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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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After the asteroid impact

How North America got its plants and animals back

My latest Mind and Matter column in the Wall Street Journal is about what happened to the cology of North America after the asteroid impact of 66 million years ago:


Last week, just as a meteorite exploded over Russia, I used this space for an email to Charles Darwin, wherever he is. I told him about the now overwhelming evidence for an asteroid impact having caused the extinction of dinosaurs. I thought he would be interested because it is a striking exception to his "uniformitarian" assumption that, in the past, evolution was shaped by the same forces still operating on Earth today.

His ghost emailed back to say that he wanted to know what happened after the impact. He always had a long-standing interest in colonization—how animals and plants find and fill vacant lots in nature. So here's my reply:

To crdarwin@evolution.hvn

Dear Dr. D (sorry for my informality last week),

When the asteroid slammed into the Yucatán Peninsula 66,038,000 years ago, North America took the brunt of the impact, because the asteroid came in from the southeast like a golf chip shot. Most of what is now the American Midwest was then under shallow water called the Bearpaw Sea, but the land on either side was devastated. The shock wave destroyed almost every living thing for hundreds of miles, then a "divot" of dust and mud buried the continent, quickly followed by tsunamis that penetrated far to the north.

The Earth's southern continents suffered far fewer extinctions, though there was still a marked drop in the abundance and diversity of animals and plants. Nonetheless, the effects of the collision were felt globally. All around the world, the dinosaurs died out—too big and specialized to cope, even if they survived the blast. With them went the last of the flying pterosaurs, many kinds of birds and mammals, lots of plants and a huge range of sea creatures.

The animals that suffered the fewest extinctions were ones like alligators, turtles and salamanders. Perhaps protected by the water, while feasting on the dead or on water plants that did not suffer so much, they were also used to withstanding long fasts.

When the dust settled, as the paleontologist Tim Flannery recounted in a fine book "The Eternal Frontier," North America was virtually devoid of flora and fauna, except in the far North and in pockets to the west of the Sierra Nevada mountains. It must have been a great brown, dusty and muddy waste. Pollen evidence shows that ferns were the first plants to bounce back, just as they were after the eruptions of Krakatau and Mount St. Helens. The pollen and spore record shows a sharp "fern spike." Soon ferns must have dominated the landscape so much that they formed endless, impenetrable thickets.

When trees did recolonize, they did so from the far north (the North Pole was under land and ice-free at this time), meaning that deciduous types of trees came to dominate even warm parts of the North American continent. Mammals reappeared within 20,000 years, probably from Asia via an Arctic land bridge. South America was off doing its own thing, farther south than it is now, and could not supply animals, apart from birds, to its northern neighbor.

All this we now know. The amazing thing about this story, Charles, is that not a jot of it was known even when I was a boy, let alone you. There is something almost miraculously exciting about the way we can recover and relive the past adventures of the planet in glorious detail thanks to modern science.

You once wrote, in a letter to your sister from the Falkland Islands in 1834, of the pleasure of "finding a fine group of fossil bones, which tell their story of former times with almost a living tongue." Today, almost 180 years later, the fossils, isotopes and spores not only speak softly of slow and gradual changes but cry out the story of a violent and truly terrible catastrophe that once transformed life on Earth.