph.D. - 10/19/2010

Question
I know about human inability to handle zero-gravity. But is it fesible to expect humans to gradually  and naturally adapt, over generations, to lower gravity such as found on Mars or higher gravities such as twice Earth-normal?  I have  been reading an SF novel(Deathworld by Harry Harrison) which describes a society of humans fully adapted to a 2G planet, having ages ago colonised it from a 1.5G planet, and was wondering if it was even possible for humans(or other animals) to give birth at lower or higher gravities than Earth-normal, as opposed to zero-G conditions.
Thanks,
Geoff

Answer
Dear Geoff,

Whether individual humans could adapt to this change in gravity over their individual lifetimes is still being researched.  There are known adverse effects to living in *low* gravity, such as loss of bone density and even increased risk of kidney stones.  You can get an overview here:

http://science.nasa.gov/science-news/science-at-nasa/2001/ast02aug_1/

Effects of higher gravity might mean that only those individuals with the capacity to live and reproduce at 2G would be the progenitors of the next generation.  If the humans were hand-selected to populate the new 2G world, and this was done on the basis of physical testing for the ability to survive and reproduce on such a planet, then you could consider the outset "interview process" a form of artificial selection.

Once the founding population got there, only those best suited to survive and reproduce at 2G would be leaving offspring.  So if any were so able, their offspring would also be likely to inherit the genetic traits that gave their parents that capability.  Over generations, the entire population could--hypothetically-shift to individuals who could live comfortably at 2G.  This is known as microevolution:  genetic change in a population without reproductive isolation from the original population (i.e., if they came back home, they should still be able to have kids with the original human population members.  If they can't, then they have evolved into a new species of humans.)

There are limits to an individual human's ability to adapt to such changes, but since we don't know all the problems of high gravity with respect to human health, it's possible that some select humans could at least *survive and reproduce* on such a planet, even if at a lower rate than on earth.

In evolutionary biology, it's important to remember that individuals adapt (to the limits of their inherited ability), and only populations evolve.  

Hope that helps.

Dana