Part 4 of What If This Never Happened
What If Chicxulub Missed Earth
The asteroid misses. What the evidence says happens next: no people, a world run by dinosaurs, what might have grown clever in it, and how the Sun ends it anyway.
Have you ever wondered what would have happened if the asteroid that ended the dinosaurs had missed?
Sixty-six million years ago a rock ten kilometres wide hit what is now Mexico. Three quarters of all species died, and every dinosaur that was not a bird. The small furry things that survived became, eventually, us.
Now take the rock away. It passes a few hours wide, and the giants never fall. This is not a story with characters. It is what the evidence lets us say about the world that would have followed: whether people could ever have appeared, what might have grown clever instead, what the continents and the Sun do next, and how it ends. Where scientists disagree, or nobody knows, it says so.
Start with what is known. Slightly over sixty-six million years ago an asteroid about ten kilometres across hit the sea floor off what is now the Yucatan peninsula and left a crater roughly two hundred kilometres wide. Three quarters of all plant and animal species on Earth died, and every dinosaur that was not a bird died with them. The animals that came through were mostly small: lizards, turtles, crocodiles, birds, and mammals no bigger than rats, most of them nocturnal insect-eaters that had spent a hundred million years keeping out of the way. In the ten million years after the impact those mammals spread into the empty world, grew large, took to the trees and the sea and the plains, and by the end of the Neogene every modern order of mammal existed. One of them, very late, was us.
Now move the rock. A few hours earlier or later in its orbit, and it misses. Nothing else about that day changes. This is not a story with characters in it. It is what the evidence lets us say, step by step, about the world that would have followed, and where the evidence runs out and guessing begins, this says so.
The first thing to know is that the dinosaurs were not living in a paradise that the asteroid interrupted. In India, at the same time, the Deccan Traps were erupting: a volcanic province that eventually covered about half a million square kilometres in lava a kilometre and more deep, with a volume of about a million cubic kilometres, over six to eight hundred thousand years straddling the boundary. Scientists have argued for decades about how much the eruptions contributed to the extinction. The current consensus is that the impact did the killing and the volcanoes were at most a contributing stress; one 2020 study suggested the eruptions may even have partly offset the cooling from the impact. So without the rock, the world of sixty-six million years ago goes on under a slow volcanic strain, with sulphur in the air and swings in temperature, but with nothing that ends it. The dinosaurs had already survived changes of that size. They keep going.
The second thing to know is that the dinosaurs were not one kind of animal. Birds are dinosaurs, and there were already birds. There were feathered hunters the size of a turkey with forward-facing eyes, and there were animals the size of a house. Take away the extinction and none of these lines is cleared out, so none of them gets the chance to fill each other's places. Evolution does not stop, but it works by editing what is there, and what is there stays there. The large plant-eaters remain the large plant-eaters. The big hunters remain the big hunters. Everything that in our world was done by mammals, from horses to whales to elephants, is done in this world by animals already doing it: by long-necked and duck-billed grazers, by armoured browsers, by marine reptiles in the sea. There is no vacancy, and mammals do not get promoted.
That is the part that matters most for the question everyone asks. Would there be people? Mammals in the Cretaceous were small because being small and nocturnal was the only way to live in a world run by dinosaurs. Our ancestors' larger brains, colour vision, grasping hands and long childhoods were all built in the open daylight of a world where the big reptiles were gone. Keep the dinosaurs and the pressure that made mammals small never lifts. Sixty-six million more years of night-time insect-hunting produces very good night-time insect-hunters. It does not produce apes. The line that leads to us needs the empty world, and the empty world never arrives. On the evidence, the answer is no. There are no people.
Would there be something else? Here the evidence thins and speculation starts, and it is worth being honest about which is which. In 1982 a Canadian palaeontologist named Dale Russell asked what might have become of a small hunting dinosaur called Stenonychosaurus, later folded into the group known as troodontids, which had one of the largest brains for its body size of any dinosaur, good eyes and grasping hands. He reasoned that if its line had survived, its brain case could by now measure eleven hundred cubic centimetres, comparable to a human's, and that its descendants might have evolved to fill the same place in the world that humans fill. He had a model built: upright, two-legged, big-headed, with a face. Most palaeontologists who have looked at it since say the same thing. The reasoning about brain size is a fair extrapolation of a real trend. The body is the problem. It looks suspiciously like a person, because a person designed it, and there is no reason an intelligent dinosaur would give up its horizontal spine, its tail and its long legs to stand like us. The honest version of Russell's idea is this: a clever, tool-using, warm-bodied bird-relative is not impossible on a dinosaur Earth, and if one existed it would look nothing like us and might think nothing like us either. Nobody can say whether it would build anything. Crows and parrots, which are living dinosaurs, use tools and solve problems and have never built a city, and they have had the same sixty-six million years we have.
So the most likely dinosaur Earth, tens of millions of years on, is a world with no cities, no roads and no lights at night, in which the largest land animals are still reptiles, the seas are still hunted by reptiles, the forests are full of birds, and the most intelligent thing alive is something we would not immediately recognise as intelligent. It is a world without written history. Nothing in it is keeping records. Everything we know about the past sixty-six million years, we know because we are here to dig it up. There, nothing digs.
Then the longer clock starts, and this part does not depend on the rock at all.
In about two hundred and fifty million years the continents, which never stop moving, are expected to gather again into one supercontinent, as they have done before. Whatever lives then lives on a single landmass with a vast dry interior and a single ocean, the kind of world the earliest dinosaurs first walked on. Their descendants would be walking on it again.
The Sun is slowly getting brighter, and that is the thing that ends the story on any version of Earth. As it brightens, the weathering of rock speeds up and draws carbon dioxide out of the air. In roughly six hundred million years the level falls below what most of today's trees and plants need to run their kind of photosynthesis. In about eight hundred million years it falls below the level the tougher grasses can use. Forests go first, then grasslands, and with the plants go the animals that eat them and the animals that eat those. Within roughly a billion years the seawater begins to be lost, some of it drawn down into the mantle and the rest boiled off into space, and the models put the loss of the oceans at somewhere between one and one and a half billion years from now. One 2024 study argued that life could hang on longer than that under the right conditions, perhaps until about 1.86 billion years, before the last plants and animals are starved of carbon and oxygen. After that there are only microbes, and then there is rock.
That is the same clock for the dinosaurs as for us. The rock that missed buys them sixty-six million years and everything they might have become in it. It does not buy them the Sun. On a dinosaur Earth the forests that die in six hundred million years are forests no one ever named, and the seas that boil off a billion years later are seas nothing ever crossed on purpose. Unless Russell's clever hunter, or something like it, learned in the meantime to leave.
There is one more thing the evidence allows. The rock that missed does not vanish. An asteroid that passes Earth stays on an orbit that crosses ours, and comes round again. On an Earth where nothing is watching the sky, that is how the story would most likely end, sooner or later, exactly as it did: with a bright line over the sea, and small furred things in the leaf litter, waiting.