If the rock had arrived twenty minutes earlier, or twenty minutes later, you would almost certainly not be reading this. That is not a way of speaking. It is what a number of scientists now believe about the worst morning in the history of life on earth.
The story of how we learned this begins with a father, a son, and a strange line in a wall of Italian rock.
In the 1970s a geologist called Walter Alvarez was working near a small town in Italy, on rock that had been laid down layer by layer for millions of years. In one place he found a thin band of clay, about as thick as your finger. Below it, the rock was full of tiny sea animals. Above it, they were gone. Something had happened in the time it took that finger of clay to form, and almost everything alive had been deleted.
His father, Luis Alvarez, was a physicist, and he had an idea about how to time it. They tested the clay for iridium, a metal that is very rare in the rocks of the earth's surface but common in the rocks that fall out of space. The clay held about thirty times more iridium than it should have. The same band was then found in other countries, always at the same moment in the record.
In 1980 the two of them published the answer they had reached: something enormous had hit the earth, and it had killed the dinosaurs. Many geologists thought the idea was ridiculous. There was no crater.
The crater was there. It had simply been looked at by the wrong people. In the 1970s, men searching for oil off the coast of Mexico had mapped a great buried ring under the sea floor and the northern edge of the Yucatán, and their maps had been filed away as a company secret. When they were finally read by the right scientists in 1991, the ring turned out to be about 180 kilometres across. It is called Chicxulub, after a village that sits on top of it.
Here is what those numbers mean. A rock somewhere between 10 and 14 kilometres wide — a small mountain — came in at about 20 kilometres a second, roughly sixty times faster than a passenger plane. It went through the air, the water and the sea floor in about two seconds. Anything within a thousand kilometres was killed at once, and stone thrown up by the impact fell back all over the planet, hot, so that the sky itself glowed like the inside of an oven. Then the dust rose, and for perhaps two years the sun could not be seen from the ground. Plants could not work in the dark; the animals that ate plants died, and then the animals that ate those animals died. About three quarters of all species on earth were lost.
And now the minutes. What made this impact so much worse than others was not only the size of the rock, but where the earth happened to have turned as it arrived. Chicxulub sat under a shallow sea, and the rock beneath that sea was full of sulfur. The impact turned that sulfur into vapour and threw it high above the weather, where it stayed for years, reflecting sunlight away. The cold came from the sulfur.
Most of the planet is deep ocean. Had the rock come down there, the water would have taken much of the force and far less sulfur would have been lifted. Had it come down on ordinary hard rock, the same. The earth turns about 1,600 kilometres an hour at that line, so half an hour either way would have put a different, safer part of the planet under it. The dinosaurs might have lost a bad century and kept the world.
It should be said that not every scientist accepts how neat this is; the argument about how much difference the sulfur made is still running. But the shape of it stands. The animals that came through were small, and many of them lived in water or underground — including our own distant relatives, who had been hiding under the feet of the dinosaurs for a hundred million years without ever getting anywhere.
The dinosaurs, by the way, are not entirely gone. Birds are dinosaurs; the pigeon on your windowsill is a survivor of that morning. But the great ones went, and the small warm animals came out of their holes into an empty world, and everything you have ever heard of happened afterwards.
There is no lesson in it. That is what makes it hard to hold in the mind. Everything you are came out of a gap of a few minutes, sixty-six million years ago, and nobody chose it.