Disappearing dinos didn't clear the way for us
The mass extinction 65 million years ago had little effect on today's mammals.
The extinction of the dinosaurs had little impact on the evolution of today's mammals, say researchers. After building a family tree of nearly every living mammal, they show that the main groups arose millions of years before the dinosaurs went extinct, and did not become dominant until millions of years after they disappeared.
The wipe-out of the dinosaurs at the end of the Cretaceous period some 65 million years ago opened up room and resources for others. But it did not specifically clear a path for the diversity of animals that would evolve into today's mammals, including humans, says evolutionary biologist Olaf Bininda-Emonds of the Technical University of Munich, Germany: "After the dinosaurs went extinct, they still didn't diversify."
There was a burst of mammal evolution just after the dinosaur extinction, Bininda-Emonds and colleagues report in Nature1. But it occurred in groups that have either gone extinct, such as a group of hoofed carnivores called the mesonychids, or which now have few species, such as the sloths.
The currently successful mammal groups, including primates, kept a quiet profile; they didn't start branching out until about 50 million years ago.
And most of the main mammal groups — including the first rodents, bats and primates — actually got their start about 20 million years before the dinosaur extinction, they add.
Bininda-Emond's team built a family tree, based on DNA data, of 99% of the 4,554 known mammal species. They put dates on the various evolutionary splits using both fossils and DNA studies that indicate how much a creature has changed and so when it parted company from its close cousins.
The DNA work shows that the first rodents and other mammals walked with dinosaurs. But none of these groups' fossil records go back that far, notes Michael Benton, a palaeontologist at the University of Bristol, UK. "The challenge now for palaeontologists is to go and find the fossils," he says.
It's not clear what caused the initial burst of mammal evolution, around 85 million years ago. There are lots of possibilities, including the separation of continents, the origin of flowering plants, or a drop in global temperatures.
The later burst of mammal evolution, 50 million years ago, coincides with one of the largest and quickest warming events in Earth history — in some places, temperatures increased by 7 °C. This seems to have driven many species extinct, and given others their chance.
As for the time around the end of the dinosaurs, it's not entirely unexpected that the mammals that got a boost didn't lead to much. Species that evolve after a mass extinction are often relatively short-lived, says Benton. "The beginning of the recovery is an unstable time," he says.
Tree of life
It's likely that both the branches and the dates of the family tree constructed by Bininda-Emond's group will change as scientists learn more, says evolutionary biologist Mark Springer of the University of California, Riverside.
"This is a reasonable first approximation," he says. "Some of the dates and relationships are probably right on, and some are probably going to move around."
For example, says Springer, the team estimates that the deepest split in the mammals' family tree, between the egg-laying monotremes (such as the duck-billed platypus) and the rest happened 166 million years ago. But some molecular analyses suggest it happened more than 200 million years ago; Springer thinks this earlier date is probably closer to the truth. If that fundamental point changes, he notes, other things will have to shift too. "That date influences everything else through the tree," he says.
"The tree as a whole is probably wrong," admits Bininda-Emonds. But, he says, having some idea of how all mammals are related to each other and when they evolved will be an important aid to future research.
And the tree will have uses besides reconstructing mammal evolution, he says. The Zoological Society of London's 'EDGE of Existence' programme, for example, is working to set conservation priorities based on which species are both evolutionarily unusual and most endangered. "The complete species-level tree is important to have," he says, to assess the evolutionary uniqueness of an animal. "There are so many things we can do with it."
- Bininda-Emonds O. R. P., et al. Nature, 446 . 507 - 512 (2007).
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