Artist's impression of how Mars colonists might look after thousands of years of life on the red planet. Joseph Ventura

In other words, becoming a multiplanet species might lead us to become multiple species.

"This

New species or not?

Six thousand years isn't long in evolutionary terms. After all, Homo sapiens has existed as

"Evolution to a new species by the classic definition of not being able to breed with humans would take a long time, probably thousands of generations and a hundred thousand years," University of Arizona astronomer Dr. Chris Impey told NBC News MACH in an email. On the other hand, he added, "changing enough to look physically distinct would be much quicker, tens or perhaps a hundred generations."

Dr. Philipp Mittercker, a theoretical biologist at the University of Vienna in Austria, said in an email to MACH that he, too, is dubious of rapid speciation.

"Speciation is a long-term process that usually requires reproductive isolation over millions of years," Mittercker said. "Some human populations had been isolated for thousands of years and are still far away from being a separate species. It is thus unlikely that humans who had colonized Mars [would] become a separate species."

Solomon acknowledged that the path of human evolution on Mars is speculative. But he told MACH in an email that "it follows from what we know about evolutionary biology" that Mars colonists might evolve faster than some think.

And the apparent absence of microbial life on Mars might play a key role.

Evidence suggests that Mars may be devoid of life, and that goes for pathogenic bacteria as well as other life forms. If humans were to establish and live within a germ-free Mars colony, Solomon said, the colonists' immune systems could eventually lose the ability to fight off infections that might be introduced to the colony by germ-carrying humans or animals visiting from Earth. That risk presumably would encourage the colonists to minimize contact including sexual contact with potentially infectious earthlings. That, in turn, could accelerate the pace at which the colonists' bodies would begin to adapt to their new world.

Surprising differences

How might these Martian people differ from their distant ancestors in other words, from us? Whether or not they evolved into a new species, they might have anatomical as well as immunological and other physiological differences. Solomon said they might have notably thicker bones (including the skull bones), which might give them a more robust appearance perhaps a bit like members of the extinct proto-human Paranthropus genus, including

Why would that be? Bones need to work against the force of gravity to stay strong.

Evolutionary pressure for beefier skeletons might be especially strong for female Mars colonists, Solomon said, given the risk of pelvic fractures during childbirth. Beefier skeletons or not, Solomon said, female colonists might come to opt for cesarean section over natural childbirth. And since the size of the human head is constrained in part by the dimensions of the birth canal, the heads of Mars colonists might become larger than what is seen in humans on Earth.

If that sounds far-fetched, consider this: recent research by Mittercker and others suggests that the rising popularity of C-sections may be allowing an

So Mars colonists might have beefy bones and big heads. Then there's the question of their eyes.

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Mars is much farther from the sun than is the Earth, and the extra distance and the lower levels of sunlight on the Martian surface could cause changes in the colonists' eyes.

"During a good day, Mars looks like an overcast day on Earth," Dr. Nathalie Cabrol, a planetary scientist at the SETI Institute in Mountain View, Calif., told NBC News MACH in an email. "Our eyes are accustomed to a certain amount of light on Earth. If there has to be some adaptation to these new ambient conditions, then either our optical system and brain will have to develop new ways of collecting more light on the retina, or we will develop new retinas or bigger eyes."

The need to protect those bigger eyes might be another reason the colonists' skulls might become more robust, Cabrol said, adding that it wasn't clear whether the changes she envisions would be evidence of a new species or simply a version of Homo sapiens adapted for life in a different environment.

Of course, evolutionary changes in humans on Mars would occur only if humans were able to reproduce and successfully raise their children in the low-gravity Martian environment. Cabrol said the colonists might need some sort of "gravity chamber" in which to reproduce and in which their offspring could spend their early developmental years in conditions closer to those on Earth.

Peculiar pigmentation

Another potential change for the Mars colonists would be their skin pigmentation.

"Because of less light," Cabrol said, "I would say that it is possible that the skin of these humans will become ... pale over time, and their hair light-toned."

Solomon sees things differently.

The Martian atmosphere is thinner than Earth's, and the red planet has essentially no protective magnetic field. Thus people living on Mars would be exposed to high levels of cancer-causing radiation even if they spent most of their lives indoors. Pigmentation helps block the effects of radiation. The deeper the color, the better the protection. Thus Solomon figures Mars people might evolve to have darker skin than anyone on Earth.

On the other hand, Solomon said, life on Mars might yield people whose skin is pigmented by carotenoids rather than our usual pigment, melanin. (Something similar has been

Cultural and technological changes

Is Solomon right, generally speaking, about the changing appearance of Mars colonists? That's impossible to say. But no matter what, experts agree that Mars colonists would likely drift away culturally and technologically from their terrestrial ancestors.

As Impey told MACH, "They will probably be aggressive in genetic engineering and self-modification (body part and organ enhancement and replacement), to the extent of embedding various monitoring and repair devices, and taking a cyborg path. This will be a very technology-forward cohort, advancing far beyond the average terrestrial society."

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Impey said it was hard to predict the psychological effects of living on Mars. But as the colonists "are removed from human affairs," he continued, "they will probably develop their own cultural norms and dialects, and start to feel very distinct or post-human."

If the colonists do change dramatically from their ancestors back on Earth, how would we view them? Would we consider them alien beings or just subtly different humans?

Solomon thinks the latter possibility is more likely.

"In the past, when there were multiple species of human around (i.e. Neanderthals, Denisovans, and Homo sapiens), we know they had sex with one another and had babies that survived," he said in an email. "That suggests to me that we view other humanlike species as being more human than animal."

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