A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

If that meteor strike hadn’t killed off the dinosaurs, how would life be different for the human race?

September 13, 2013

Dear Cecil:

If there had never been a dinosaur extinction due to a meteor strike, would human life have evolved into what it is today? Clearly the meteor that struck Earth had an enormous impact and changed the course of evolution, but is there anything to indicate human lineage development before or after the little big bang?

Cecil replies:

Well, I’ll tell you one thing. They’d have to take the The Flintstones out of the cartoon section and refile it as reality TV.

Beyond that we get into some pretty woolly speculation. We’re trying to guess hypothetical planetary-level changes over millions of years, when we aren’t really sure what actually did happen. Never fear, this is the Straight Dope. Woolly speculation is what we do.

What we’re talking about is a massive extinction of terrestrial life about 66 million years ago called the Cretaceous-Paleogene extinction, or the K-Pg event. More than 70 percent of all animal species were wiped out, making K-Pg the fifth-worst extinction in history. Bear in mind we’re talking loss of species here, not necessarily numbers of individual critters. During an extinction, the species that perish are those that can’t adapt, leaving the planet to those that can.

No question, something went horribly wrong on earth around the time of K-Pg. The scientific consensus is that a major asteroid or comet strike occurred, but evidence abounds of multiple catastrophes, perhaps additional strikes or massive volcanic eruptions. One possibility is that our planet passed through a field of space debris; another is that a much larger object with a long orbital period broke into pieces, several of which crashed into earth during a relatively short span of geologic time.

The point is, the idea that a single cosmic collision threw up huge clouds of debris that froze everything on the planet is way too simple. Drastic climate changes were happening for as much as a million years prior to the K-Pg event, including huge drops in sea level, abrupt warmings, and six major global freezes. One ice age reduced average global temperature by more than 14 degrees Fahrenheit less than 100,000 years before K-Pg.

Still, there’s no disputing the pterosaurs, large marine reptiles, and dinosaurs (except proto-birds) were wiped out during K-Pg. Up to 15 percent of all marine families were killed off, although 80 to 90 percent of sharks and other fish survived. Turtles, crocodiles, lizards, and snakes survived, and amphibians were largely untouched. In North America up to 57 percent of plant species disappeared, and some parts of the globe saw a 90 percent loss.

Mammals had coexisted with dinosaurs for quite a while prior to K-Pg, but after it many species, notably placental mammals, exploded in significance. Previously mammals had been relatively small, due undoubtedly to competition from dinosaurs. Over the next 35 million years, with their competition out of the game, mammals increased dramatically in size.

And why not? In some ways it was a golden age. The Eocene epoch, beginning roughly 56 million years ago, saw a rise in global temperatures and the spread of vegetation, covering the earth in forest. Atmospheric oxygen levels increased significantly, peaking at about 35 million years ago. Land area increased, providing more habitat. And mammal size increased too, culminating circa 30 million years ago with the indricotherium, the largest land mammal known: 18 feet tall at the shoulder and weighing 30 tons, it looked like a rhino crossed with an okapi, but bigger than a one-car garage.

But here’s the thing. Having attained dino scale, mammals then dialed back somewhat, becoming smaller on average. Why? Probably because the environment became harsher and more variable. The Miocene epoch, ranging from 23.5 million to 5.3 million years ago, was markedly cooler than previous eras, with more distinct seasons. This was followed by the even colder period that began 2.7 million years ago: after North and South America joined, intensifying the Gulf Stream and global precipitation, the earth’s axial tilt then shifted, resulting in more than 20 ice ages.

These changes probably doomed dinosaurs regardless. Mammals, being warm-blooded, were far better equipped to survive the cold, and their more efficient jaws made it easier to chew and extract energy from limited resources. Placental mammals had further advantages — extra nurturing in the womb allowed more advanced physical features to develop.

So chances are the world wouldn’t look much different today had the K-Pg event never happened. That’s disappointing, I know — childhood dreams of a pet stegosaurus die hard. We can of course arbitrarily declare that dinosaurs would have persisted somehow, and conjure up scenarios where bipedal apes in warm climes compete with fast, small, carnivorous dinosaurs.

My heart isn’t in it, though. As it is, human hunters wiped out numerous species bigger, stronger, and faster than themselves; chances are we’d have done the same to dinosaurs if any had lasted long enough to become our prey.

Still, let’s not get smug. It’s one thing to adapt to climate change when it’s spread over thousands of millennia. We’ll see how well we manage when it happens in 50 years.

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References

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Jablonski, David, and W. G. Chaloner. "Extinctions in the Fossil Record [and Discussion]." Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 344.1307 (1994): 11-17.

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Keller, Gerta, et al. "Main Deccan volcanism phase ends near the K–T boundary: Evidence from the Krishna–Godavari Basin, SE India." Earth and Planetary Science Letters 268.3 (2008): 293-311.

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O'Leary, Maureen A., et al. "The placental mammal ancestor and the post–K-Pg radiation of placentals." Science 339.6120 (2013): 662-667.

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Vajda, Vivi, and Stephen McLoughlin. "Extinction and recovery patterns of the vegetation across the Cretaceous–Palaeogene boundary—a tool for unravelling the causes of the end-Permian mass-extinction." Review of Palaeobotany and Palynology 144.1 (2007): 99-112.

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Wilf, Peter, Kirk R. Johnson, and Brian T. Huber. "Correlated terrestrial and marine evidence for global climate changes before mass extinction at the Cretaceous–Paleogene boundary." Proceedings of the National Academy of Sciences 100.2 (2003): 599-604.

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