"Virtually no biologist thinks about the failure to originate as being a major factor in the long term causes of extinction," said Charles Marshall, director of the UC Berkeley Museum of Paleontology and professor of integrative biology, and co-author of the report. "But we found that a decrease in the origin of new species is just as important as increased extinction rate in driving mammals to extinction."

The effects of such a decrease would play out over millions of years, Marshall said, not rapidly, like the global change Earth is experiencing from human activities. Yet, the findings should help biologists understand the pressures on today's flora and fauna and what drove evolution and extinction in the past, he added.

The results, published June 20 in the journal Science Express, come from a study of 19 groups of mammals that either are extinct or, in the case of horses, elephants, rhinos and others, are in decline from a past peak in diversity. All are richly represented in the fossil record and had their origins sometime in the last 66 million years, during the Cenozoic Era.

The study was designed to test a popular evolutionary theory called the Red Queen hypothesis, named after Lewis Carroll's character who, in the book "Through the Looking Glass," described her country as a place where "it takes all the running you can do, to keep in the same place."

In biology, this means that animals and plants don't just disappear because of bad luck in a static and unchanging environment, like a gambler losing it all to a run of bad luck at the slot machines. Instead, they face constant change -- a deteriorating environment and more successful competitors and predators -- that requires them to continually adapt and evolve new species just to survive.

Though the specific cause of declining originations and rising extinctions for these groups is unclear, the researchers concluded that the mammals' death was not just dumb luck.

"Each group has either lost, or is losing, to an increasingly difficult environment," Marshall said. "These groups' demise was at least in part due to loss to the Red Queen -- that is, a failure to keep pace with a deteriorating environment."

Marshall and former UC Berkeley post-doctoral fellow Tiago Quental found that the animal groups were initially driven to higher diversity until they reached the carrying capacity of their environment, or the maximum number of species their environment could hold. After that, their environment deteriorated to the point where there was too much diversity to be sustained, leading to their extinction.

"In fact, our data suggest that biological systems may never be in equilibrium at all, with groups expanding and contracting under persistent and rather, geologically speaking, rapid change," he said.

Marshall and Quental, who is now at the University of Sao Paolo, Brazil, will present their results in two talks this Saturday, June 22, at the Evolution 2013 meeting in Snowbird, Utah.

Note: If no author is given, the source is cited instead.

• Extinction
• Nature

• Ecology
• Global Warming

• Evolution
• Early Mammals

• Mesozoic
• Extinction event
• Cenozoic
• Smilodon
• Cretaceous-Tertiary extinction event
• Conservation status