An artist's rendering of the hypothetical placental mammal ancestor,… (Carl Buell )
What's cute and furry and has a name only a paleontologist could love?
A tiny theoretical creature dubbed the "hypothetical placental mammal ancestor" is stealing the hearts of some evolutionary biologists — and annoying others — as it raises new debate over just when our early mammal ancestors began diversifying across the globe.
In a paper published this week in the journal Science, an international team of researchers described how they used a vast database of fossil and anatomical data, as well as DNA evidence, to reverse-engineer an ancestor to the largest group of living mammals.
The scientists theorized that this hypothetical critter would have begun to diversify into new species of mammals after the demise of the dinosaurs roughly 66 million years ago — a concept at odds with some current thinking.
"This is fairly novel, to reconstruct an ancestor," said study coauthor and paleontologist Mike Novacek, the provost of science at the American Museum of Natural History in New York. "I don't know of any other."
Placental mammals, including humans, make up the largest branch of the mammalian family tree. Only marsupials and a small number of egg-laying mammals are excluded.
For the last several decades, scientists have debated just when it was that placental mammals began their explosive proliferation. Estimates based on DNA, or so-called molecular evidence, suggest that placental mammals appeared in the mid-Cretaceous period when dinosaurs like Tyrannosaurus rex and Triceratops still trod the Earth.
However, scientists have yet to uncover evidence of placental fossils during this period, according to the authors of the new study. They argue that this is because placental mammals began to diversify only several hundred thousand years after the demise of non-avian dinosaurs, in the Paleocene epoch.
"An environment that radically changed may have offered more opportunities for a new group to take form and radiate," Novacek said.
Using a vast online database called Morphobank, the team of 23 scientists built a composite of the hypothetical ancestor based on the morphology, or appearance, of existing and extinct animals, along with the DNA data. Study coauthor Maureen O'Leary, a paleontologist at Stony Brook University in New York, likened the process to reassembling a crime scene from thousands of bits of evidence.
Over a period of six years, the researchers developed a highly specific picture of an unassuming little animal whose descendants would one day include whales, bats, lions and people.
"The placental ancestor was a scampering species that had a diet of insects, a fleshy nose, a light underbelly in its fur, and a long tail," O'Leary said. "It was larger than a mouse, but smaller than a rat."
Based on their findings, the scientists said placental mammals began their spread about 36 million years later than estimates based on the DNA-derived molecular clock model.
The research team also contradicted the molecular clock-based theory that placental mammals developed into different species because of the fragmentation of the supercontinent Gondwana, millions of years before the dinosaurs vanished. In addition, the study concluded that a subset of placental mammals known as Afrotheria, which includes elephants and aardvarks, did not originate in Africa as commonly believed; instead, authors say they arose in the Americas.
Not all evolutionary biologists have fallen in love with the furry, hypothetical bug eater, however. Argument over the rise of placental mammals is likely to continue for some time.
"The bottom line is that this study is not convincing and will not settle the debate," said Mark Springer, an evolutionary geneticist at UC Riverside who was not part of the research team.
Using fossils to establish branches of a family tree can result in mistaken groupings of species, Springer said, because some traits are passed along by related species while other traits develop in parallel with more distantly related species. This is known as convergent evolution, and Springer said he was not convinced that study authors had addressed that issue.
Study coauthor Novacek said he did not expect the paper to end discussion on the matter, but he hoped it would add weight to an ongoing argument.
"I think this will reinforce the notion that the fossil record is not in agreement with what's been reconstructed by molecular clocks," Novacek said. "As data accumulates, the case for this late origin becomes better and better."
Another debate is brewing over whether the hypothetical creature deserves a less cumbersome moniker.
"People who think this organism is cute have asked me for a name," Novacek said. "I tell them we can't give it a Latin name, because it never really existed."