You, me, and our Grandma Eve

Our setting is eastern Africa, more than 100,000 years ago. Local people kneel around a woman as she lies dying. We can’t see her face, so we don’t know her age. Maybe she’s old, and the two women holding her hands are her daughters, or maybe she’s still a teenager, and her daughters are those two young girls playing in the dirt. Why do we care about this woman? Because - spoiler alert - this woman, Eve, is your long-dead grandma. Her family dynasty extends to every person alive today, and that includes you.

Image:Acacia tortilis, by JMK / CC BY-SA 3.0 / via Wikimedia Commons

You see, Mitochondrial Eve, to use her full name, died leaving at least two daughters, who were healthy enough to each have their own daughter. Those daughters had a daughter, who had a daughter, and so on, all the way to your mother, who had you.

Her glamorous name comes from where her legacy lies. When an egg is fertilized, the embryo receives most of its genetic data, the nuclear DNA, from the mother and father equally, half from each. Such a thorough DNA shake up every generation is what makes us all so different, and also what dilutes family traits down the generations. However, one type of DNA sits apart from the rest, in the energy generators of the cell known as the mitochondria. This mitochondrial-DNA (mtDNA) is passed directly from mother to child unchanged, which makes it easier to follow it back through centuries and millennia of mothers. The only differences down the generations arise due to mutations. We’ll come back to those mutations. But first, let’s rejoin Mitochondrial Eve’s community to see how it is possible that 7 billion people today are all related to this one woman.

Imagine there were six young women in Eve’s community. We’ll call them A, B, C, D, Eve and F. “A” was Eve’s best friend, but she died of a fever before she had kids. Look at the chart and see how her blue dot is the end of the line. No point reaching for your hanky, she left no children and hence no mtDNA, so we’re moving on. This mother-and-child chart shows what happened to each of the women’s descendants over the next 6 generations. A blue dot is a female, and a green dot is a male. A blue dot with no line below it represents a girl or woman who died without children. Since a man does not pass on his mtDNA, he is the end of the mtDNA line and I have not included his offspring, nor have I shown the male partners of the women. Sorry, fellas, you don’t even co-star in this one, but I promise you a walk-on role before the end.

After six generations, Eve is the only woman of her generation whose mtDNA still survives in this community, even though are many more people - “D’s” oldest son may have had six healthy daughters with “C’s” daughter, and Eve’s son may have left a thriving family, but none of that shows on this particular family tree. At this time the human population numbered in the thousands rather than millions or billions, and Eve's mtDNA continued to spread. By the time humankind journeyed into the rest of the African continent and around the world, every successful population took with it Eve’s mtDNA.

We know this thanks to those mutations I mentioned, the times when a stretch of DNA changes accidentally. Mutations occur randomly, but over very many generations they tend to build up at a steady rate. That means the number of differences in mtDNA between you and me are a measure of how closely we are related, of how long ago our female-line ancestors parted company. If you take the two most unrelated people in the world and calculate how long ago their female-line ancestors parted, you get to Eve’s daughters. Eve, everybody’s grandma.

Is there a male equivalent, everybody’s grandpa? Yes, actually, and his name is no better. “Y-chromosomal Adam”. You might guess where his name comes from. Girls receive an X-chromosome from each parent, but only boys get Dad’s Y chromosome, which is passed down father to son, intact except for mutations.

The jury is still out on how close in time these Adam and Eve characters lived, but please don’t get any notions that the two may actually have been partners. This ain’t no romcom.

These two individuals are important to us because they are places where all humans meet. As such, they are not only part of our shared human story, but they help us actually piece that story together. For example, following the mtDNA trail backwards in time to the first major branching of the female line leads us to Africa, adding to other evidence that humankind arose there. Since then our family has spread to every nook of Africa and the world, resulting in diverse populations around the globe. In a time when our web connections tend to reach further than those of our family tree, Mitochondrial Eve and Y-chromosomal Adam remind us that, in the end, we are all one family.

Further reading:

Nature, “Genetic Adam and Eve did not live too far apart in time”

National Geographic Genographic Project, Map of Human Migration

New Scientist, "Found: Closest Link to Eve, our Universal Ancestor"

Smithsonian.com, “No, a Mitochondrial eve is not the first female in a species”