Sifting Through Photographs of Our Ancestors to See Evolution

Find a picture of yourself. Now take a picture of your father and place it on top. Then find a picture of his father, your grandfather. Then place on top of that a picture of your grandfather's father, your great-grandfather. You may not have ever met any of your great-grandfathers. I never met any of mine, but I know that one was a country schoolmaster, one a country doctor, one a forester in British India, and one a lawyer, greedy for cream, who died rock-climbing in old age. Still, even if you don't know what your father's father's father looked like, you can imagine him as a sort of shadowy figure, perhaps a fading brown photograph in a leather frame. Now do the same thing with his father, your great-great-grandfather. And just carry on piling the pictures on top of each other, going back through more and more and more great-great-greats. You can go on doing this even before photography was invented: this is a thought experiment, after all.

How many greats do we need for our thought experiment? Oh, a mere 185 million or so will do nicely!

[...]

The near end of the bookshelf has the picture of you. The far end has a picture of your 185-million-greats-grandfather. What did he look like? An old man with wispy hair and white sidewhiskers? A caveman in a leopard skin? Forget any such thought. We don't know exactly what he looked like, but fossils give us a pretty good idea... Your 185-million-greats-grandfather was a fish. So was your 185-million-greats-grandmother, which is just as well or they couldn't have mated with each other and you wouldn't be here.

Let's now walk along our three-mile bookshelf, pulling pictures off it one by one to have a look at them. Every picture shows a creature belonging to the same species as the picture on either side of it. Every one looks just like its neighbours in the line - or at least as much alike as any man looks like his father and his son. Yet if you walk steadily from one end of the bookshelf to the other, you'll see a human at one end and a fish at the other. And lots of other interesting great-... great-grandparents in between, which, as we shall soon see, include some animals that look like apes, others that look like monkeys. others that look like shrews, and S(SO on. Each one is like its neighbours in the line, yet if you pick any two pictures far apart in the line they are from humans back far enough you^u come to a a fish. Flow can this be?

Actually, it isn't all that difficult to understand. We are quite used to gradual changes that, step by tiny step, one after the other, make up a big change. You were once a baby. Now you are not. When you are a lot older you'll look quite different again. Yet every day of your life, when you wake up, you are the same person as when you went to bed the previous night. A baby changes into a toddler, then into a child, then into an adolescent; then a young adult, then a middle-aged adult, then an old person. And the change happens so gradually that there never is a day when you can say, 'This person has suddenly stopped being a baby and become a toddler.' And later on there never comes a day when you can say, 'This person has stopped being a child and become an adolescent.' There's never a day when you can say, 'Yesterday this man was middle-aged: today he is old.'

[...]

You are Homo sapiens and your 50,000-greats-grandfather was Homo erectus. But there never was a Homo erectus who suddenly gave birth to a Homo sapiens baby.

So, the question of who was the first person, and when they lived, doesn't have a precise answer. It's kind of fuzzy, like the answer to the question: When did you stop being a baby and become a toddler? At some point, probably less than a million years ago but more than a hundred thousand years ago, our ancestors were sufficiently different from us that a modern person wouldn't have been able to breed with them if they had met.

Whether we should call Homo erectus a person, a human, is a different question. That's a question about how you choose to use words - what'* called a semantic question. Some people might want to call a zebra a stripy horse, but others might like to keep the word 'horse' for the species that we ride. That's another semantic question. You might prefer to keep the words 'person, 'man and 'woman for Homo sapiens. That's up to you. Nobody, however, would want to call your fishy 185-million-greats-grandfather a man. That would just be silly, even though there is a continuous chain linking him to you, every link in the chain being a member of exactly the same species as its neighbours in the chain.

A great thought-experiment that takes us all the way back to when our ancestor was a fish, but shows us that the neighbors of any ancestor looked identical.