Human evolution is complicated, to say the least.
It's complicated by love, which makes us want to keep people alive. It's complicated by science and technology, which give us the power to do so. It's complicated, sometimes, by politics. And it's complicated by our environment, which is relatively stable, meaning we haven't needed many significant adaptations for thousands of years.
But what happens when that changes? What would happen to a population of humans - generations of humans - who left Earth to find new lives in the vast wonder of space?
The environment out there, beyond the protective environment of our home planet, is very different from the one we've evolved in for millions of years. It's quite conceivable that our species would become something altogether different.
Fictional bones and natural selection
For decades, this potential transformation of humanity has been fertile ground for speculative fiction.
In James S. A. Corey's wildly popular The Expanse series, and Robert Heinlein's The Moon is a Harsh Mistress, away from the strictures of gravity we see humans evolving to be unnaturally tall, brittle-boned creatures. In Becky Chambers' To Be Taught If Fortunate, we see the reverse - bones thickening on high-mass exoplanets, to support a higher body mass.
We see a lot of longevity as imagined technologies extend our lives. Iain M. Banks' Culture novels are a brilliant example of this, wherein humans develop the ability to live for centuries, and become cosmic dilettantes. In Bruce Sterling's Schismatrix, humans have genetically engineered themselves to such an extent, they're pretty much a new species.
While the specific outcomes and pathways vary quite wildly in science fiction, the concept itself - human metamorphosis away from Earth - isn't that far-fetched. It may not seem like it, but as we go about our day-to-day lives, humans are still evolving.
Scott Solomon, an evolutionary biologist at Rice University, has written a book on the continuing human evolution, and spent a lot of time thinking about how we might evolve in the future. According to him, migrating away from Earth is bound to change us.
"For evolutionary change to occur, you need genetic variation and you need natural selection," he told ScienceAlert.
"When you have those two things for a population that's facing a very big change in environment, evolution can happen quickly. We will have all the pieces in place if we move to, say, Mars."
Space will hurt you
Environmental pressures are going to be a huge part of our transition from an Earth-bound species to a space-faring one. That's already evident. Just a temporary jaunt in space physically changes people. Astronauts, even after a few months in space, can take years to restore the bone density lost in microgravity. Some changes occur even faster.
Kira Bacal is a medical scientist and practising physician at the University of Auckland in New Zealand. But she spent several years working at NASA's Johnson Space Center working in aerospace medicine.
"There are some things that happen on an extremely rapid timeline," she told ScienceAlert.
There is, for example, the baroreceptor reflex. It regulates our blood pressure, keeping it at a constant level in response to external changes. It's the reason you don't faint when you stand up, keeping your blood from pooling in your feet due to gravity. In microgravity, this reflex is immediately impaired because you don't need it.
Changes also happen in the vestibular system - the finely tuned mechanisms that help us keep our balance and control eye movements, along with the brain processing involved.
"Suddenly you're in a situation where if you drop something from your hand, it doesn't go anywhere," Bacal explained.
"So the whole expectation of what's going to happen, the processing of the sensory inputs you're getting, the way it feels to move through space, no pun intended, is very different."
Other changes that take place over time in microgravity include the loss of bone density; without the constant stress gravity places on your bones, they lose density at about 10 times the rate of osteoporosis. There are also anatomical changes to the eye, microstructural changes in the brain, and even changes in the gut microbiome.
Although these physiological changes give us some idea of the environmental pressures that could shape the evolution of space-faring humans, they only affect individuals to different degrees, and seem to revert to normal when the astronaut returns to Earth, even if it sometimes takes a few years.
Between nature and nurture
So, how fast could we expect to see permanent evolutionary adaptations in Homo galacticus? To understand the timescales involved, terrestrial precedent can help us once again. Two recent examples, detailed in Solomon's book Future Humans, are disease resistance and adaptation to the lower oxygen levels of high altitudes.
In the tropics, where malaria is most common, there's also a higher incidence of sickle cell anaemia. That's because sickle cell, a hereditary disease, involves a gene that protects against malaria - so people most likely to survive malaria and reproduce are also sickle cell carriers. And different populations of people living at high altitudes have developed different adaptations to cope with low oxygen levels.
Both of those are relatively recent, taking place over the last few tens of thousands of years - practically an eyeblink in evolutionary terms.
But it's not just the environment that shapes the path of our evolution. Culture - the way we live and the choices we make - also plays a role, and it can speed things along quite substantially.
"We can see things that have happened even in just the last several hundred years," Solomon said. "For example, quite a few studies have found that the timing of reproduction is evolving in modern human populations, and it's evolving in ways that are often surprising to people."
