My son, Billy, was born three weeks and two days ago. People say he looks like me, but he very obviously looks like a baby, or at a pinch Sir Winston Churchill; he sleeps, and feeds, and poos, and when he wants us to help in the process of one of those things, he cries. He’s an ordinary baby – objectively and unarguably better than all other babies ever, of course, but not qualitatively different. Four limbs, 20 digits, blue eyes, good lungs, comical facial expressions.
But an “ordinary baby” is an extraordinary thing. He is the product of a direct unbroken chain of information transfer going back 1.5 billion years, and, in a complicated fashion, far, far more. His tiny, pot-bellied little body is made of some number of trillions of cells – liver cells, brain cells, smooth and striated muscle cells, red blood cells, macrophages. Each of them is a descendant of two fused cells, my sperm and his mother’s egg which combined their divided chromosomes to form an embryo. And those cells are themselves descendants – through cell division in my body, and his mum’s, and before that the odd unzipping dance of meiosis – of the eggs and sperm in his grandparents, and their parents, and so on. (If you want to read more about this, I recommend Adam Rutherford’s lovely book Creation.)
His grandfather, my dad, was born in 1952 – three generations of Chiverses born under Queen Elizabeth II, a fact I find both strange and comforting. But I want to look further back than that.
My wife and I both have entirely British ancestors for several generations; the likelihood is, then, that the backwards trace of our ancestries would bump into each other pretty rapidly, certainly within the last thousand years; the 30 or so generations in those last 1,000 years would mean that we’d each have over a billion ancestors, more than the world population at the time. We’re all cousins of varying degrees. What that means is that at some point, going backward, you reach the most recent common ancestor (MRCA) of all humans; estimates put that at somewhere between 2,000 and 3,500 years ago, depending on how much interbreeding there was between different populations, although I have seen other estimates of up to 10,000 years. At some point in that area, there lived a man or woman who was not only the N-greats grandparent of Billy, and the (N-1)-great grandparent of me, but of you, and Barack Obama, and of Miley Cyrus, and every other human being alive. All the way through those thousands of years, those cell divisions were carrying on; the vast majority of cells were an evolutionary dead end, because they made skin cells or liver cells and died with the body. But a tiny minority were sperm and egg cells that joined to form new humans; the information in the chromosomes is shuffled and passed on, shuffled and passed on.
Go back a bit further, and you reach the ACA point – all common ancestors. That’s been estimated as between 7,000 and 5,000 years ago. At that point, everyone who was alive then is either the ancestor of everyone who’s alive now, or the ancestor of nobody at all. Billy’s great-great-etc grandparents, at some point perhaps around the birth of the Sumerian civilisation, included 80 per cent or so of everyone alive; the rest were dead ends. (I should admit, for the sake of perfectly reasonable people in the comments who have pointed this out, that the ACA and MRCA figures are probably underestimates as they assume no completely isolated human populations – but there were, such as the Tasmanian aboriginals.)
Further back, those chromosomes zipping and unzipping (by the way, at this stage, we meet lots of people who are the ancestors of everyone but who also have precisely none of their genes still extant in the population. That’s just a quirk of how the genetic shuffling works), the same germ line of dividing cells linking Billy to his ancient ancestors. We’re still talking about utterly modern humans at this stage; the sort of humans who you could drop into a London pub without attracting the slightest comment, assuming you either changed their clothes or made sure it was Dalston. Keep going back, to about 50,000 years ago. Humans are still human, but until now they use far less sophisticated tools – barely differentiated stone hand-axes – and they probably don’t know how to make fire. Language may have barely begun (or it may have been much older). Then at around 50,000 years ago, there seems to have been a “great leap forward” – a flowering of culture, sophisticated art, long-distance trade. What caused this is unknown, although language has been put forward as a candidate. Billy’s forebears, all living in Africa at this point, start a journey towards modernity.
