The Two People We're All Related To

You have the materials within you, right now, to unlock the story of your deep and distant ancestry. And mine too. This is partly because you have mitochondria in your cells. And you got them only from your mother, not your father. And if, in your 23rd pair of chromosomes, you have an X and a Y, like me, instead of an X and an X, then you got that Y chromosome only from your father. Together, these two facts mean that there is an unbroken line of mothers and mothers of mothers who passed down 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 is a lineage of parents and parents of parents who passed down their Y chromosome, uninterrupted, leading 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 strange genetic quirk, 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 come from. We are only beginning to understand the legacy of these two people to whom we are 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 are relics of the time, more than two billion years ago, when our ancestor was single-celled. And at some point, he engulfed another single-celled organism and began using it as an energy supply. As a result, today mitochondria still have their own genomes, albeit very short ones.

This is your mitochondrial DNA or mtDNA. And it is only transmitted from the mother, because eggs have many mitochondria, but sperm have only a few and are destroyed after fertilization. Meanwhile, the Y chromosome is the smaller of the two sex chromosomes, X and Y. People with one X and one Y, rather than two Xs, 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 quite short and they do not recombine either. Here's what that means: In the process of creating sperm and eggs, our chromosomes align and exchange information.

Matching chromosome pairs swap arms or legs with each other. This molecular two-way is known as recombination and means that the offspring will have a slightly different combination of genes on each of their chromosomes than their parents had. Basically, this is how sex creates new genetic variations. But Y chromosomes are much smaller than X chromosomes. And unlike the rest of our chromosomes, it does not match its partner. So it doesn't recombine with the X. And the mitochondrial genome doesn't recombine with anything either. Because he has no partner to combine with. All this means that these two fragments of genetic information are passed, almost unchanged, from parents to children.

Which makes them traceable over time. That's why, for decades, scientists have been studying these two pieces 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 have learned about ourselves from mitochondrial DNA is the history of human migration. Although passed from mother to child without recombination, mtDNA slowly accumulates mutations. And as those mutations are transmitted within a population, they begin to form a genetic pattern within that group. This allows scientists to organize us into genetically similar groups, called haplogroups.

Anyone who has used a DNA testing kit has heard of them. So if you and another person share most of these mitochondrial mutations, then you belong to the same haplogroup. And decades of mtDNA research have shown that the vast majority of haplogroup diversity exists within 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 rest of the world is represented by parts of a single haplogroup! That's L3. So if you are not of African descent, you belong to L3, which contains many subgroups, such as K, M, N, and R, found among populations outside of Africa.

But there are even more L3 subgroups in Africa. So what does all this tell us? Well, on the one hand, it is taken as genetic evidence for what is known as the “out of Africa” hypothesis: the hypothesis that modern humans originated in Africa and spread around the world. This model was first developed by anthropologists around the 1980s, based on skeletal evidence, specifically, the first anatomically modern humans found in southern and eastern Africa. And today this mitochondrial data is considered molecular support for that idea, starting with a famous article published in the journal Nature in 1987. That article detected the first signs of these genetic patterns, based on mtDNA samples from just 147 people from five different geographical areas. populations.

But, among other things, that study showed us that there is a great diversity of haplogroups in Africa, because that is 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 immigrants became the founders of their own genetic lineages, which are found within the L3 haplogroup. But there was still an older origin population in Africa that they used to be a part of. Now, we can also use changes in our mitochondrial DNA to estimate when certain lineages diverged from each other.

This method is known as the molecular clock, which we have already mentioned before. It is based on the idea that mutations occur in mtDNA at a fairly regular rate. But since this rate of change is not the same throughout humanity, it is necessary to calibrate the clock, for example with the help of well-dated fossils and even the DNA of ancient fossil humans. Using this method, scientists have traced mutations in all the major lineages of haplogroup L3 people appearing outside of Africa. Where those non-African groups converge in time, we find the first humans to leave Africa. And the data suggests that this occurred about 70,000 years ago.

And going back even further, it appears that all known haplogroups converge on a single female ancestor who lived approximately 200,000 years ago. So our mitochondrial ancestor can tell us a lot about where we came from and when. But we also have to talk about what she can't tell us. She is not the first woman of our species, nor the first anatomically modern human being, nor anyone really special. For one thing, there is evidence of modern humans dating back to 300,000 years ago in North Africa. So we know that our species existed long before this woman lived, for thousands of generations.

But her mtDNA simply did not survive to this day. The fact that a woman passed her mitochondrial genome on to all of us is really just a matter of chance. Think of it this way: in any given generation, a woman can have sons but not daughters. And if she only has sons, that means none of her mitochondrial DNA will be 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, over the last 200,000 years, by pure chance. Now, naturally, there are many limitations to what mtDNA can tell us.

The dates they provide us are not very precise. And the genomes themselves are small and represent a small fraction of the information that is in our entire genome. And, of course, they only tell us this about half the population: women! So while mtDNA was crucial as an early source of genetic data, as sequencing methods began to improve, scientists began studying the other stretch of DNA that doesn't recombine: 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 lineages, the Y chromosome can tell us about the migration patterns of some groups of men.

For example, a pair of studies conducted in 2010 and 2013 sequenced both the Y chromosomes and mtDNA of 2,740 people across Indonesia. And the results showed that a surprising amount of Y chromosome DNA came from very far away places (such as China, India, Arabia and even Europe), especially from the western islands of Indonesia. On the island of Borneo, for example, the presence of haplogroup Y known as O-M7 appears to be the fingerprint of the immigration of men from Han dynasty China, about 2,000 years ago. But! In those same men, their mitochondrial DNA looked more like local haplogroups. This suggests that, at least for the last few thousand years, men had arrived from elsewhere and mated with local women.

And, as far as how far back this Y chromosome goes, the latest calibrations of the molecular clock now suggest that our Y chromosomal ancestor lived about 200,000 to 300,000 years ago. Like 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 they have on our genomes. One idea is that there might have been a human population boom about 200,000 years ago in Africa, when our species was doing very well. If that were the case, then both people's offspring might have been more likely to survive and pass on their DNA.

Or, in the case of our ancestor Y, it could be that he was something like Genghis Khan, having many, many, many children, some of whom were children who also had many, many, many children. But the story that these two people can tell us ends when they were born, because we cannot trace their genetic trace further back in time. So to investigate the origins of anatomically modern humans, we need earlier data sources. Remember: the Y chromosome and the mitochondrial genome represent only a small fragment of the human genome. To understand the full range of human diversity, we need to study... the full range of human diversity.

Fortunately, it is the 21st century and we no longer have to sequence small stretches of individual genomes by hand. We can sequence entire genomes quickly. So, as our technology and methods improve, we may soon be able to go beyond the lives of these two ancestors, into an even deeper past. But even when we do, each of us will continue to carry the molecular legacy of a man and a woman who managed to leave their mark on all of humanity. Thank you for joining me today on this truly amazing story. And a BIG thank you to our eontologists: Jake Hart, Jon Ivy, and mah boi STEVE!

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