Ancestry by sex

How individuals and populations tell different stories

Imagine a far away land, a long time ago, in which the plains were vast and across them came flowing marauding hordes on horseback. The people on horseback did not come in peace. The people on horseback had come for conquest. The people on horseback were men.

The local people, being fond of their lives and their lifestyles, their farms and their families, fought back, but to little avail. The local men, mostly, were slaughtered. So too were the children. Many of the women were raped. Some of the invading men stayed in the new land, and some went back to the lands and families that they had left behind. Children born of rape in the new landscape; children born of love—or at least not rape—in the old. Two different sets of women, mothers to all of the children. One set of men, who fathered children in both places.

Or perhaps these men who came flying across the vast plains on horseback came from a land which held little for them to return to. Perhaps, indeed, that land had a tradition of extreme polygyny¹, in which a very few, very powerful men, controlled and monopolized the majority of women, leaving most men without reproductive options.

Stories like this have occurred throughout human history. Stories like this are one reason that many have arrived at the surprising conclusion that, population-wide, humans have more female than male ancestors. Many men fail to reproduce. They are slaughtered in war—either set on a mission to invade a foreign land, or fight in place when an army arrives on their doorstep. But for those men who do reproduce, they will have noticed that it doesn’t take much for a man to make a baby. A baby will be far more likely to grow into a skilled and wonderful human being if her father does stick around, and actually provide parenting to the child, but at minimum, the requirements of a man to produce a baby is rather easy. Enjoyable too, it would seem.

At the population level, we have sex ratios that are roughly equal at birth—one male baby is born for every one female baby born—but the ratio starts skewing fairly quickly, and leave human populations with an excess of females by old age. Until yesterday, evolutionarily speaking, pretty much all women would bear children, whereas many men would not, and not by choice. While most people alive now live in cultures that nominally adhere to expectations of monogamy—pair bonds between two individuals, both of whom are involved in the raising of offspring—the relative prevalence of polygamy in times past has meant that, while most women have borne children, many men have not.

This, then, is the basis for the idea that we have many more female than male ancestors. It is a much-repeated conclusion, which is true in one way, and false in another, depending on what you are accounting for—populations, or individuals. Confusion about when it is true and when it is false is why it is invoked nearly every time I say the following:

Every human being has an equal number of male and female ancestors.

How can both things be true? If more women than men have reproduced throughout history—and humans therefore have more female than male ancestors—how can it be true that any given individual human has an equal number of both female and male ancestors?

Consider this:

How many genetic mothers do you have?

Do you have the number in your head, fixed? Perhaps you want to write it down, to make sure you don’t cheat?

Good. Next question:

How many genetic fathers do you have?

Write that down too, on a piece of paper. Keep your numbers hidden for now.

Okay, now, on the count of three, we’re all going to show our pieces of paper. All of them, all at once. One, two, three, now turn them over….

That’s a whole lot of ones I’m seeing. Everyone seems to have had just the one genetic mother, and the one genetic father. You know why? Because that is the way that sexual reproduction works. One egg from a mother, plus one sperm from a father, produces one (or occasionally two) zygotes. It’s never two eggs that combine to make a zygote, nor is it ever two sperm. Neither happen. You have one genetic mother, and one genetic father, regardless of how mixed and modern your family may be. The roles that we play in each other’s lives are far more expansive than are the roles of the original genetic contributions to our existences.

Going back at least 500 million years, and likely well over a billion, we have had one long, uninterrupted line of sexual reproducing ancestors in which, with every single successful reproductive event, one mom, plus one dad, made a baby (or babies).

Therefore you’ve got an equal number of female ancestors as male ancestors².

How does that square with the idea that, at the population level, more females than males have left their genetic mark?

The difficult part of this to grok, for those who aren’t accustomed to it, is simultaneously holding in your head the idea of the individual, and the population.

Most humans walk around thinking about individuals: what do individuals do, how do they make decisions, what is their impact on the world? Evolutionary biologists such as myself think about individuals, but we also think in terms of populations. Common parlance generally doesn’t distinguish between the two, however, and the widespread, oft-repeated conclusion that “we have more female than male ancestors” is taken to be about individuals, because that’s what most people think about, but it is actually about populations.

Both of these statements are true:

• Individual humans have an equal number of female and male ancestors.

• Human populations have more females than males in their lineage.

Imagine a population of 10,000 people in which extreme polygyny is the norm, which has an even sex ratio—half are men, and half are women. Of the 5,000 men, only 1,000 reproduce—they monopolize the reproductive activities of all 5,000 of the women. Each of the men who is mated has, on average, five wives. Whereas each of the 4,000 men who is not mated, has on average zero wives. And all 5,000 of the women are mated. On average, they each have one husband, and none of them has more than one husband. There is no variance in the number of husbands that women have in this hypothetical, extreme polygynous system.

Reducing the numbers to an absurdity—polygyny likely wouldn’t persist under these conditions—imagine a strongly polygynous population of 10 people: five men, five women. Using the same made-up but plausible ratios above: only one of those five men gets to mate, and he gets to mate with all five of the women. The other four men leave no descendants—no genetic trace at all in future generations. Future generations have, as their founding heritage, five mothers, and one father. But every single individual in those future generations has one mother, and one father. No individual has five mothers and one father, even though the population had that ratio of parents at its founding.

You have one genetic mother, and one genetic father, as has every human being throughout all of time; therefore you yourself have the same number of male as female ancestors. At the population level, though, more women than men have reproduced, and so more ancient women than men are represented in our modern genetics. Both things are true.

People are fascinated by sex and by ancestry, so this particular topic has traction. But the more important message here is that there is clarity to be gained by becoming well versed in thinking on both the individual and population levels, in being able to hold both entities in your head at the same time. You are both an individual, and you belong to several populations, including but not limited to your family, and your town, and your country. The tension between these distinct ways of understanding ourselves is real, but both are useful and true. Consider what other phenomena you might try to understand more holistically, with both an individual lens and a population-level one. Chances are that this will allow you to see into other people’s heads with more clarity, and have more compassion for places where you disagree.

Be good to one another, and happy solstice to all.

1. Most people say “polygamy,” but “polygyny” is more precise. Polygamy (poly – many, gamy – marriage) refers to any mating system in which individuals of one sex tend to have many more mates than do members of the other sex. Polygamy comes in two flavors: polygyny (poly – many, gyn – female, in which individual males monopolize the reproductive efforts of multiple females) and polyandry (poly – many, andr – male, in which individual females monopolize the reproductive efforts of multiple males). Across the animal kingdom, polygyny is common; polyandry is not. See my very brief primer on mating systems from August 2021.

2. There are, as always in such discussions, edge cases and exceptions that are both true, and do not falsify the central premise. One such edge case here is that sometimes, social rules breakdown so acutely that a man (P, for parental generation) fathers a child, a girl (the F1 generation), and then, many years later, that same P man goes on to father another child (F2) with his own F1 daughter. In such a case, the F2 child has a mother and a grandmother who are distinct (two female ancestors), but a father and grandfather who are the same individual (one male ancestor). This however, and happily, is exceedingly rare.