This story was updated at 3:17 pm EDT March 14.
The dinosaurs needed to go extinct for the mammals, and humans, to arise. At least that's what researchers have been thinking for decades. But, a new study indicates that at least one group of ancient mammals was already expanding 20 million years before the dinosaurs were wiped from the Earth.
Analysis of ancient mammal teeth indicates they were able to take off not because dinosaur die-off made way for them, but because they discovered a new food source that others weren't consuming.
Some of the world's earliest mammals were the multituberculates, a group of small rodentlike animals that first emerged on Earth about 165 million years ago. For the next 80 million years, they stayed small, seeming to evolve slowly while living in a limited number of habitats and eating insects. Researchers once thought the animals were being held back by dinosaurs, which out-competed them for food sources.
Tooth for thought
The fossil record shows that after the mass extinction event 65 million years ago, dinosaur diversity and density dropped and overall mammal diversity and density increased. Researchers used to think that the mammals succeeded in the absence of the dinosaurs. [Gallery: Evolution's Most Extreme Mammals]
To test this theory, the researchers looked closely at a specific group of early mammals. They analyzed 48 sets of fossilized teeth from multituberculates. The teeth were scanned into a computer program, which analyzed their complexity.
"If you look at the complexity of teeth, it will tell you information about the diet," study researcher Gregory Wilson, of the University of Washington, said in a statement. "Multituberculates seem to be developing more cusps on their back teeth, and the bladelike tooth at the front is becoming less important as they develop these bumps [or cusps] to break down plant material."
This increasing "dental complexity" allowed them to eat a more diverse range of foods, including the first flowering plants called angiosperms, which started to appear 140 million years ago.
The fossil teeth suggested that about 85 million years ago, body size of these mammals increased and more and more new species appeared, especially after they switched to eating plants.
"These mammals were able to radiate in terms of numbers of species, body size and shapes of their teeth, which influenced what they ate," Wilson said.
This matches up with other mammal groups, including recently discovered species from the Late Cretaceous (between 100 million and 65 million years ago) showing highly specialized adaptations and increasing diversity.
The multituberculates went extinct about 35 million years ago, but not before they spread across the globe. It's been suggested that modern rodents and other animals drove them to extinction because they were competing for the same food.
The study was published today (March 14) in the journal Nature.
Editor's Note: This story was updated at 3:17 pm EDT March 14 to correct the name of the researcher's institution.