What are you going to believe--bones or genes? Studying human and chimp DNA overturned conclusions about our origins and evolution that had been drawn from fossils-this molecular evidence suggests that the last common ancestor lived about 6 million years ago, whereas fossil evidence had dated that fateful split at as long ago as 14 million years, as Newsweek described in our "Evolution Revolution" cover-and now it is doing the same for mammals as a whole.
The first true mammals-the class of warm-blooded, furry vertebrates that bears live young and nurses them-appeared in the late Jurassic, about 70 million years ago, according to fossil evidence. You've probably seen pictures of these early, rat-like creatures cowering the shadows of the dinosaurs that ruled the planet back then. According to the conventional wisdom, mammals never got a chance to diversify because the giant reptiles were taking up most of Earth's available niches; only when the asteroid hit the Yucatan peninsula 65 million years ago, wiping out the dinosaurs, did mammals get their chance. (For a quick refresher course on this, rent the 2000 Disney movie "Dinosaur," in which our saurian hero, orphaned by the asteroid hit, is raised by lemurs who are poised to inherit the Earth.) The new research confirms that many of the "genetic ancestors" of today's mammals survived the meteor impact.
But the asteroid hit didn't much improve their evolutionary prospects, according to new molecular evidence reported in today's issue of the journal Nature. The basic strategy of molecular paleontologists is to compare the DNA of lots of mammal species (there are 4,500 living today), some closely-related and others less so. DNA changes (at least neutral ones, those that neither help nor harm a species' survival odds) occur at a fairly regular rate. That means you can count DNA differences, and work backwards to infer how long they've been accumulating.
The answer: mammals' family tree began branching like crazy only 50 million to 55 million years ago-in others words, 10 million to 15 million years after T. rex breathed his last. Only then did groups of modern mammals such as primates and rodents and hoofed animals start diversifying.
With the planet theirs for the taking, what took them so long? The short answer is, scientists don't know. But they have one guess: climate change. After the asteroid impact, certain mammals did diversify and evolve quickly, including Andrewsarchus (a wolf-like cow), but they eventually went extinct. Other mammals, such as sloths and armadillos, actually became less diverse after the asteroid. As Andy Purvis of Imperial College London out it, "for the first 10 or 15 million years after the dinosaurs were wiped out, present day mammals kept a very low profile, while these other types of mammals were running the show. It looks like a later bout of 'global warming' [10 million years after the dinos' demise] may have kick-started today's diversity, not the death of the dinosaurs."