Why did mammals evolve to enormous sizes - then get smaller again?

After the dinosaurs were wiped out by a massive meteor strike 65 million years ago, tiny mammals ruled the Earth. But they quickly evolved to be as big as dinosaurs, then evolved to be smaller again. What happened?


This is what paleontologists have long wondered. Scientists knew that extremely large mammals evolved within a few million years after the end of the dinosaurs. As evolutionary biologist Patrick Stephens puts it:

There is a much better fossil record for mammals than for many other groups. That's partly because mammals' teeth preserve really well. And as it happens, tooth size correlates well with overall body size.

But how many of these mammals there were, and why they evolved, have been a mystery until a group of international scientists pooled all the information they had about these giant mammals into one database.

What they discovered was extraordinary: Giant mammals, long believed to be somewhat rare, were common across the entire planet. It seems they grew to fill an ecological niche left by the dinosaurs, aided by a cooling climate and greater amounts of land mass that supported large body sizes. Even more interesting is that mammals didn't reach some "upper biomechanical limit" to their body sizes - they could have grown much larger. The only thing that prevented truly mega-mammals from evolving were climate and available food resources.

According to a summary of the research, published today in Science:

The researchers found that the pattern was indeed consistent, not only globally but across time and across trophic groups and lineages-that is, animals with differing diets and descended from different ancestors-as well. The maximum size of mammals began to increase sharply about 65 million years ago, peaking in the Oligocene Epoch (about 34 million years ago) in Eurasia, and again in the Miocene Epoch (about 10 million years ago) in Eurasia and Africa.

"Having so many different lineages independently evolve to such similar maximum sizes suggests that there were similar ecological roles to be filled by giant mammals across the globe," said [researcher John] Gittleman. "The consistency of the pattern strongly implies that biota in all regions were responding to the same ecological constraints."

Global temperature and the amount of land available as an animal's range are two ecological factors that appear to correlate with the evolution of maximum body size, but Gittleman warned against assigning cause and effect. "A big part of science is seeing patterns, and then producing new hypotheses and testing them," he said. "We have now identified this pattern very rigorously."

The point is, climate change could - over millions of years - lead to giant monsters. So there's something for future geoengineers to aspire to.

You can read more about the scientists collaborating on giant mammal work via their website, or read their paper via Science