Let me draw this chapter, and the previous one, to a conclusion. Selection - in the form of artificial selection by human breeders - can turn a pye-dog into a Pekinese, or a wild cabbage into a cauliflower, in a few centuries. The difference between any two breeds of dog gives us a rough idea of the quantity of evolutionary change that can be achieved in less than a millennium. The next question we should ask is, how many millennia do we have available to us in accounting for the whole history of life? If we imagine the sheer quantity of difference that separates a pye-dog from a peke, which took only a few centuries of evolution, how much longer is the time that separates us from the beginning of evolution or, say, from the beginning of the mammals? Or from the time when fish emerged on to the land? The answer is that life began not just centuries ago but tens of millions of centuries ago. The measured age of our planet is about 4.6 billion years, or about 46 million centuries. The time that has elapsed since the common ancestor of all today's mammals walked the Earth is about two million centuries. A century seems a pretty long time to us. Can you imagine two million centuries, laid end to end? The time that has elapsed since our fish ancestors crawled out of the water on to the land is about three and a half million centuries: that is to say, about twenty thousand times as long as it took to make all the different - really very different - breeds of dogs from the common ancestor that they all share.
Hold in your head an approximate picture of the quantity of difference between a peke and a pye-dog. We aren't talking precise measurements here: it would do just as well to think about the difference between any one breed of dog and any other, for that is on average double the amount of change that has been wrought, by artificial selection, from the common ancestor. Bear in mind this order of evolutionary change, and then extrapolate backwards twenty thousand times as far into the past. It becomes rather easy to accept that evolution could accomplish the amount of change that it took to transform a fish into a human.
But all this presupposes that we know the age of the Earth, and of the various landmark points in the fossil record. This is a book about evidence, so I can't just assert dates but must justify them. How, actually, do we know the age of any particular rock? How do we know the age of a fossil? How do we know the age of the Earth? How, for that matter, do we know the age of the universe? We need clocks, and clocks are the subject of the next chapter.
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