An identikit picture of the small furry ancestor that gave rise to humans and most other mammals has been pieced together by scientists.
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 the dinosaurs 65 million years ago.
From this small beginning sprang every "placental" mammal that gives 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 consist of kangaroos and other marsupials, and egg laying monotremes such as the duck-billed platypus.
An artist's impression of the earliest placental ancestor has been published based on the results of a six-year long investigation by scientists.
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'."
Colleague Dr Maureen O'Leary, from Stony Brook University in New York, compared the work to a detective investigation, saying: "Just like with a crime scene, the new tools of DNA add important information, but so do other physical clues like a body or, in the scientific realm, fossils and anatomy. Combining all the evidence produces the most informed reconstruction of a past event."