Expert: Dana Krempels, Ph.D. - 1/2/2011

Question
Hi,
I was rereading the SF "Deathworld" series by Harry Harrison. I was wondering just how much stronger a human fully evolutionarily adapted to a 2G environment after generations would be by comparison to a 1G-standard Earthman. Also, how many generations such an adaptation would need in order to be implemented fully. Any suggestions welcome.

Thanks,
Geoff

Answer
Dear Geoff,

I think there are too many variables here to make a confident prediction about exactly *how much* stronger a 2G human would be than a 1G human, especially if one specifies that the difference has been evolutionarily attained.  This is different from an individual human *adapting* to 2G gravity over his/her lifespan:  the increase in strength would be limited by the human's own genetic makeup and the plasticity of muscle and skeletal mass changes it would allow.  It would also depend on the age at which the person was introduced to 2G gravity, since a child would probably still have the developmental plasticity to make permanent changes in skeletal and muscle mass more appropriate to a 2G world than a fully developed adult.

There is also no way to know for sure how many generations it would take for a founding population of humans introduced to a 2G environment to "fully adapt", evolutionarily, to such change.  It might be only a few generations.  It might be never.  All would depend on the genetic makeup of the founding population, and how natural selection might act on them.  Their skeletons might become less dense, their muscles more efficient, or a combination of other factors.  There are many ways that a population might end up "facing" this challenge.  But it all boils down to whoever has the most offspring.  Natural selection works thus:

1.  Organisms are capable of producing huge numbers of genetically variable offspring
2.  These offspring must compete for limited resources and/or against environmental challenges.
3.  Those best equipped to exploit resources and meet challenges will leave the most genes to the next generation, and the population will shift to the individuals bearing those adaptive genes.

So the traits that best allowed individual humans to withstand 2G and reproduce more than their "weaker" conspecifics would "win" the game of natural selection.  Gradually (or maybe not so gradually, depending on a LOT of other factors), the population would shift to have more members with those genetically-determined adaptive features.  What those might be is anyone's guess.  Higher muscle efficiency (with not too much increase in mass)?  Lower skeletal density?  Muscles that might be bigger and able to move and support a *heavier* skeleton?  Higher levels of hemoglobin to provide oxygen for those efficient muscles?  There's a huge array of possible traits that might contribute to the new population being "stronger", and so nearly impossible to predict what might actually happen.  So the SF author picks a scenario and runs with it.  :)

Then there's the issue of culture.

If the Founder human population were to change only due to the 2G gravity, and not invent any cultural/scientific ways to avoid the gravity problem, physical change might evolve more quickly than if the humans invented ways to allow the "weaker" members of the population to survive and reproduce despite the gravity challenge.  (For a parallel situation, consider that people with Type I diabetes can now survive and reproduce quite well, given the proper medication.  Before treatments were invented, they had a very low rate of survival/reproduction, and so their genes left the gene pool.  Now they stay, and our population changes accordingly.  Any other treatable genetic "difference" can be considered in the same light.  Culture can change the playing field completely.)

Remember that--as far as we know--evolution proceeds by means of natural selection and genetic drift.  While there could be some epigenetic changes (i.e., changes in the DNA "packaging" and expression from one generation to the next without actual changes in the DNA sequence, but which still can have profound effects on the physical aspects of the organism inheriting those epigenetic changes) in a single generation due to the difference in gravity, there's no way to predict that.

If you would like to learn more about natural selection, genetic drift, and epigenetics, here are some good sites I hope you'll enjoy:

http://evolution.berkeley.edu/evolibrary/article/evo_25

http://evolution.berkeley.edu/evosite/evo101/IIIDGeneticdrift.shtml

http://www.talkorigins.org/faqs/genetic-drift.html

http://www.evolutionnews.org/2009/06/epigenetic_inheritance_can_evo021341.html

I know you were probably hoping for a more definitive answer.  But when someone is writing an SF with a premise like that, then s/he is probably just making a lot of assumptions and hoping the reader will go along for the ride.

Hope that helps...at least a little.  :)

Dana