Evolution, extinction and asteroids

My latest Mind and Matter column for the Wall Street Journal, published the day after a big asteroid missed the earth by 17,000 miles and a smaller one blew out windows in Russia, is about the huge one that extinguished the dinosaurs just over 66 million years ago:

The future has a richer past than the past did. By this I mean that one of the great benefits of modern science is that it enriches our knowledge of the past. Imagine how thrilled Charles Darwin would have been to learn this week that it's now all but certain that the extinction of the dinosaurs was caused by an asteroid (much bigger than the one that missed us this week) slamming into Mexico about 66,038,000 years ago. In fact, I might send him an email to explain.

To: crdarwin@evolution.hvn

Hi, Charlie (if I may),

You know how you championed the cause of "uniformitarianism" in geology? For instance, that fossils of sea creatures on mountain tops weren't put there by big catastrophes, like Noah's flood, but by unimaginably slow and gradual changes of the same kind we see today acting over immensely long periods. Well, you were mostly right, but there's now an important exception.

Don't worry, it doesn't involve Noah. In 1980, father-and-son scientists, Louis and Walter Alvarez, found a thin layer of enriched iridium in rocks from the end of the Cretaceous period, when the dinosaurs died out. Iridium is rare on Earth, more common in space. Maybe, they said, a comet or an asteroid hit the earth, and the ensuing dark, cold, acidic conditions killed off the dinosaurs and a lot of other creatures.

Many geologists and zoologists resisted the idea, if only because it seemed a return to the special pleading of catastrophism that your friend the geologist Charles Lyell had first challenged. But in 1990, based on the work of a geologist named Glen Penfield, a crater was identified—a 110-mile-wide circle dating to the same period as the extinction event and buried deep beneath limestone centered on Chicxulub, on the northern shore of the Yucatán Peninsula.

Other evidence accumulated—shocked quartz grains, spherules of molten rock, charcoal from forest fires, tsunami beds, even a few fragments of the object itself, a carbonaceous chondrite asteroid. The layer of clay that separates the age of the dinosaurs from the one that followed bears witness to a lot of sediment in the air and water. A rock, 6 to 9 miles across, had slammed into the shallow sea, instantaneously opening a 2-mile-deep crater in the Earth's crust with the force of a billion Hiroshimas (which I'll explain later).

By the turn of the 21st century, even the scientists most committed to gradualism had to admit that a very big collision had happened. But many resisted the conclusion that it had wiped out the dinosaurs, chiefly because the dates seemed not to match: Several studies put the impact 180,000 years too early. Others said it came too late.

Now Paul Renne at the University of California at Berkeley and colleagues have used an argon/argon dating technique to narrow down the timing of both the extinction (in a layer of coal from Montana) and the impact (in specks of molten rock from Haiti). Using the technique, which depends on the radioactive decay of a potassium isotope into argon gas, Dr. Renne was able to establish that the two events coincided, in his words, to within "a gnat's eyebrow": to within 11,000 years of each other.

Dr. Renne and colleagues do concede that the climate had become more unstable before the crash, with a sharp cooling of eight degrees Celsius evident in North Dakota about 100,000 years earlier, perhaps because of huge volcanic eruptions. That might (or might not) have weakened the dinosaurs' resistance and made them more vulnerable, but there's now little doubt that it was the impact that finished them off.

Charlie, I have to dash now, but ping me if you want another email next week about what happened after the asteroid hit and how North America, which bore the brunt of the devastation, gradually got its plants and animals back.