You have the materials inside you, right now, to unlock the story of your deep, distant
ancestry.
And also mine.
That’s partly because you have mitochondria in your cells.
And you got them only from your mother, not your father.
And if, on your 23rd pair of chromosomes, you have an X and Y, like I do, rather than
an X and an X, then you got that Y chromosome only from your father.
Together, these two facts mean that there’s an unbroken line of mothers and mothers’
mothers who passed down the DNA in their mitochondria for hundreds of millennia, creating a biological
thread that connects you to a single female ancestor, regardless of your gender.
And it also means that there’s a lineage of fathers and father’s fathers who passed
on their Y chromosome, uninterrupted, leading back to a single male ancestor.
Now, I know what this might sound like.
I’m not talking about the first two people.
I’m talking about two humans who lived at different times in the distant past -- about
200,000 to 300,000 years ago.
I’m talking about two people who never met, but who, because of this odd quirk of genetics,
combined with some unique evolutionary circumstances, managed to pass on a very small fraction of
their genomes to you.
And to me.
To all of us.
And this is an incredibly powerful tool for studying where we all came from.
We’re only beginning to understand the legacy of these two people to whom we’re all related
-- a legacy that goes back some ten thousand generations.
Let’s talk about where this legacy begins, in your own cells.
Your mitochondria are the small structures that produce energy for your cells.
And they’re relics from the time, more than two billion years ago, when our ancestor was
single-celled.
And at some point, it engulfed another single-celled organism and started using it as an energy
supply.
As a result, mitochondria today still have their own, if very short, genomes.
This is your mitochondrial DNA, or mtDNA.
And it’s only passed down from the mother, because egg cells have lots of mitochondria,
but sperm cells only have a little, and they’re destroyed after fertilization.
Meanwhile, the Y chromosome is the smaller of the two sex chromosomes, X and Y.
People with an X and a Y, instead of two X’s, are physiologically male.
And there’s a reason we study mitochondrial genomes and Y chromosomes to understand our
ancestry.
Actually, two reasons.
Because they have two important things in common:
Their genomes are both pretty short, and they also don’t recombine.
Here’s what that means: In the process of creating sperm and egg cells, our chromosomes
line up and exchange information.
Matched pairs of chromosomes swap arms or legs with each other.
This molecular do-si-do is known as recombination, and it means that offspring will have a slightly
different combination of genes on each of its chromosomes than its parents had.
This is basically how sex creates new genetic variations.
But Y chromosomes are much smaller than X’s.
And unlike the rest of our chromosomes, it doesn’t match its partner.
So it doesn’t recombine with the X.
And the mitochondrial genome doesn’t recombine with anything either.
Because it doesn’t have a partner to combine with.
All of this means that these two snippets of genetic information get passed on, almost
unchanged, from parent to offspring.
Which makes them traceable through time.
So for decades, scientists have been studying these two bits of information.
And they tell two stories about our history that are slightly different but still complement
each other.
For example, one of the most important things we’ve learned about ourselves from mitochondrial
DNA is the story of human migration.
Even though it’s passed on from mother to child without recombining, mtDNA does slowly
accumulate mutations.
And as those mutations get passed on within a population, they start to form a genetic
pattern within that group.
This allows scientists to organize us into genetically similar groups, called haplogroups.
Anyone who’s used a DNA test kit has heard of these.
So if you and another person share most of these mitochondrial mutations, then you belong
to the same haplogroup.
And, decades of research into mtDNA has shown that the vast majority of haplogroup diversity
exists inside Africa.
For example, there are several haplogroups that are only found in Africa, or among people
of African descent.
These are groups like L0, L1, L2, and L4, 5, and 6.
But!
The whole rest of the world is represented by parts of only one haplogroup!
That’s L3.
So if you’re of non-African descent, you belong to L3, which contains lots of subgroups,
like K, M, N, and R, which are found among populations outside Africa.
But there are even more subgroups of L3 found within Africa.
So what does all of this tell us?
Well, for one thing, it’s taken as genetic evidence for what’s known as the “out
of Africa” hypothesis -- the hypothesis that modern humans originated in Africa, and
spread throughout the world.
This model was first developed by anthropologists around the 1980s, based on skeletal evidence
-- specifically, the earliest anatomically modern humans that were found in southern
and eastern Africa.
And today this mitochondrial data is seen as molecular support for that idea, starting
with a famous paper published in the journal Nature in 1987.
That paper detected the first signs of these genetic patterns, based on mtDNA sampled from
just 147 people from five different geographic populations.
But among other things, that study showed us that there’s such a great diversity of
haplogroups in Africa, because that’s where our genetic populations are oldest.
