The shrew-like creature weighed less than half a pound, had a long tail and ate insects. It evolved some 200,000 years after a massive asteroid impact led to the extinction of dinosaurs 65 million years ago.
From this small beginning sprang every "placental" mammal - which give birth to mature live young - including dogs, cats, rodents, whales and humans.
Placental mammals are the largest branch of the mammalian family tree, with more than 5,100 living species. Non-placental mammals comprise kangaroos and other marsupials, and egg-laying monotremes such as the duck-billed platypus.
Experts recorded 4,500 physical traits for 86 mammalian species, including 40 that are now extinct. The features, which include the presence or absence of wings, teeth, and bone types, produced a data set 10 times larger than any used before to study mammalian ancestry.
Combined with molecular information from DNA samples, it allowed the scientists to pinpoint the likely start of the story of placental mammals.
Recent work has suggested that the group's origins date back long before the death of the dinosaurs. The new US research, reported in the journal Science, confirms an earlier hypothesis that our ancestors only flourished after the dinosaurs departed, leaving ecological niches for them to fill.
Dr Jonathan Bloch, associate curator of vertebrate palaeontology at the Florida Museum of Natural History, who co-led the study, said: "With regards to evolution, it's critical to understand the relationships of living and fossil mammals before asking questions about 'how' and 'why'.
"This gives us a new perspective of how major change can influence the history of life, like the extinction of the dinosaurs. This was a major event in Earth's history that potentially then results in setting the framework for the entire ordinal diversification of mammals, including our own very distant ancestors."
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