A half-pound, furry-tailed animal that probably ate bugs may have been the earliest ancestor of mammals, a group of more than 5,100 species including humans, according to the largest ever study of the group’s ancestry.

Researchers mapped genetic data with fossil evidence, creating a new family tree with the hypothetical ancestor, according to a study in the journal Science. They focused on placental mammals, such as humans, dogs and cats, that develop a placenta to nourish the young before a live birth.

Scientists have debated the time period when these mammals came into existence. Fossil evidence suggested that after the Cretaceous-Paleogene extinction event when the dinosaurs died, mammals quickly evolved to fill now-empty ecological niches. Genetic data suggested mammals may have evolved earlier. The two were combined for the finding, which uses a dataset that’s more than 10 times larger than any previous study of mammalian anatomical relationships.

“There are many more species to add, and I want to continue to look for fossils that will fit on this tree and tell us more about it, either on the dinosaur or mammal side,” said Maureen O’Leary, a study author and paleontologist at the Stony Brook University School of Medicine.

The bug-eating ancestor was extrapolated from traits observed from 86 species, 40 of which were fossils.

About 4,500 characteristics were mapped, including anatomy, such as types of hair, brain structures, tooth formation and the presence of wings. Behavioral traits were added as well. That was combined with existing genetic data to provide scientists with a better approximation of the hypothetical family tree trunk.

The new data, which will be publicly available online for researchers and the public, suggests that placental mammals, a group of furred animals that don’t lay eggs or carry their young in pouches, came into existence about 200,000 years after the dinosaurs were extinct.

Adding information on marsupials, animals such as kangaroos that carry their young in pouches, could also be helpful, O’Leary said. Marsupials are more closely related to placental mammals, and mapping their tree may provide more information on how the groups diverged.

Egg-laying mammals, such as the platypus, are called monotremes, and are more distantly related to the other two mammal groups, O’Leary said.

The database used to map the 86 species that were the basis for the family tree will provide a “playground” for other scientists, said Mike Novacek, provost of the American Museum of Natural History, and a study author.

“If you wanted to study the ear region in mammals, that can be a platform,” Novacek said. He hopes future work will expand the dataset to include more species, both living and extinct. “Virtually everywhere on the tree, there are interesting problems.”