NARRATOR: This episode is supported by 23andMe.
It's fun to think about humanity settling
the galaxy, outposts of familiar Homo sapiens
spread among the stars.
But there may be nothing at all familiar about these distant
future space farers.
Human populations on other planets
may quickly evolve into things that look nothing
like humans as we know them.
And it'll start with Mars.
We may or may not ever become an interstellar species.
It's likely, however, that we'll at least take a shot
at becoming interplanetary.
Unless we manage to self-destruct
in the next little while, humans will be around for a long time.
At some point, perhaps soon, perhaps in the far future,
we will try to colonize Mars.
That colony will face monumental challenges to its survival.
But if those challenges are met, we
may reach a time when many generations of humans
have lived and died on the red planet.
Those people will be very different to us.
How will they change?
Will Martian humans or human Martians actually evolve?
Will they eventually become a species
distinct from Homo sapiens?
Humans have never stopped evolving.
Whenever a human population is isolated,
characteristic genotypes develop.
Sets of genes defining skin pigmentation, height and build,
immunities, even facial structure define the group.
The more isolated the population and the more extreme
the environment, the quicker this adaptation happens.
For example, humans of Tibet developed genetically coded
larger lung capacities, faster breathing, and higher
These traits developed soon after they
moved from neighboring China to the high-altitude Himalayas
around 3,000 years ago.
Mars is quite a bit more isolated and extreme
than the Himalayas.
How might we adapt there?
First, let's think about this evolution thing.
It's important to remember that evolution doesn't just
morph a species into the optimal form for its environment.
Evolution is blind.
It's an emergent effect of mutation and natural selection.
Mutations occur at a steady rate due to DNA copying errors
and through chemical and radiation damage.
Mutations that improve the function of a gene
or add in a useful trait will be more likely to spread
through a population.
Those that reduce function get weeded out.
By natural selection also maintains what traits we have.
If a genetic trait is no longer useful,
then random mutations will gradually
destroy it, like sight and pigmentation for cave fish
or tails and appendices for humans.
Use it or lose it.
So what traits are likely to be enhanced or developed on Mars
and which are likely to decay?
Mars is very different to Earth.
But the differences that we adapt to
will depend enormously on to what degree our technology
addresses those differences.
Let's start with the one that's hardest to fix--
Mars has a surface gravity 38% that of Earth.
This could have different effects.
Low gravity requires less bone and muscle strength
to function normally.
Strength may not be as strongly selected for as it is on Earth.
Perhaps over generations, Martian humans
would become intrinsically weaker.
But there's another effect to consider.
We know that the zero-G experienced
in orbital or interplanetary space leads
to decreased bone density and muscle mass in astronauts.
We'd see this in the first Martian colonists also.
Over the years of a human life on Mars,
this could be a huge health issue.
So genetic adaptations that counter the risks
may be very strongly selected for.
Perhaps the initial selection will be for people
with especially strong bones and high muscle mass, people
who can afford a little strength loss and still be healthy.
This is an especially compelling scenario
when you consider that bone strength makes
us more resistant to injury.
And a really important issue may be childbirth.
A mother's pelvis needs to be able to withstand
significant pressure that has nothing to do
with the gravitational field.
Extremely brittle bones may greatly increase both infant
and maternal mortality.
That would be a huge selection pressure in any population
without very consistent access to safe c-sections.
So will Martians become stockier or skinnier?
Over time, people are likely to develop a resistance
to low-G wasting, after which strength may go down.
But until then, we may expect strong selection
for very robust individuals.
That same low gravity may also affect height.
It takes a lot of effort for our hearts to pump blood
from our feet to our heads.
And that's why lying down feels so good.
In low-G, the heart doesn't have to work so hard.
So it's less of a disadvantage to be very tall.
In fact, it may be a real advantage.
After long zero-G missions, astronauts
lose significant muscle mass in their hearts.
