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past, and look at some of the stations on the way back to that fish. Suppose we have just arrived in our time machine at the station labelled ‘Six Million Years Ago’. What shall we find there? So long as we make a point of being in Africa, we’ll find our 250,000-greats-grandparents (give or take some generations). They’ll be apes, and they might look a bit like chimpanzees. But they won’t be chimpanzees. Instead, they’ll be the ancestors that we share with chimpanzees. They’ll be too different from us to mate with us, and too different from chimpanzees to mate with chimpanzees. But they will be able to mate with the passengers we took on board at Station Five Million Nine Hundred and Ninety Thousand Years Ago. And probably those from Station Five Million Nine Hundred Thousand Years Ago, too. But probably not those who joined us at Station Four Million Years Ago.
    Let’s now resume our ten-thousand-year hops, all the way back to Station Twenty-Five Million Years Ago. There we shall find your (and my) one-and-a-half-million-greats-grandparents – at an approximate estimate. They will not be apes, for they will have tails. We would call them monkeys if we met them today, although they are no more closely related to modern monkeys than they are to us. Although very different from us, and incapable of breeding with us or with modern monkeys, they will breed happily with the all-but-identical passengers who joined us at Station Twenty-Four Million Nine Hundred and Ninety Thousand Years Ago. Gradual, gradual change, all the way.
    On we go, back and back, ten thousand years at a time, finding no noticeable change at each stop. Let’s pause to see who greets us when we reach Station Sixty-Three Million Years Ago. Here we can shake hands (or should that be paws?) with our seven-million-greats-grandparents. They look something like lemurs or bushbabies, and they are indeed the ancestors of all modern lemurs and bushbabies, as well as the ancestors of all modern monkeys and apes, including us.
    They are as closely related to modern humans as they are to modern monkeys, and no more closely to modern lemurs or bushbabies. They wouldn’t be able to mate with any modern animals. But they would be able to mate with the passengers we picked up at Station Sixty-Two Million Nine Hundred and Ninety Thousand Years Ago. Let’s welcome them aboard the time machine, and speed on backwards.
    At Station One Hundred and Five Million Years Ago we’ll meet our 45-million-greats-grandfather. He is also the grand ancestor of all the modern mammals except marsupials (now found mostly in Australia, plus a few in America) and monotremes (duckbilled platypuses and spiny anteaters, now found only in Australia/New Guinea). He is equally closely related to all modern mammals, although he may look a bit more like some of them than others.
    Station Three Hundred and Ten Million Years Ago presents us with our 170-million-greats-grandmother. She is the grand ancestor of all modern mammals, all modern reptiles – snakes, lizards, turtles, crocodiles – and all dinosaurs (including birds, because birds arose from certain kinds of dinosaur). She is equally distantly related to all those modern animals, although she looks more like a lizard. What that means is that lizards have changed less since her time than, say, mammals have.
    Seasoned time-travellers as we are by now, it isn’t far to go until we hit the fish that I mentioned earlier. Let’s make one more stop on the way: at Station Three Hundred and Forty Million Years Ago, where we meet our 175-million-greats-grandfather. He looks a bit like a newt, and is the grand ancestor of all modern amphibians (newts and frogs) as well as of all the other land vertebrates.
    And so to Station Four Hundred and Seventeen Million Years Ago and your 185-million-greats-grandfather, the fish we met before. From there we could go on even further back in time, meeting more and more distant great-grandparents,