BRITAIN:Humans and other modern mammals do not owe their existence to the asteroid that wiped out the dinosaurs, new research suggests.
For decades experts have assumed that our ancestors took over the earth after seizing the opportunity offered by the extinction of their reptilian rivals.
But a new "tree of life" tracing the history of the estimated 4,500 species of mammal alive today challenges this theory.
The study, conducted by an international team of scientists including experts from the Zoological Society of London and Imperial College, indicates that the genetic ancestors of modern mammals did not begin to dominate the planet until 10 or 15 million years after the dinosaurs disappeared.
It was a sudden increase in the earth's temperature, rather than the demise of the dinosaurs, that spurred mammals to diversify and spread, the scientists believe.
Prof Andy Purvis, from Imperial College London's Division of Biology, said: "It looks like a later bout of 'global warming' may have kick-started today's diversity - not the death of the dinosaurs.
"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."
Dinosaurs were at the peak of their reign when a giant asteroid smashed into the earth off the coast of Yucatan, Mexico, 65 million years ago.
Huge predators such as Tyrannosaurus rex dominated the planet and made the world a dangerous place for the much smaller mammals alive at the time, which kept a low profile.
The early ancestors of today's mammals existed at least 85 million years ago, but in fewer numbers.
After the asteroid impact scientists believe the earth underwent a form of "nuclear winter" caused by millions of tons of dust blocking out the sun. The blast may also have triggered a chain reaction of earthquakes and volcanic activity. Temperatures will have plummeted, while chemical reactions in the atmosphere generated acid rain.
Although the dinosaur extinction is still debated, most experts believe the creatures simply could not survive the conditions, and the impact on food chains of the massive loss of vegetation.
Mammals, which were warm-blooded and did not need so much food to survive, were better able to adapt.
They began taking advantage of their situation. But according to the new study, it was other groups of ancient mammals besides those we know today that first jumped into the gap left by the dinosaurs.
Most of these groups, represented by animals such as the wolf-like cow Andrewsarchus, have since died out. Those remaining, including the group containing sloths and armadillos, have declined in diversity.
The vast majority of modern mammal ancestors continued a slow pace of development for millions of years after the dinosaurs were gone.
Dr Olaf Beninda-Emonds, from the Technical University of Munich, Germany, lead author of the study published today in the journal Nature, said: "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."
The evolutionary tree was constructed by comparing the DNA of living species and looking at the fossil record. Because genetic changes occur at a relatively constant rate, like the ticking of a clock, scientists can estimate the time it has taken for species to diverge from a common ancestor by counting the number of mutations in their DNA.