A life on low-G Mars could be at serious risk
for early cardiac problems.
Having a little extra height would keep the heart
more active and healthy.
Perhaps Martian humans will end up much taller
than their earthling forebears.
OK, gravity isn't the only difference
between Mars and Earth.
What about this pesky lack of a decent atmosphere?
Well, that's not something we can easily evolve to deal with.
We'll always need some degree of technology
both for a survivable air pressure and oxygen
level and to protect us from space radiation.
But if these technologies aren't perfect or consistent,
we may see some adaptation.
This is especially true if we eventually terraform Mars
because probably whatever atmosphere we build won't be
as thick as Earth's.
There are many potential adaptations
to low oxygen environments.
Different mutations have arisen in Tibet
versus the Chilean Andes versus the Ethiopian highlands.
In all cases, these humans are able to sustain more activity
with less oxygen. So we might expect future Martians
to be incredible endurance athletes.
Mars's thin atmosphere and lack of an ozone layer
also means the surface is bombarded
with hard UV radiation.
It's deadly and requires artificial protection.
But again, assuming this isn't completely
mitigated with technology or terraforming,
we may see rapid evolution of darker skin pigmentation.
On the other hand, the less intense sunlight
at Mars compared to Earth means it'll be harder
to produce vitamin D. This factor is believed
to result in the paler complexions found
at high latitudes on Earth.
So will Martians be dark or pale?
It depends which of these effects
we better handle with technology.
Perhaps we'll have lily-white Martians in underground cities
and much darker Martians on the surface.
Even more dangerous than the UV are high-energy cosmic rays
and solar particles.
These bombard the surface due to the sparse atmosphere
and the absence of a protective magnetic field.
Unless humans stay underground or in well-protected shelters
pretty much all of the time, they
will see an increase in DNA damage from these.
That's going to mean increased mutation rates.
The most obvious thing this will result in is a lot more cancer.
Perhaps Martians will evolve defenses.
We have natural DNA repair and cancer-fighting mechanisms.
These may become enhanced.
Most mutations are bad.
But sometimes they are beneficial.
An increased mutation rate may mean evolution proceeds faster
on Mars than it does on Earth.
The final really important issue is that Mars is sterile.
As far as we know, it has no microbes whatsoever.
The human immune system is constantly
being exercised by exposure to Earth's biosphere, which
contains countless bacteria and viruses.
This results in a massive selection
pressure to keep our immune system up to date.
In an environment absent those pathogens,
the genes that code for immunity will gradually
mutate into uselessness.
Use it or lose it.
Future Martians will be highly susceptible
to earthly diseases.
And this effect may actually speed up their divergence
from the humans of Earth.
See, even if Martian colonies are extremely well-resourced
and have technologies to mitigate
a lot of the challenges of Mars, there
will be an inevitable drift between the Martian and Earth
When populations of the same animal species
are isolated from each other, they drift apart genetically.
At first, it's just cosmetic.
But eventually, there's a drift in function
and ultimately in their capacity to breed together.
At that point, we consider the populations
to be separate species.
Speciation has occurred.
This effect will be amplified on Mars
because the intermixing with Earth populations
will be minimal.
Earth will quickly become a deadly place for Martians.
They'll find Earth's nearly three times
higher gravity incredibly uncomfortable.
And any stray viral or bacterial infection could kill them.
This later will also make them very wary about allowing
earthling visitors to Mars.
The inevitable divergence between Earth and Mars
will eventually lead to speciation.
But that's a very slow process.
Homo sapiens have been around for 200,000 years
in some pretty wildly different environments.
This has led to an incredible variety of physical appearance.
Yet, we're all still Homo sapiens.
Our cousins, Homo neanderthalensis, Homo erectus,
Homo floresiensis, and others evolved, speciated,
from a common ancestor.
And they shared Earth with us for millennia.
They've since gone extinct, leaving us
as the only species of the genus Homo.
