Isolated on islands with limited resources, species undergo rapid size changes. While Homo floresiensis ('hobbits') and pygmy elephants shrank, other species like Komodo dragons and tortoises became giants. This evolutionary pressure applies to any isolated population, including future human colonists on Mars.

People tend to marry and befriend those who are genetically similar, a process that amplifies genetic inequality in the next generation. This is compounded by geographic sorting, where individuals with genetic propensities for success migrate away from disadvantaged areas, leaving them 'doubly disadvantaged, genetically and environmentally.'

Living in a sterile Martian habitat, colonists would only be exposed to a tiny fraction of Earth's microbes. Their immune systems would be unprepared for Earth's vast microbial diversity, making a return journey potentially fatal. This creates a permanent biological quarantine that would accelerate human speciation.

A human born and raised in Mars's one-third gravity would likely not develop the bone density and muscular strength required to withstand Earth's gravity. The physical stress would be painful and potentially debilitating, effectively trapping them on their home planet for life.

Despite decades of spaceflight, there is almost no research on conception, pregnancy, childbirth, and child development in a low-gravity environment. Our assumption that humans can successfully reproduce off-world is a massive, unverified leap of faith and the single biggest unknown for establishing a permanent settlement.

To shield against radiation and meteorites, Martian habitats will likely be built underground, not in glass domes. A society that lives its entire existence underground, reliant on artificial light and disconnected from an open sky, would develop a psychology profoundly different from Earth's.

Since Mars cooled and had water before Earth, Avi Loeb argues life likely started there first. This primordial life could have been transported to Earth inside rocks ejected by asteroid impacts. This makes humans descendants of Martian microbes and Elon Musk's mission a 'return to our childhood home'.

Women raised in one-third gravity may have bones too brittle for natural childbirth, risking fatal pelvic fractures. If C-sections become the norm, the evolutionary pressure that limits a baby's head size to fit the birth canal is removed. This could lead to the rapid evolution of larger-headed humans.

On Earth, we have non-genetic ways to improve lives. For a child born on Mars who can't escape the high-radiation, low-gravity environment, genetic engineering might be the only way to alleviate suffering. This flips the ethical question to whether it's unethical *not* to intervene genetically.