Hamster-sized mammal topped our family tree

Scientists have unveiled a detailed portrait of the earliest ancestor of dogs, horses, humans and all other mammals that don't lay eggs or carry their young in a pouch — even though it hasn't yet been found in the fossil record.

The scampering, insect-eating animal was about the size of a hamster, with a long tail and a narrow snout, an international team of 23 scientists conclude in a study published Thursday online in the journal Science.

Their analysis also determined that the earliest ancestor of placental mammals — which nourish their unborn using the organ inside the uterus called the placenta — lived about 200,000 to 400,000 years after the dinosaurs died out. That is about 36 million years later than DNA evidence had suggested, and it means nothing resembling modern-day mammals such as lemurs or squirrels ever scurried among the dinosaurs.

Detailed family tree constructed

By and large, the ancestral animal looks similar to what scientists had previously guessed, said Mary Silcox, a professor of anthropology at the University of Toronto Scarborough, and co-author of the study.

However, Silcox noted that the reconstruction was not based on guessing. Since no fossils have yet been found of the tiny animal, its characteristics were extrapolated based on an exhaustive analysis of 4,500 physical traits found among 86 mammals, about half of them living and about half of them from the fossil record, thought to be descended from it. That resulted in a detailed family tree of all placental mammals — with different traits on different branches, mapping how they changed over time — and their earliest ancestor at the top.

"This was based on data," Silcox said. "It's a much more powerful approach than just kind of making up a story based on your expectations."

That said, she added, there were some surprises. "The brain is actually advanced over what I expected," she said.

Silcox's main role in the study was to be in charge of the 1,400 teeth traits that were analyzed. She said that was very important because most mammal fossils are actually teeth.

"The hardest substance in the body for a mammal is enamel, and therefore it tends to survive better than anything else in the fossil record. So there are a lot of species that we only know from teeth," she said. "Fortunately, teeth are incredibly informative."

For example, they provide information about how big the animal was and what it was eating.

Animal traits database is online

In addition to figuring out what the animal looked like, the study also estimated when it lived. 

DNA evidence has suggested the earliest placental mammal arose about 100 million years ago, while dinosaurs still roamed the Earth. Dinosaurs went extinct about 65 million years ago during a catastrophe that wiped out 70 per cent of all species.

But based on how mammals' traits changed over time, the research team concluded that the placental ancestor didn't exist until 200,000 to 400,000 years after the dinosaurs died out.

Silcox noted that the estimates based on DNA are calibrated against certain events in the fossil record.

"So at some point, they have to believe in our data," she said.

She suggested that perhaps some of the assumptions used to generate the estimates from the DNA are wrong.

The data used in the study, which was led by Maureen O'Leary, researcher at Stony Brook University and the American Museum of Natural History, is compiled in a huge database of animal traits called MorphoBank that is publicly available online.