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! Now, Lemme give you my two cents. Actually, “Two Cents” is a new series
from PBS Digital Studios about money and YOU. Financial experts (and husband and wife team)
Philip Olson and Julia-Lorenz Olson guide you through the complex world of personal
finance, from the kitchen table to the Stock Exchange. You’ll get practical knowledge about how
to spend, save and earn, and even insights into how your brain is hardwired to react
to economic problems. Money might make the world go round, but it
doesn’t have to make your head spin. So check the link in the description below
to subscribe to “Two Cents”! Now, what do you want to learn about? Leave me a comment, and don’t forget to
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Adam and Steve
One of them is definitely your mum
That's super interesting information and amazing scientific work.
...... our parents
Well presented duder.
PBS Eons is the shit!