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People tend to be fascinated by their ancestors –
we wonder what they were like, what they did, and what they’d think of us.
I can’t be alone in wondering if I’m related to someone famous, or historically significant.
Like, what if I’m descended from royalty?
Did my ancestors make anything that’s still here today?
And while family trees tend to get murkier the further back you go,
we do know that at some point, all of our family trees connect up,
and there’s someone who we’re all related to.
We’re the same species, after all, so we have ancestors that we all share.
And the very first person that we can all call an ancestor
is called the most recent common ancestor for our species.
While we’ll probably never know their name,
there are a lot of ways
that we can narrow down when they lived.
And as it turns out, that’s probably not as long ago as you’d think.
Think about a family tree.
The most recent common ancestors of you
and your siblings would be your parents,
while the most recent common ancestors
for you and your cousins would be your grandparents.
The most recent common ancestor is like that, but for every person alive.
So how can we find out who that person is,
or when they lived?
In the era of DNA ancestry tests telling us that we’re 2% Sri Lankan or 3% Scandinavian,
you’d think we could just use genetics to reveal
who our most recent common ancestor is and where or when they lived.
But it’s actually really hard to trace your ancestry that far back
using genetics alone because of how you inherited those genes.
For 22 of our 23 pairs of chromosomes,
we only get half of each parent’s genes,
plus genes get reshuffled on chromosomes
and swapped around during meiosis,
which is the process that makes our egg or sperm cells.
And which genes you get is random,
meaning that two siblings can have a really different assortment
of genes from each other,
even though they’re both equally related to their parents.
So genes actually end up dropping out of the pool pretty frequently,
which means that to look at an unbroken lineage using genetics alone,
you have to look at DNA that doesn’t get reshuffled or split up.
Fortunately, we happen to have two ways to do that.
And there’s one for each biological sex, which is kind of convenient.
Both of these types of unshuffled DNA
can tell us about our distant ancestry,
although even using one or both lines of evidence
won’t get us to the most recent common ancestor, for reasons we’ll explain later.
For biological females, we can look at mitochondrial DNA,
which gets passed down directly
from a mother to all of her offspring,
since the offspring’s mitochondria originate
from within their mother’s egg cell.
For biological males, we can study the Y chromosome
because it's passed from fathers to their sons and is also conserved basically intact.
So you can trace that DNA back
to the most recent female or male that we can genetically tie
to all biological females or males alive today.
These unknown individuals
have earned the Biblical nicknames
Mitochondrial Eve and Y-chromosome Adam.
They didn’t necessarily know each other though, Biblically or otherwise.
They might not have even lived at the same time as one another.
We can work backwards to figure out roughly
when Mitochondrial Eve and Y-chromosome Adam lived
by counting up the mutations in mitochondrial or Y-chromosome DNA,
and multiplying that number by the average rate of mutation
in order to calculate how long it would take for that many mutations to arise.
Current estimates say that Mitochondrial Eve
lived about 99 to 148 thousand years ago,
and Y-chromosome Adam lived about 120 to 156 thousand years ago.
Now, unlike their namesakes,
Eve and Adam are by no means the first people of our species,
nor were they the only people alive at the time.
They’re just the most recent people that had the mitochondrial or Y-chromosome DNA
that all current mitochondrial and Y-chromosome DNA is descended from.
And, even though they are common ancestors to all humans,
they aren’t our most recent common ancestors - not by a long shot.
That’s because our most recent common ancestor isn’t sex specific!
Someone could be related to humanity’s
most recent common ancestor through their mother’s mother’s father’s mother’s father,
but they have to be related to Mitochondrial Eve
through their mother’s mother’s mother’s mother, and so on.
Basically, the most recent common ancestor
is a shortcut connection that our genetic analyses alone just can’t trace.
And, there are millions of potential lineages that could connect yo
u to the most recent common ancestor,
but only one single lineage that can connect you to Eve or Adam.
So if we can’t find out who this person is using genetics,
we have to use a different strategy to figure out
when our most recent common ancestor lived- math!
There’s a general equation to figure out
when the most recent common ancestor lived for a population or species that mates randomly,
which gets more accurate as that population’s size increases.
The equation assumes that the population randomly mates,
meaning any given male has roughly equal odds of mating with any female in that group.
But humans don’t mate randomly. At all.
We mate within social groups and our geographic area -
and not only that,
but the geographic areas where we’ve even been able
to go have changed dramatically throughout history.
As port cities opened or closed, trade relationships
between the various nations changed, and technology
to get from place to place changed too.
All of that affected how humans could get around,
and, uhh, get around.
So to figure out our most recent common ancestor,
you have to factor in all that, too.
In 2004, researchers tried to do just that.
They used population sizes, geography, birth rates, and historical migration patterns
to model how frequently people
from one area might reproduce with people from another area.
Now, these are estimates about human populations
living hundreds or even thousands of years ago,
so it’s tough to model these trends with complete certainty.
The research team ran the model under several different scenarios
to come up with an estimated time range.
And the model suggests that the most recent common ancestor of everyone
currently alive lived around 1400 BCE,
and possibly as recently as 55 CE! .
For context about how recent that is,
we’re talking about someone who could have used a sundial to tell time, made glass,
or had the opportunity to read a surgical manual.
So to answer my question about what my distant ancestors’ lives were like,
it turns out that they could have been doing all those things, too.
And to go a little further back, the Great Pyramid of Giza
was built around 2500 BCE,
so it’s entirely possible that I am related
to any of the people hauling rocks across the desert,
thousands of years ago.
We could all be related to them -
every single one of us.
All of this math and genetics
is just more evidence that the lines that we might draw separating people today
are largely arbitrary.
If we can go back just a few thousand years to when Great-Great-....
However many greats, Grandpa could have been working
on one of the world’s great wonders,
how different can we all really be?
After all, we’re family. See you at the reunion! And at this year’s end of year family reunion,
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