Your Ancestors
Prompt: A visualisation and exploration of your reverse family tree, showing the scale and complexity and the genetic intermixing going on, through human history back to the dawn of life.
Two parents, four grandparents, eight great-grandparents. Keep doubling and within thirty generations you have more ancestors than people who have ever lived. The resolution to that paradox is one of the most beautiful ideas in genealogy. Keep going and the tree merges into a single thread that runs from you, through every human who ever lived, through fish and worms and single cells, to a moment around four billion years ago that nobody alive remembers.
1. Your reverse family tree
A normal family tree starts with an ancestor and fans forward through their descendants. A reverse family tree starts with you and fans backward through your parents, grandparents, and so on. Every step back doubles the number of slots in the tree.
The tree above stops at four generations because the page would otherwise have to be a kilometre wide. The doubling, though, does not stop.
2. The doubling paradox
Slide the generation count and watch the slot total. A generation is roughly 30 years on average, so 30 generations is about 900 years, give or take. By then the tree is asking for more ancestors than have ever lived.
At 30 generations the tree needs more than a billion ancestor-slots in a world that contained perhaps 400 million people. At 40 generations it needs a trillion in a world of less than 300 million. The numbers very quickly stop being possible.
Where the orange line crosses the green one, your tree is asking for more ancestor-slots than there were people. Something has to give.
3. Pedigree collapse
What gives is the assumption that every slot is a different person. It is not. The same ancestor shows up in the tree many times over, sometimes thousands of times over. This is pedigree collapse: the same nodes feeding multiple lines of descent down to you.
Why it happens
For most of history, the people you might marry were a tiny subset of the world: those who lived in the same village or parish, spoke the same language, were not too closely related but were also not strangers from another continent. Within those pools, distant cousins married distant cousins, generation after generation, without anyone noticing or caring. Royal lines were extreme cases, but the same physics applies to peasants.
How much collapse?
| Generations back | Tree slots | Distinct ancestors (typical) | Collapse ratio |
|---|---|---|---|
| 10 (~300 years) | 1,024 | ~1,000 | almost none |
| 15 (~450 years) | 32,768 | ~25,000 | noticeable |
| 20 (~600 years) | ~1 million | ~200,000 | ~5× |
| 25 (~750 years) | ~33 million | ~1–2 million | ~20× |
| 30 (~900 years) | ~1 billion | maybe 5 million | ~200× |
Distinct-ancestor counts are rough order-of-magnitude estimates for a population mixing on a regional scale. Your figures depend on which population you descend from, and on island and isolated communities pedigree collapse hits much faster.
4. Genetic ancestors versus genealogical ancestors
A subtler twist: even the ancestors who really existed in your tree may have contributed nothing measurable to your DNA. You inherit roughly half your DNA from each parent, but the shuffling is coarse. By the time you go back ten generations or so, most of your genealogical ancestors are not genetic ancestors. Their slot is real. Their DNA in you is zero.
The mechanism: each parent passes you 23 chromosomes that are themselves stitched together from their two parents' chromosomes by recombination, with roughly 30 crossover points per generation. Each generation back, a chromosome is chopped into ever-smaller pieces, and many pieces get lost in the shuffle. By around 10 generations back, you can expect to carry no detectable DNA from a randomly chosen ancestor in your tree.
Why this is not depressing
You still descend from all of them. They each played a small role in the chain that produced you, even if the specific stretches of DNA they contributed have been recombined out of existence. Genealogy is about the lineage of bodies, not the lineage of base pairs.
5. The most recent common ancestor
Take any two living humans. Walk back up both reverse family trees until they meet. The meeting point is your most recent common ancestor (MRCA). The remarkable result, first sharpened by mathematicians Joseph Chang, Douglas Rohde and Steve Olson in the early 2000s, is that the MRCA of everyone alive today probably lived only a few thousand years ago.
| Reference point | Approximate date | What it is |
|---|---|---|
| MRCA of all living humans | ~2,000–5,000 years ago | The most recent person whose descendants include everyone alive now. |
| Identical Ancestors Point | ~5,000–15,000 years ago | The point at which every person then living is either the ancestor of everyone alive today, or of no one. |
| "Y-chromosomal Adam" | ~200,000–300,000 years ago | MRCA along the strictly male-to-male line. Just one thread of the tree. |
| "Mitochondrial Eve" | ~150,000–200,000 years ago | MRCA along the strictly female-to-female line. Another single thread. |
The first two are dizzying. If you read this in the UK, in Lagos, in Jakarta, in Lima, in Auckland: provided your family did not stay on a truly isolated island, you share a great-great-great-...-grandparent with every other reader, perhaps as recently as the Bronze Age. Genghis Khan, Charlemagne, Nefertiti, Confucius, the unnamed farmer next to them: any of these who left living descendants at all are almost certainly your ancestor.
