New Theory Downplays Effect of Dinosaur Extinction

The long-held belief that ancient humans and modern mammals inherited the Earth only because the dinosaurs were wiped out by a devastating asteroid has been overturned by scientists.

An unprecedented "tree of life" that pieces together the evolutionary histories of 99% of modern mammals has revealed that nearly all of those that capitalised on the loss of the dinosaurs have long since died out.

The most primitive ancestors of modern humans and mammals alive today instead went through a period of rapid expansion and diversification some 10 - 15m years after the dinosaurs disappeared, when the Earth experienced a sudden surge in temperature.

The reign of the dinosaurs is believed to have come to an abrupt end when a giant asteroid thumped into the Yucatan peninsular, in Mexico, 65m years ago.

The cataclysmic impact unleashed an enormous cloud of millions of tonnes of dust that blocked out the sun and saw temperatures plummet, creating a "nuclear winter". The impact may also have triggered earthquakes and stirred dormant volcanoes.

The violent change in environment is thought by many scientists to have driven the dinosaurs, including giant predators such as the Tyrannosaurus Rex, to extinction. But smaller mammals, which required less food, may have fared better, and were able to adapt more easily to the new conditions.

The study, published in the latest issue of Nature, shows that some of the most ancient species of mammals leapt into the gap left by the dinosaurs. Most of those, such as the wolf-like cow, Andrewsarchus, have since died out, while others, including sloths and armadillos, are edging towards extinction.

The ancestors of nearly all the mammals alive today apparently "lay low", evolving and spreading out very slowly, over millions of years after the death of the dinosaurs.

"The big question now is what took the ancestors of modern mammals so long to diversify," said Ross MacPhee, an author of the paper based at the American Museum of Natural History.

"It's as though they came to the party after the dinosaurs left, but just hung around while all their distant relatives were having a good time. Evidently we know very little about the macroecological mechanisms that play out after mass extinctions."

The scientists pieced together the vast evolutionary tree of mammalian life for the past 160m years by combining 2,500 partial family trees constructed using fossil remains and molecular data.

"It looks like a later bout of 'global warming' may have kick-started today's diversity, not the death of the dinosaurs," said Andy Purvis, a co-author of the paper at Imperial College, London.

"This discovery rewrites our understanding of how we came to evolve on this planet, and the study as a whole gives a much clearer picture than ever before as to our place in nature," he added.

Olaf Beninda-Emonds, from the Technical University of Munich, Germany, who led the study, added: "The end result is that the mammals we know today are actually quite old and just flew under the radar of everything that was out there, be they dinosaurs or now other 'archaic' mammals as well, for a lot longer than most people suspected.

"This is just the first of many insights, if not surprises, about mammalian evolution to be mined with the help of this tree."