In the case of the French-Canadian population of Île aux Coudres, for instance, detailed church registers dating back to the 18th century showed a curious trend - the average age at which women gave birth to their first child went down from 26 to 22 in a 140-year span. This reproductive age seems to be heritable, and women who reproduce younger have more children that also reproduce younger, coming to dominate the population. That's natural selection.
But in other places, that age of first reproduction is rising, as women choose to delay having children for various reasons, now that those regions have readily available means to do so. That's culture - and technology - at work.
"This is a good example of natural selection and culture in society having sort-of tug of war over the same characteristics," Solomon said. "Some people have gone so far as to say that [culture] has replaced natural selection, but I think it's quite clear that it hasn't replaced natural selection, it's just changed it."
Culture, technology and natural selection will be important for space-faring humans, too. Will there be artificial gravity or not? Science fiction puts forward that lack of gravity will result in bird-boned, brittle humans, but Solomon believes otherwise - simply because of the strain childbirth puts on our bones.
Not only is the birthing process hard, but minerals to grow the baby are often taken from the mother's bones, resulting in lower bone density. So the women more likely to survive pregnancy and childbirth in space might have denser bones to start with, allowing them to live to have more dense-boned children. Technology and natural selection could both play a role there.
Will there be radiation shielding? How strong will it be? Because radiation can drive mutation (and cancer), and being exposed to it could produce some unexpected evolutionary pathways. But darker skin is more resistant to dangerous radiation, so that could play a role, too.
These - and other, smaller influences - are all going to have varying effects, sometimes playing tug of war with the same trait to shape a human optimised for their space environment.
By and large, we can't really predict what our space-faring descendants will look like, because we don't know all the factors that are going to come into play.
The founder effect
Even with all those unknowns, decisions made before those pioneers set off into the infinite final frontier - decisions we might see made in our lifetimes, in fact - will have more of an impact than we might know.
As Solomon explains, it's yet another effect we've already watched unfold on Earth - the founder effect.
"The people that are the founders will have a very significant influence on the long-term makeup of the human population in space," he said.
"It plays out on Earth all the time. Every time a new island pops up out of the sea there are going to be some plants and some insects and other species that will eventually make their way there. And whatever characteristics and traits they happen to have are going to be the characteristics that are going to be present in that population."
We can already see hints of how it might play out for spacefaring humans. Earlier this year, NASA put out a call for astronaut applications - and one of the requirements is a Masters degree. That means people who are wealthy enough to be highly educated. That means, in America at least, probably white people.
Not every country has the resources for a human space program, or can train astronauts. Sometimes the decisions on who gets to go to space may be politically motivated.
People can also get selected based on physical traits, which is starting to sound a bit too much like eugenics, if the plan is to travel space for multiple generations.
"A lot of how we develop and what we develop is affected, not so much by 'is there gravity' or 'isn't there gravity', but by who they decide make acceptable astronauts," Bacal said.
A mid-point as a test case, she points out, is the notion of the commercialisation of space. Miners, for instance - spending stretches in low gravity, returning to Earth in between jobs. It takes much longer to regain bone density than it does to lose it, so it could be possible that space miners never gain enough time to fully recover, resulting in early-onset osteoporosis.
"That could - as it already is in terms of the astronaut corps - impact who gets to work there or who gets chosen for your 10-generation generation ship. You might say, 'look, we're going to choose people who are less likely to be susceptible to bone density loss'," she said.
"That has ethnographic implications. It has gender implications." And these will need to be considered very carefully if we want to avoid a situation where specific groups of people are barred from space because of their race or gender.
We may never become a true space-faring species. It's possible we'll never leave the Solar System. But we're also not likely to stay here on Earth forever.
Space agencies are already making plans for at least one permanent base on the Moon. We've sent several robotic missions to Mars, and plans for a crewed mission are underway. A permanent Mars colony isn't out of the question, either.
These are harsh, literally alien environments, with low gravity and intense radiation. They're entirely different from our home planet, where we've been evolving for millions of years. These places will inevitably have an impact on the human body.
So will the technology we have available, how we travel, and the decisions we terrestrial humans make about who gets to go. But not all differences are visible to the eye.
"You know, there are people that are better at coping with radiation here on Earth. You can't tell by looking at them, but they might live longer than the rest," Solomon said.
"I think a lot of the changes are things that will take place inside the body. I think there would be some outward changes, but a lot of more subtle, internal changes."
In the end, those future humans, the inheritors of the stars, may not look as different from us as we might expect.