What "Mitochondrial Eve" is not
Mitochondrial Eve was not the only woman alive. She was not even the only woman alive whose descendants survive. She is the most recent woman whose mitochondrial DNA, passed mother-to-child unchanged except for occasional mutations, traces back to her unbroken. Every other woman alive at the same time may also be your ancestor through some other path in the tree. The "Eve" label is misleading in exactly the way you suspect.
6. Genetic intermixing
If everyone has roughly the same ancestors going back far enough, why do humans look different? The answer is that the genome is a vast space, the differences between populations are small fractions of it, and the same ancestors can contribute different shuffled draws to different descendants.
Ancient DNA, sequenced from skeletons over the past fifteen years, has shown that the modern populations of Europe, India, the Americas, East Asia and Africa are each themselves blends of older groups that mixed and re-mixed. Western Europeans, for example, are largely a three-way mix of Pleistocene hunter-gatherers, early Anatolian farmers (around 7,000 years ago) and Bronze-Age steppe pastoralists (around 5,000 years ago). South Asians are blends of ancient North Indian and ancient South Indian populations, themselves shaped by older migrations. There is no pure anything. Mixing is the rule, isolation the exception.
The 0.1% that does the work
Any two humans differ at roughly one base pair in a thousand. Of that variation, about 85% is found within any single local population. Only a small slice maps onto the differences between continents, and even that slice is mostly statistical: gradients, not boundaries. Most of you is shared with most of everyone.
7. Deep time: out of Africa and before
Push the reverse tree past a few thousand years and the human story merges into a single thread. Around 50,000 to 70,000 years ago, the ancestors of every non-African alive today were a relatively small group that left East Africa. Before that, all human ancestry runs through Africa.
Push further. Roughly 600,000 years ago, the lineage leading to modern humans split from the lineage leading to Neanderthals and Denisovans. Most non-Africans carry about 1–2% Neanderthal DNA, and some populations carry a few percent Denisovan, the legacy of brief interbreeding episodes when our ancestors met theirs.
Keep pushing. About 7 million years ago, your lineage shares an ancestor with chimpanzees: a small ape, almost certainly living in central African forest, neither human nor chimp but the common ancestor of both. Around 25 million years ago the broader ape lineage splits off from old-world monkeys. Around 65 million years ago, after the asteroid impact that ended the non-avian dinosaurs, small shrew-like mammals diversified into the lineage that produced primates.
8. The dawn of life
Keep going. The chain of bodies producing bodies, of single cells dividing into other single cells, runs unbroken from you to the first living thing on Earth. Below is a compressed walk along that chain. Each line in the table is an ancestor of yours in the strictest sense: not a relative, not an example, but a creature in the direct line.
| Roughly | What your direct ancestor looked like |
|---|---|
| 200 kya | An anatomically modern human in East Africa. |
| 2 Mya | An early Homo, walking upright, making stone tools, brain about half the modern size. |
| 4 Mya | An australopithecine, bipedal but small-brained, eating fruit and tubers on the African savanna. |
| 7 Mya | A small ape in African forest. Common ancestor with chimpanzees. |
| 25 Mya | A primitive ape, tail-less, in the canopy. |
| 65 Mya | A small, nocturnal, insect-eating mammal, surviving the asteroid impact. |
| 200 Mya | A shrew-sized proto-mammal, warm-blooded, hidden from the dinosaurs. |
| 320 Mya | A reptile-like amniote laying tough-shelled eggs on dry land. |
| 375 Mya | A lobe-finned fish hauling itself through swamp on muscular limbs. Tiktaalik's cousin. |
| 500 Mya | An early jawless fish in Cambrian seas, with a notochord that would become a spine. |
| 550 Mya | A worm-like bilaterian, the body plan from which almost all animal life descends. |
| 700 Mya | A simple multicellular animal: something like a sponge or a placozoan. |
| 1.5 Bya | A single-celled eukaryote: a cell with a nucleus and mitochondria, the result of one ancient cell engulfing another. |
| 3.5 Bya | A prokaryote in a hot ocean: a tiny membrane-bound cell, copying its genetic material and dividing. |
| ~4 Bya | The last universal common ancestor of all life on Earth, LUCA. Before that: chemistry on its way to becoming biology. |
The thing to sit with is that every link in that chain succeeded: each ancestor lived long enough to reproduce, and each child survived long enough to do the same. Run the chain back four billion years and not a single link broke. The base rate for survival in any given generation is low. Multiplied across that many generations, the prior probability of you being here is essentially zero. You are here anyway.
Look up from this page at the next stranger you see. The same is true of them. Somewhere not all that long ago in the scheme of things, the two of you share an ancestor in the strictest sense. Probably not so long ago at all.
Sources include Adam Rutherford, A Brief History of Everyone Who Ever Lived; David Reich, Who We Are and How We Got Here; Chang (1999) on common ancestors; Rohde, Olson and Chang (2004) on the recency of the MRCA; Richard Dawkins, The Ancestor's Tale. Numbers are illustrative; the deep-time dates carry wide uncertainty.