If I take ten thousand healthy adult humans, strap them into a colony ship, and fire them off on a three-million year journey to a distant star, what will the creatures climbing out of the colony ship look like at the end of the journey?

The colony ship is built using near-future technology, and has a full range of exercise equipment and medical equipment required for humans to survive and give birth to reasonably healthy children in a zero-g environment. The exercise section of the ship rotates, generating centrifugal force to make activities like running and swimming possible, but the majority (98%) of the ship has no form of artificial gravity.

The ship is an oblate spheroid with a 2 km long axis and a 1 km short axis. Engines and technical areas run through the middle of the vessel, with commercial and habitation layers wrapping around in cylindrical layers. The outermost layers of the ship are designated farm/nature areas, with a functioning self-regulating biosphere that keeps the atmosphere stable and the colonists fed. Major layers on the ship are fixed and cannot be altered, but the ship has been designed with smaller bulkheads that can be moved around internally according to the wishes of the occupants. The ship travels via a large array of solar sails, while internal power comes from a large array of hydrogen fusion generators, which are restocked via small transport craft that harvest fuel from gas giants and interstellar gas clouds en route.

No effort has been spared in selecting and equipping the colonists to survive their journey. The inhabitants of the ship have a wide range of expert knowledge required for running and maintaining the ship, and the initial generation has been selected to be as psychologically compatible as ten thousand people can be. They've been selected from groups of people around the world to keep the genetic pool as diverse as possible, and all individuals have been screened against genetic dispositions for disease and disability. They are not, however, obeying any sort of selective breeding schedule to guide their evolution, which will be driven by the fitness of individuals to survive in space. How will evolution shape the colonists?

  • Note: I'm not looking for answers like those in this question:worldbuilding.stackexchange.com/questions/3781/…, which deal with low (but not zero) gravity, or this question: worldbuilding.stackexchange.com/questions/23912/… which deals with designing a creature to live in zero-g. – ckersch Feb 22 '16 at 16:59
  • You may have handwaved away a critical part of the puzzle by accident. It is currently not known whether a child can be born in zero G, with most of the evidence suggesting it actually cannot be done. Given that, the effect of evolution might be at a far deeper level than one might look at first. Alternatively, evolution may be more shaped by the particular medical equipment needed to make a zero-gravity pregnancy possible than it is by the zero-gravity itself! – Cort Ammon Feb 22 '16 at 19:26

There is some speculation on this, and our friends at NASA have some things to say. Specifically Patty Currier, from 1999, talks about babies in space, and Michael from Vsauce talks pretty well about low-g or zero-g living in space, and finally NASA's article "Gravity Hurts (So Good)" are good initial sources for this.

Here some of the effects of gravity the body:

  • The head will be thicker, because more fluid pressure gets to the head, because there is no gravity to pull the fluid out of the head (and into the feet). So babies will develop thicker skulls.
  • Legs will be underdeveloped, because they don't experience the stress of holding a body up.
  • Hands will likely be just fine. They will likely be used just as they would on earth, so they will develop "normally."
  • It is estimated that 40-60% of bone loss can happen in low-gravity environments. (About 1% per earth-month!)
  • Muscle mass loss of up to 20%
  • Due to the increased pressure of fluid in the head, overpressure in the eyes can lead to various eye conditions.
  • This same pressure in the head, in rats, have lead to more plaques in the brain which lead to alzheimer-like diseases.

So what does this mean? Assuming humans can actually have babies in space, it looks like the space-adapted humans will have:

  • decently developed hands
  • less muscle and bone mass.
  • Poor eyesight
  • significantly more height than average (Fully developed humans can grow up to 3% taller just by living in space.)
  • less overall blood and body fluid, because it's easier to pump less blood around if that less blood still satisfies the needs of the body.
  • Develop a method of combating Alzheimer's disease.

Very likely, if they ever entered an area with actually gravity, it would be very painful for them until they developed the muscles needed to move about as usual. This could possibly be deadly. The good news; only their extremities would be useless, their ribcages and skulls would likely be fine.

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