Elon Musk wants to get a self-sustaining colony going on Mars in his lifetime—his current goal is to get that started in the next four years with crewed flights, albeit not yet with colonists along. I agree with him in the goal of a self-sustaining colony and with his rationale—that getting off planet in a sustainable way using only the resources available on the second (and further) planets is critical to the survivability of homo sapiens.

I have questions, though.

What will be the long-term effect of living lifetimes in Mars’ gravity field, which is roughly 38% that of Earth’s? What effect would that great difference have on human (and food animal) gestation?

At the very bottom of human metabolism, of Earth-adapted metabolism in general, are a suite of minerals that plants, animals, essential bacteria need. Many of these are used in trace amounts only, but they seem to be critical. Will all of these minerals be present on Mars? What will be the effect, even if the minerals are present in some amount, on the bacteria on which all the other life depends, and on the plants on which all the animals depend?

And one more question: say a self-sustaining colony has been a going concern for some number of generations. With those generations’ adaptation (focusing here solely on the human population) to Mars’ gravity and to that planets’ mineral suite—especially in the latest generations there—will those folks ever be able to come back to Earth and live and operate in our much higher gravity field? Will those folks even still be homo sapiens, or will they be something different—homo mars? It seems likely the two populations still would be capable of interbreeding, much like homo sapiens with homo neanderthalensis and with denisova hominin.

Getting sustainably off planet will facilitate our ability to survive natural disasters and our own machinations, and thereby extend the life of homo sapiens as a species. But evolution won’t be stopped by getting off planet. That will only generate new pressures that guide evolution, new pathways for evolution to follow. And that includes the evolution of homo sapiens, even here on Earth.