Go back. A hundred and fifty or so thousand years earlier, we stop calling Billy’s ancestors “Homo sapiens” and start meeting older species of humans – although, of course, the line is an arbitrary one; every daughter is the same species as her mother. The change is far too gradual to spot. Slowly we move into Homo heidelbergensis, Homo antecessor, Homo erectus (the tall-standing man), Homo habilis (the handy man). As we walk backwards through time the humans become heavier-set and hairier, with smaller brains, looking steadily more like the apes with which we share an ancestor. At about 2.5 million years – again, arbitrarily; the line is hotly debated and blurred – scientists stop referring to them as “Homo”, human, and start calling them “Australopithecus”, southern apes, like Lucy, the famous ancestor discovered in 1974. Further back and further back, to about 6.3 million years ago, when we meet someone, or something, who is the ancestor of both my son Billy and the chimpanzees in Dorset’s Monkey World.
By the way, if you’re wondering where all these dates come from, they have various sources. One is something called the “molecular clock”; the amino acids in any given protein change at a relatively stable rate through evolutionary time, a rate which has been determined by comparison with other dating methods (radioactive dating, geological layers, etc), so by comparing the proteins in two species you can estimate how recently they shared an ancestor. (And if you like this stuff, Richard Dawkins’s The Ancestor’s Tale does the amazing backwards journey through time much better than I can.)
I’ll speed the process up now. Zoom back through common ancestors with gorillas (about 9 million years ago), and orang utans (15 million). Then the split with smaller apes, then the split between Old World monkeys and apes, and then Old World monkeys and New World monkeys, around the 30 million year mark. By the time you get back to the end of the dinosaurs, 65 million years ago, Billy’s great-great-etc grandad was small, probably nocturnal, squirrel-like and doing its best to survive on insects and so on while a giant asteroid smashed the skies into darkness and slayed all the giants. (Except the ones who later became birds. The domestic chicken is by any measure more like Tyrannosaurus rex than was its fellow “dinosaur” triceratops, as XKCD points out.)
Further back. The first mammal-like things appeared between 220 and 160 million years ago. Billy’s ancestors split from the ancestors of birds and modern reptiles about 250 million years ago. Three hundred million years ago his ancestor was a lizard-like creature; 365 million years ago it was an amphibian, not unlike a modern salamander or newt. By 400 million years ago it was more like a lungfish or mudskipper, capable of coming out of the water but essentially an aquatic creature.
Back through a hundred million years or so of fish, variously armoured and scaled and naked, to the first vertebrates; ugly, slimy jawless fish-things like the modern lamprey or hagfish. At about 530 million years ago, Billy’s ancestor is a thing called Pikaia (or at least something like it): a proto-vertebrate with a sort of streak up its back where a spine will one day evolve; about 550 million years ago is the first creature with a brain, a flatworm thing.
More on human evolution
That’s the time of the famous “Cambrian explosion”, when the first really complex life starts to appear in the fossil record. Before that it was sponges, before that single-celled organisms. About 1.2 billion years ago, sexual reproduction begins; before that it is meaningless to talk in terms of Billy’s “great-grandfathers” and “grandmothers”, it’s just asexually cloning blobs. And about 1.5 billion years ago, there’s the great moment when an archaeon, a sort of single-celled life form, enveloped a bacterium, giving rise to the eukaryotic cell (a complex thing with lots of tiny subsections, like mitochondria and a nucleus) which is the ancestor of everything from elephants to yeast infections, via oak trees and great white sharks. Billy, by this stage, shares an ancestor with everything alive isn’t a bacterium or an archaeon. All the way back to that point, you can trace a direct line, of cell division after cell division, from this impossibly ancient speck to the nerve cells in Billy’s fingertips.
Life goes back beyond this, although it becomes impossible to trace and largely meaningless to talk about “descent” since the prokaryotes – the simple, single-celled organisms, the bacteria and archaea – swap genes between them readily, so it stops being a tree of life and becomes a sort of tangled thicket. But the process remains the same, of DNA copying itself, replacing itself, back to what is called the Last Universal Common Ancestor, Luca – a prokaryote of some form that split, one daughter of which became all bacteria, one daughter of which became all archaea. And one of those archaea then became the ancestor of all complex life.
No one knows when Luca lived – the guesses cover billions of years. We know that the first life arose about four billion years ago, when the world was only half a billion years old and was still battered and boiling by the creation of the solar system. But we know that if we could find it, we can play the tape forward, and it would spring forth into an uncountable multitude of forms, skylarks and earthworms and mangrove trees and dinosaurs and humans. And, eventually, via cell division and (eventually) sexual reproduction, in an unbroken if confused line, into my son Billy. Happy four billionth birthday, son.
More by Tom Chivers