The Grandma TimelineA thought experiment that shrinks a hundred thousand years down to a crowd you could count — one mother at a time

The arithmetic is almost insultingly simple, which is exactly why it lands. A generation, in this toy model, is twenty-five years — roughly the age at which a mother has her daughter. Divide any stretch of time by twenty-five and you get the number of generational steps, the number of "grandmas," between then and now. A thousand years ÷ 25 = 40. That's it. The reason a millennium feels vast is that we instinctively count it in years, and years are the wrong unit for human lineage. Switch to grandmothers and the same span suddenly fits inside a family reunion.

This works because a maternal line is a chain of single links, not a sprawling tree. Trace only mother-to-mother and each century is just four steps. The whole of the last millennium — every plague, coronation and revolution your history book covers — is bridged by forty consecutive women, mother to daughter, hand to hand. The vastness was an illusion of the unit. Below, you can run the same calculation on any moment you like.

Once you have the unit, you can lay out the whole human story as a column of numbers, and the effect is genuinely startling — because the numbers stay so small. The classical world of Rome and the Caesars is about eighty grandmothers back. The pyramids, built before Rome was even a village, sit at roughly one hundred and eighty. Writing itself — the invention that begins "history" as opposed to prehistory — is only around two hundred and twenty grandmothers ago.

The lesson hiding in this table is humbling. We talk about civilisation as though it were ancient and deep, but in grandmother-units it is shockingly thin: fewer than five hundred women separate us from the people who first planted seeds instead of chasing them. Everything we call history — cities, religions, empires, science — happened within the lifespans of a few hundred mothers passing the world to their daughters. We are not far downstream of the beginning. We are practically standing next to it.

So far the grandmothers have been lined up end to end, but real generations overlap — and that makes the chain feel even shorter. With a hundred-year lifespan and twenty-five-year steps, a single moment can hold five living generations: the math is simply 100 ÷ 25 = four gaps, which means five women alive together. Five-generation family photographs are rare but real; the newborn at one end and the centenarian at the other are separated by a century, yet they share a room, a moment, a touch.

This overlap turns the timeline into something you can almost reach through. Memory leapfrogs down the chain: a child who sits on her great-great-grandmother's lap is touching someone who, as a child herself, sat with her great-grandmother — and so a handful of overlapping lifetimes can carry a living memory across two or three centuries with only a few human links. The forty grandmothers back to the year 1000 aren't forty separate, sealed-off lives. They are a continuously overlapping relay, each woman's life shingled over the last, so that the "distance" to the Middle Ages is only about eight or nine overlapping lifespans deep.

That's the quiet magic of counting this way. The past stops being a foreign country reachable only by time machine and becomes something more like a long room full of people standing close enough to pass a whisper, hand to hand, all the way back.

This is where the feature connects back to The Egg. Your mitochondria — the tiny power plants inside your cells — are inherited almost entirely from your mother, packed into the egg you grew from. So the chain of grandmothers isn't only a chain of women; it's a chain of egg cells, each one formed inside the body of the last. Follow that single maternal thread back far enough and every living human's converges on one woman: Mitochondrial Eve, the most recent common ancestor of us all through the purely female line, who lived in Africa on the order of 150,000–200,000 years ago.

Put that in grandmother-units and the number refuses to be frightening. At twenty-five years a step, ~180,000 years is about 7,200 grandmothers; at the real maternal average (more on that next), closer to 7,800. Either way — call it seven or eight thousand women. That is the entire direct maternal ancestry of any person alive, from today back to the origin of Homo sapiens. Seven thousand mothers. You could seat every one of your grandmothers-to-Eve in less than half a basketball arena and photograph the whole of your species' maternal history in a single frame.

One necessary caution, because this is exactly the kind of fact that gets garbled: Mitochondrial Eve was not the first woman, nor the only woman alive in her time, nor anyone's idea of a biblical Eve. She is simply the most recent woman from whom everyone alive descends along an unbroken mother-line — and which woman holds that title actually shifts over generations as maternal lineages die out. The name is poetic shorthand, not paleontology. But the chain is real, and it is breathtakingly short.

No model this tidy survives contact with reality unscathed, so here are the honest dents. First, the generation length isn't a constant. Averaged across both parents, recent genomic work puts the human generation interval at about 26.9 years over the last quarter-million years — but it has drifted up and down with climate and culture, and it differs by sex. Because we're counting grandmothers, the relevant figure is the maternal one: ~23.2 years. Which is the delightful part — our breezy "25" turns out to be a hair high for the mother-line, so if anything the chains are a touch longer (and the grandmothers slightly more numerous) than the clean version suggests, not shorter.

Second, the hundred-year lifespan is a modern luxury. For most of history, far fewer women reached old age — though the usual "people died at 30" claim is itself misleading, skewed by infant deaths; a woman who survived childhood often saw sixty or seventy. Third, and most important: this traces one thread. You don't have a single line of ancestors but a vast branching tree — two parents, four grandparents, and so on, doubling each generation. The grandmother-chain follows only the strict mother-to-mother edge of that tree. It's a real line, but it's one of countless.

And yet none of this breaks the wonder; it sharpens it. Even with the real numbers, the dawn of our species sits only six to eight thousand mothers back — a countable crowd, not an abyss. That is the thing worth carrying away from this whole book about the female: the egg, the womb, the long maternal relay are not abstractions lost in deep time. They are a short, unbroken, hand-to-hand chain of women, each one made inside the last, reaching from you to the very beginning — close enough, almost, to touch.