Meet our earliest common mammalian ancestor

Say hello to your greatest grandparent. Cute, furry, long-tailed and with a penchant for insects ? it sounds like something we would keep as a pet rather than be related to. But it seems that such a creature was the last shared ancestor of placental mammals ? a group including all living mammals apart from marsupials and those that lay eggs.

An exhaustive analysis, combining genomic information and fossil evidence, suggests that our ancestor lived soon after the asteroid impact that wiped out the dinosaurs 66 million years ago. Its descendants rapidly spread around the globe and evolved into all the major placental groups seen today, from bats to whales and from mice to men.

Molecular biologists and palaeontologists have long argued over the origin of placental mammals. For a long time, the accepted theory, based on fossil evidence, was that mammals had diversified after the dinosaurs were wiped out, exploding into myriad different species once the world was no longer dominated by these giants.

However, recent genetic analyses have suggested that our earliest ancestor would have walked with the dinosaurs ? living about 100 million years ago, and diversifying into more than 20 different lineages in the succeeding 35 million years.

But palaeontologists have found no fossils from any modern placental group that date from before the impact, suggesting that the ancestor of living mammals evolved after the dinosaurs were gone.

Family get-together

In an attempt to resolve the conflict, Maureen O'Leary of Stony Brook University in New York state and colleagues built a family tree combining genetic information with data on more than 4500 anatomical traits ? the most ever used in this way ? from 46 living species and 40 fossil species.

None of the fossils dating from before the impact could be classified as modern placental mammals. The oldest fossil they identified as being a modern placental lived 200,000 to 400,000 years after the impact. Their analysis showed that rapid evolution followed, producing the first members of the major placental lineages, such as primates and rodents, about 2 to 3 million years later.

The team's genetic analysis suggests that the last common ancestor was a small insectivore weighing between 6 and 245 grams that climbed trees and had a long furry tail (see image above).

"We have no real idea" where it originated, or how its descendants spread around the world, O'Leary says. And although the reconstruction looks like a rodent, "its teeth were completely different".

Boom or bust

Placental mammals weren't the only mammals to diversify after the impact. Marsupials experienced a similar but smaller burst of evolution following the asteroid hit, with a single lineage leading to all living species, O'Leary says. As did the multituberculates ? a group of rodent-like mammals that died without descendants 35 million years ago.

Palaeontologist Donald Prothero at the Natural History Museum of Los Angeles County, is delighted with the results. "This is more in line with what the fossil record has told us for a long time," he says.

"This is a landmark piece of work," says Stephen Brusatte of the University of Edinburgh, UK. "The fossil record and the molecular clock are starting to converge. That's just what we want."

However, molecular biologist Mark Springer of the University of California at Riverside, who carried out the genetic analysis that suggested our ancestors diversified while the dinosaurs were still living, is still sceptical. "It does not convince me that the placental diversification commenced in the Palaeocene", the first 10 million years after the extinction, he says. "The tree based on morphology? is in striking disagreement with the molecular tree."

Resolving the dispute is likely to require the discovery of new fossils. Brusatte and others are currently looking for mammal fossils from the Palaeocene Period, hoping to catch the earliest stages of the evolution of our earliest ancestor.

Journal reference: Science, DOI: 10.1126/science.1229237

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