So when a small group of people migrated out of Africa, they only represented some of the
genes in the total human gene pool.
Those migrants became the founders of their own genetic lineages, found within the haplogroup
L3.
But there was still an older, source population in Africa that they used to be a part of.
Now, we can also use changes to our mitochondrial DNA to estimate when certain lineages split
off from each other.
This method is known as the molecular clock, which we’ve mentioned before.
It’s based on the idea that mutations occur in mtDNA at a pretty regular rate.
But since that rate of change isn’t the same across all of humanity, the clock needs
to be calibrated, like with the help of well-dated fossils and even the DNA of ancient fossil
humans.
Using this method, scientists have traced the mutations in all of the major lineages
of people from haplogroup L3 that appear outside of Africa.
Where those non-African groups converge in time, we find the earliest humans that left
Africa.
And the data suggest that this happened around 70,000 years ago.
And going back even further, it appears that all known haplogroups converge at a single
female ancestor who lived roughly 200,000 years ago.
So our mitochondrial ancestor can tell us a great deal about where we came from, and
when.
But we also have to talk about what she can’t tell us.
She isn’t the first woman of our species, or the first anatomically modern human, or
anyone really special, for that matter.
For one thing, there’s evidence of modern humans as far back as 300,000 years ago in
northern Africa.
So we know our species was around long before this woman lived, for thousands of generations.
But their mtDNA just didn’t make it to the present day.
The fact that the one woman passed on her mitochondrial genome to all of us is really
just a matter of chance.
Think of it this way: In any given generation, a woman might have sons but not daughters.
And if she only has sons, that means none of her mitochondrial DNA will get passed on.
So our mitochondrial ancestor is the only person who managed to have one or more female
offspring, who in turn also had female offspring, in an unbroken line, for the past 200,000
years, by sheer chance.
Now, naturally, there are lots of limitations to what mtdna can tell us.
The dates they provide us aren’t very precise.
And the genomes themselves are small, representing a tiny fraction of the information that’s
in our whole genome.
And, of course, they only tell us about half the population: females!
So while mtDNA was crucial as an early source of genetic data, as sequencing methods started
to improve, scientists began studying the other non-recombining stretch of DNA: the
Y chromosome.
Much of this work was done in the early 2000s.
And, just as mtDNA can shed light on the growth and spread of certain maternal bloodlines,
the Y chromosome can tell us about the migration patterns of some groups of men.
For example, a pair of studies in 2010 and 2013 sequenced both Y chromosomes and mtDNA
from 2,740 people across Indonesia.
And the results showed that a surprising amount of Y chromosome DNA came from far away -- like
China, India, Arabia, and even Europe -- especially in Indonesia’s western islands.
On the island of Borneo, for instance, the presence of the Y haplogroup known as O-M7
seems to be the fingerprint of immigration of men from Han Dynasty China, about 2,000
years ago.
But!
In those same men, their mitochondrial DNA more closely resembled local haplogroups.
So that suggests that, at least over the past few thousand years, men had been arriving
from elsewhere and pairing up with local women.
And, when it comes to how far back this Y chromosome goes, the latest molecular clock
calibrations now suggest that our Y chromosomal ancestor lived from about 200,000 to 300,000
years ago.
Much like with our mitochondrial ancestor, this guy must have had at least one male offspring,
who in turn had more males, in an unbroken line for hundreds of millennia.
Now, we don’t really understand why these two individuals left the indelible mark that
they have on our genomes.
One idea is that there might’ve been a boom in the human population around 200,000 years
ago in Africa, when our species happened to be doing very well for itself.
If that were the case, then the offspring of both of those people may just have been
more likely to survive, and pass on their DNA.
Or, in the case of our Y ancestor, it could be that he had a sorta Genghis Khan thing
going on, having many many many kids, some of whom were sons who also went on to have
many many many kids.
But the story that these two people can tell us ends when they were born, because we can’t
trace their genetic trail any further back in time.
So, to probe the origins of anatomically modern humans, we need earlier sources of data.
Remember: The Y chromosome and the mitochondrial genome represent just a small fragment of
the human genome.
To understand the whole range of human diversity, we need to study...the whole range of human
diversity.
Luckily, this is the 21st century, and we no longer have to sequence tiny stretches
of individual genomes by hand.
We can sequence whole genomes, and quickly.
So as our technology and methods improve, we may soon be able to reach beyond the lives
of these two ancestors, into the even deeper past.
But even when we do, each of us will continue to carry the molecular legacy of one man and
one woman, who managed to make their mark on all of humanity.
Thanks for joining me today for this truly amazing story.
And BIG thanks to our Eontologists: Jake Hart, Jon Ivy and mah boi STEVE!
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