The asteroid strike that wiped out the dinosaurs 65 million years ago is often credited with prompting the rise of the mammals we see today - including primates like us.
But a new study says the effects of the dinosaurs' demise have been greatly exaggerated. Modern-day mammals, researchers say, displayed an initial burst of evolutionary diversity up to 100 million years ago - while the dinosaurs were still roaming prehistoric swamps.
And the mammals showed a second burst between 55 million and 35 million years ago - long after the dinosaurs had disappeared.
Now scientists think the mammals that benefited directly from the death of the dinosaurs have long since died out. And they've pushed back the evolutionary birth date of up to 40 types of existing mammals by at least 35 million years.
For example, fossil records indicate that the oldest whale is about 53 million years old, while the oldest rodents are 56 million years old and the oldest rabbit goes back 50 million years.
But the new research indicates that whales, rodents, rabbits and a number of other mammals all have direct ancestors that go back about 93 million years - far older than any fossil remains discovered.
"These modern lineages had to have started, and had ancestors, as far back as 100 million to 85 million years ago," said Ross D. MacPhee, curator of vertebrate zoology at the American Museum of Natural History in New York and co-author of the study that appeared in yesterday's edition of the journal Nature.
The second burst of new species of mammals occurred about 50 million years ago as a prehistoric warming trend was turning much of the planet into a tropical hothouse, the researchers say. But scientists said it will take more study to determine whether that bout of global warming actually caused the change.
"It could have been the changes in temperatures. It could have been those changes in climate altered the landscape, creating interconnected forests that meant these species had wider ranges and better habitats," said Andy Purvis, a lead author of the report and an evolutionary biologist at Imperial College, London. "The truth is, we really don't know."
Why the dinosaurs disappeared remains a mystery. But most experts today believe that a worldwide layer of iridium and other rare minerals, first detected in the 1980s, is evidence of a huge asteroid strike that wiped out the ancient reptiles 65 million years ago.
Some experts believe there may have been several asteroid strikes, and others blame volcanic eruptions. "But most paleontologists believe a single asteroid caused it," said Peter Sheehan, curator of geology at the Milwaukee Public Museum.
Previous studies showed mammals evolved with the earliest dinosaurs some 210 million to 220 million years ago. But they were primitive mammals - few were larger than cats. And researchers believed that very few of those had any direct links to the mice, dogs, cats and monkeys we know today.
In their study, the researchers used previously dated fossil remains and DNA extracted from dozens of these early mammals to compile a tree of life that identifies the lineage of 99 percent of today's 4,554 identified mammalian species.
They used 2,500 previous trees of life and 66 gene mutations found in fossils to estimate the dates when offspring split from their ancestors to form new species.
In an accompanying report, two experts from New Zealand not associated with the study called the scale of the new evolutionary tree "groundbreaking."
The research is intended to help scientists identify the types of mammals most at risk of extinction. Future research will focus on finding the mechanisms that caused modern mammals to flourish and diversify so long after the dinosaurs disappeared, Purvis said.
But the conclusions drew a skeptical reception from some paleontologists, who argued that if so many mammals have been around for so long, there would be more fossil evidence of their existence.
"It's difficult to hypothesize a 35 million year record of evolution when there's no record of them anywhere. It's not like we haven't been sampling anything," said Ken Rose, an expert on anatomy and paleontology at the Johns Hopkins University School of Medicine. "Either all of the fossil record is misleading us, or something else is going on here."
The authors say that they are only clarifying what fossil studies have shown, not contradicting them. But they expect criticism.
"There will be people who hate it, no question," said MacPhee, the co-author.
Previous research had already established that it took millions of years after the dinosaurs disappeared for modern mammals to begin to flourish, Rose said.
Ancient mammal fossils, collected over the years throughout North America, show a 10-fold increase in mammalian diversity immediately after the dinosaurs' demise, but those species have died out, experts say.
What is new in the study - and harder for some paleontologists to accept - is that so many species of modern mammals go back to the age of the dinosaurs.
"It's virtually impossible for me to believe that living lineages of mammals had diversified as much, in the age of dinosaurs, as these guys would have us believe," said Chris Beard, curator of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh.
For example, the study concludes that eight families of rodents coexisted with the dinosaurs. But Beard said fossil records show that rodents first appeared in Asia about 5 million years after dinosaurs disappeared.
"If they're right, the fossil record is worse than many people believe," Beard said.
Paleontologists say the problem could be that scientists using fossils base their findings on an animal's physical characteristics, such as the size or position of the teeth or jaw.
But in the Nature report, researchers used DNA to establish genetic links between ancient and modern mammalian species that might have looked nothing like one another.
"From our fossil record, we can trace an animal like a deer back pretty far, but not beyond a point where they didn't look like a deer," Sheehan said.
Here's when some species made their debuts: 500 million years ago: 1st fishes
470 million years ago: 1st land plants
300 million yeas ago: 1st reptiles
210-220 million years ago: 1st dinosaurs/mammals
150 million years ago: first birds
70 million years ago: first primates
56 million years ago: first rodents
53 million years ago: first whales
50 million years ago: first rabbits
20 million years ago: first chimpanzees
5-6 million years ago: early human hominids appear
200,000 years ago: modern humans appear
TIMELINE SOURCES: Nature, worldbook.com, sciencedaily.com, discovery.com and the Smithsonian Institution.