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Let's use a Mars-like as an example of a planet that interstellar travelers may settle on. Assume that the spacecraft will provide Earth gravity and atmosphere.

Comparing this new planet to Earth, it has .5g (1/2 the gravity) and has .5bars of atmosphere, or 1/2 the pressure of Earth. Everything else is the same. Water, light, heat, techtonics, length of day and gases in the atmosphere.

In building this world, my travelers build a settlement, and begin having children.

My questions may be basic, but in this first generation that is born on this planet:

1) Will they grow taller? And by how much? 2) Will their lungs develop the capacity to breath unaided? 3) Will their muscles and bones develop naturally for the new world conditions, or will genetics get in the way? If so, how and what?

My last question is we know humans in zero gravity or even on Mars will lose bone and muscle, heart weakens. We suspect if humans born on Earth were to find such a planet, they will need to be aided in breathing like mountain climbers.

Is it realistic to believe that human muscle/bone loss will stop when it reaches what it needs to function at .5g and .5bars of atmosphere, or will our genetic code continue the signal of bone/muscle loss?

It may be a lot to think about, but the scenario I am looking at is what life will be like for the first generation born on a new world that is different from Earth.

And if anyone has ideas on whether the second generation will have it easier, I'd like to hear about that too.

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    $\begingroup$ Welcome to the site, George. I have to ask: are you a time-traveler Hollywood producer? Because this feels suspiciously like the premise of a movie that came out last week (Feb 3)... $\endgroup$ – Frostfyre Feb 9 '17 at 21:41
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    $\begingroup$ This reminds of conversations about how many generations it takes to breed a wolf into a chihuahua. $\endgroup$ – cobaltduck Feb 9 '17 at 21:50
  • $\begingroup$ @Frostfyre Neither, nor do I follow movies that much. I looked at that link and do not recognize the movie, but the Martian would have a harder time on Earth. If he weighed 100 lbs on Mars, he'd be 300+ on Earth, or that is what it would feel like to him. Mine is actually a thought experiment I came up with. I'm not only a computer scientist, but a cosmologist as well. $\endgroup$ – George McGinn Feb 9 '17 at 22:43
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  1. They will grow slightly taller.

Genes won't allow any huge differences - our spines are still stuck at a relatively set height - but less compression will allow a few inches of growth, according to this source.

  1. Their lungs will not catch up.

Such low gravity and pressure are survivable, as clarified by the comments, but pressure suits and masks may be more comfortable. People can acclimate to survive - their bodies can respond and stay that way - but there is no reason for our genes to change.

Evolution is driven, along with other things, by natural selection. What can survive will, what isn't fit to survive won't, and over time, whole species change to be better suited to the environment. However, if humans are at the point where they can colonize other planets, those "unfit" to survive will survive anyways.

Nothing prevents those unable to breathe as easily from being able to reproduce. If an individual is not able to acclimate, they can get a pressure suit and a mask - and reproduce just like everyone else. Therefore, there is no evolutionary pressure, or incentive, to adapt. It's not impossible for it to occur with mutations, but when there's no reason to adapt, don't expect adaptation.

  1. Their muscles and bones can adjust.

People on your world will definitely appear smaller - they don't need heavier bones or stronger muscles, so they will not develop them. This probably isn't a problem if they don't try to go back home, but in the event that they must, excercise and nutrition can help rebuild what has been "lost".

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    $\begingroup$ Even just a year in space on a grown human can make them taller: cnbc.com/2016/03/02/… I do agree with what you said as that was going to be my answer mostly but ya, beat me to it :P I think the organs will need a few generations to adjust while the body structure will adapt quickly. $\endgroup$ – ggiaquin16 Feb 9 '17 at 22:01
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    $\begingroup$ Lungs will catch up, maybe not all the way up. 0.5 bar is not a deadly low pressure, it's approximately like 5500m altitude on Earth. Breathing is problematic, but possible for extended period of time. $\endgroup$ – Alexander Feb 9 '17 at 22:38
  • $\begingroup$ @notstoreboughtdirt not so immediately as 2 to 3 generations. I suppose I use the wrong term with "few" but the point still stands. No need to be nit picky for the sake of creating argument. $\endgroup$ – ggiaquin16 Feb 9 '17 at 22:38
  • $\begingroup$ @zxyrra That applies to those who land. So you're saying a child born in that condition will not develop larger lungs or another method of greater Oxygen intake? Sorry that was a question, not just a comment, but in otherwords, people born and raised in Denver, Colorado have a hard time breathing when they get to sea level here in Florida, because the atmosphere is thicker. $\endgroup$ – George McGinn Feb 9 '17 at 22:49
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    $\begingroup$ @Zxyrra, I can imagine that in a few generations humans can successfully adapt to living at 0.5 bars. Present day highlanders living in Himalayas, for example, don't actually live at 5500m, but can function at this and higher elevations much better than outsiders. $\endgroup$ – Alexander Feb 9 '17 at 23:32

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