But perhaps a new species of human is ahead.
Give Martian colonies some tens of thousands
of years and some rapid evolution
and Homo martiansis may enter the scene--
tall, strong-boned yet slender, enduring yet disease-prone
and cosmetically very different.
Perhaps they'll be the first in a long line
of descendent species that spread their way
planet to planet, then star to star across the reaches
of space time.
Thanks to 23andMe for sponsoring this episode.
The name 23andMe comes from the fact
that the human DNA is organized into 23 pairs of chromosomes.
23andMe is a personal genetic analysis company
created to help people understand their DNA.
You'll be able to see which regions around the world
your ancestors come from, learn how DNA impacts
your health, facial features, hair, sense of taste and smell,
and sleep quality.
Learning this information starts with just spitting into a tube.
As it happens, I just got my 23andMe results back.
I knew my heritage was mostly Irish, some English,
no doubt some other bits.
The results did pretty much confirm
this with some surprises.
One weird thing is that I'm not entirely Homo sapien after all.
Actually, none of us are.
Anyone with European or Middle Eastern ancestors
has some Neanderthal DNA.
People from other parts of the world
have traces of different Homo genus cousins.
I have a bit more Neanderthal DNA than most.
In fact, more than 89% of other 23andMe participants.
I should have guessed.
I do have a rather luxurious head of hair.
It was really interesting peering
into my ancestry this way.
I don't feel like it defines me.
But it does represent a long, fascinating story
of human evolution and history that ended in me, in us.
But it doesn't stop here.
Perhaps future Homo martiansis will
be amused by the fraction of Homo sapien's DNA
that they have.
Go to 23andMe.com/spacetime to support the show and learn more
about your personal DNA story.
As always, thanks so much to all of patreon supporters.
This week, a special huge thanks to Samuel Dean Jacintho,
whose contribution at the big bang level
is being used to fund a top secret genetic engineering
The first batch of Homo martiansis
will be shipped to Mars as soon as that Musk
guy finishes the spaceship.
And I wanted to give a shout out to a brand new PBS series,
Above the Noise.
It's a critical dive into the science behind the news.
You should check it out.
Especially check out their great interview with Adam Savage.
He used one of the best descriptions I've ever heard
of what science actually is and what it isn't.
And now, your comment on last week's
episode on the great American eclipse.
Martin Ketling asks whether a DIY
of Arthur Ellington's confirmation
of general relativity would be possible.
Well, the answer is yes.
But it ain't easy.
Eddington measured the very slight
offset in the positions of stars around the limb,
the edge, of the sun due to the powers of their light bending
in the suns gravitational field.
The difference in position is something like 1.7 arcseconds.
So you need a position precision significantly better than that.
The typical blurring of Earth's atmosphere
is one to two arcseconds, which makes it tough.
But with a clever experiment, it's doable.
You need to compare the positions of stars
on either side of the sun during the eclipse and then
several months later when the sun has moved
from that part of the sky.
The stars should be a few arcseconds further apart
during the eclipse.
You'll also need a decent astroimaging
camera and a relatively high-end telescope.
But it doesn't need to be a research-grade scope.
It's not an easy experiment.
But with enough preparation, it's possible.
venkata asks why the eclipse shadow
moves from east to west rather than west to east
as you might expect due to the rotation of the Earth.
In fact, it's the moon's orbit around the Earth that results
in the shadow's movement.
The moon orbits the Earth once a month,
which means it moves about 0.5 degrees per hour.
That's its own angular diameter on the sky.
And it's also the sun's angular diameter.
So it takes one hour for the moon
to fully eclipse the sun and another hour to move past it.
And that's from one position on the Earth.
So the shadow actually moves as the moon moves.
Some of you objected to my pronunciation of Oregon.
I probably pronounced the O--
Or-ee-gone-- totally wrong.
Until I move to the States, I've only
ever read the name Oregon, usually right before dying