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Head to givingwhatwecan.orgscishow to figure out how to get the  most impact for your donation. [ intro ] 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, you can come together to do some good with Giving What We Can. They’re an organization that  aims to help you find and donate to the charities that can do the  most good per dollar donated. Giving What We Can isn’t asking for your money and they don’t take a cut from your donation.

Their website is full of free  resources to find great charities. And they spend their resources working with experts to identify the most impactful causes and the best charities working on them. They’ve identified over 30 top rated charities  and funds working on high impact causes, like reducing poverty and illness,  addressing climate change, and advocating for farmed animals.

And their top-rated organizations have been vetted by expert evaluators so you know your  donations will be used effectively. To make the most impact you can  when you donate this holiday season, you can head to givingwhatwecan.org/scishow. Thanks to Giving What We Can for  supporting this SciShow video! [ outro ]