SciShow is supported by 23andMe.
Whether it’s SpaceX or Lockheed Martin or NASA, eventually someone’s going to start
sending people to Mars.
And when that happens, we might decide to set up a colony there, too.
People could be born, raised, have their own families, and die without ever setting foot
on Earth.
Sending people across the void of space to a desolate planet is hard enough, but founding
a remote colony comes with its own special challenge: low genetic diversity.
If we’re going to create new societies on other planets, we need to know how many people
to send to avoid the problems that come from inbreeding.
Whenever a subset of people goes off and starts their own society, they’re going to be less
genetically diverse than the much bigger population they came from.
That loss in diversity is known as the Founder Effect.
The differences can be amplified over generations, especially if the new population is small.
The smaller the population, the harder it is to find someone to have babies with that
you’re not related to, so you end up with a lot of inbreeding.
It all depends on which founders produce how many offspring, and which alleles, or variations
of a particular gene, they happen to pass on.
Some traits can completely disappear from the gene pool, and rare diseases can become
super common.
That’s happened before.
The Amish in Pennsylvania, for example, are much more likely than other Americans to have
Ellis-van Creveld syndrome, a type of dwarfism, because one of their founders carried the
allele for it.
And the South African Afrikaner community, which was started by a small group of predominantly
Dutch settlers in the 17th century, has a higher frequency of Huntington’s disease.
To completely avoid the consequences of inbreeding, a founding group needs to have enough people
so that, genetically speaking, it’s basically as diverse as the original group.
And it has to be able maintain that diversity over time.
In population genetics theory, that magic number is known as the effective population size.
It’s a really broad estimate that varies by species, and is based on things like how
often random genetic mutations happen, which members of the population mate, and how many
kids everyone has.
For humans, the effective population size is thought to be at least several thousand,
if not 10-20 thousand.
But the more people we have to send to start our space colony, the harder and more expensive it’ll be.
So scientists have tried to calculate the bare minimum number of people we’d need,
and they’ve come up with a few different estimates — along with a couple of other
strategies that could help.
In 2002, an anthropologist named John Moore tried to calculate the population you’d
need for a 200-year interstellar mission to colonize a hypothetical Earth 2.0.
Using a computer program based on the reproductive behavior of actual pre-industrial communities,
he simulated different scenarios to see what would happen to the population over time.
He found that, if you choose pairings and make other reproductive decisions really carefully,
you’d only need 75-90 young couples to start a colony.
So, less than 200 people in total.
But other anthropologists have argued that such a small number would only work if their
descendents were able to make babies with new humans at the end of their journey.
Which could happen—I mean, assuming Earth hasn’t suffered some sort of armageddon,
there’s no reason we couldn’t send another mission to Mars in a century or two, if not
much sooner.
Or the colonizers could pack a collection of frozen sperm and eggs from people who never
actually leave Earth, allowing them to mix in new genes over time.
Another issue with Moore’s estimate is that it’s only enough under the most ideal conditions.
For one thing, it doesn’t leave any wiggle room for disasters that kill off a significant
percentage of the population.
It also doesn’t take into account the fact that small pre-industrial groups occasionally
interbred with neighboring communities.
That means the actual number of people you’d need to maintain a healthy population might
be much higher.
In a 2014 paper published in Acta Astronautica, one researcher estimated that you’d really
need somewhere between 14,000 and 44,000 people.
If we followed that advice, our Martian colony might be sort of like Iceland.
Iceland was founded around 1100 years ago by about 10,000 people.
Thanks to the Founder Effect, the country’s modern population of about 300,000 is one
of the least genetically diverse in all of Europe.
So Icelanders keep meticulous genealogical records.
There’s even an online genetic database that acts like one big family tree, with information
on 95% of people born in the past 3 centuries.
If you’re in it, you can see how anyone else in the database is related to you.
And it’s more than just an incest-prevention tool — the database has also helped identify
genes associated with certain diseases, like Alzheimer’s and cancer.
So, thanks, Iceland.
If we want to colonize another planet, a similar database to keep track of things would probably
come in handy.
And so might those sperm and egg vials.
But before we worry about that, we should probably focus our efforts on sending any
number of people to Mars safely.
It might be a while before we colonize Mars, but you can learn about your genetic ancestry
RIGHT NOW by checking out 23andMe, a service that lets you learn where your ancestors originated
and how your DNA affects things like your facial features or your sleep quality!
The holidays are coming up too, which means you're probably going to be spending a lot
of time with family.
The information you get from 23andMe can make for some fascinating dinner conversation,
like I found out that even though I’m pretty sure my ancestors came from southern China,
their DNA is categorized more as southeast Asian than as Chinese.
Good luck explaining that one Uncle Joe...
I don't have an Uncle Joe.
And 23andMe is super easy to do: You just spit in the provided tube, send it in, and in 6-8 weeks
you get your results online and can dig through a lot of cool information.
There's even a Thanksgiving sale going on right now, through November 23 so head on
over to 23andMe.com/SciShow and check out their Thanksgiving Family Offer today!