250 million is the new 40: Mammals may already be halfway done on Earth, study finds

You can say this about mammals: We've had a good run.

Over the 250 million years since the first mammals diverged from reptiles and birds, we hairy, warm-blooded vertebrates have come to dominate the Earth, with rodents and humans and ungulates and whales living on nearly every inch of the planet.

But the end to that reign may come much sooner than traditionally thought, suggests a new study published this week in the journal Nature Geoscience.

Scientists have long thought that life on Earth will continue until the planet enters its "runaway greenhouse" phase — a state in which water vapor and other greenhouse gasses in the atmosphere become so dense that heat can't escape the surface and the oceans boil away, like the planet Venus. That's 2 or 3 billion years away.

Mammals will run into problems much sooner due to our limited ability to resist heat, the study concludes. Our mechanisms to avoid overheating — like sweat, circulatory systems and the ability to physically move to cooler environments — can only go so far, and evolution can only move so fast, said Alexander Farnsworth, a climate researcher at the University of Bristol and lead author on the study.

Eventually, "Earth could reach a tipping point rendering it uninhabitable to mammalian life," researchers wrote.

The study puts mammals' end date at about 250 million years from now — roughly the same amount of time since mammals first appeared.

It's difficult to make predictions about what the planet will look like such a long time from now; for comparison, modern humans have only existed for about 50,000 years and have changed the planet dramatically in that time.

Still, Farnsworth and his colleagues describe three major factors that will drive up temperatures on Earth.

The big one: a future supercontinent. Over Earth's history, continents have undergone a cycle of converging into supercontinents, then again dispersing into the separate continents as we see today. Predictions suggest the next cycle will bring together our current continents into a supercontinent, called Pangea Ultima, centered along the equator.

"Instead of having continents scattered around the world, all of them are now inhabiting most of the tropics. And, of course, we know the tropics is a very, very hot and humid place to live. So that's already going to have a big impact," Farnsworth said.

Adding to that will be a surge of volcanic activity, the study says. Historically, supercontinents have coincided with an increase in volcanism that will release more carbon dioxide into the atmosphere — perhaps double the current amount, researchers predict.

And all the while, as the sun ages, it will emit more energy — an estimated 1% more every 100 million years — adding even more heat to the equation.

Altogether, Farnsworth said, much of the supercontinent would have average monthly temperatures in excess of "40, 50 or even 60 degrees centigrade" — the equivalent of 104, 122 or 140 degrees Fahrenheit.

"It's not going to be a very happy place for most species to live," he said.

Perhaps only 8% of Earth will be habitable for mammals, according to the study. And competition will be stiff for those slivers of habitable land — which would be located mostly along the edges of what are currently Canada, Russia and Chile. Reptiles could re-emerge as the dominant type of life, or birds could win out.

Mammals' best hope for survival through Earth's Pangea Ultima era could look something like the North African gerbil, he said — a small rodent that thrives in the harsh conditions of the Sahara desert. "It doesn't need too much moisture, too much water availability or too much food," he said. "Who knows? Maybe they radiate outwards and mammals re-evolve from these lineages to something new."

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