The international six-year study used a massive trove of data, including genetic and physical traits from both modern and prehistoric species, to reconstruct the extended family tree of mammals.
The researchers focused on the "placental mammals" a branch of species that includes humans, horses, whales and many others.
The project has helped scientists better understand how and when modern placental mammals evolved, and, importantly, has traced the starting point to after dinosaurs went extinct 65 million years ago.
The researchers say the new information will help them study a vital question facing the world today: How mammals may have survived past instances of climate change and how that might help us face the warming now underway.
"Species like rodents and primates did not share the Earth with non-avian dinosaurs but arose from a common ancestor - a small, insect-eating, scampering animal - shortly after the dinosaurs' demise," said lead author Maureen O'Leary of New York's Stony Brook University.
The new conclusion overturns an earlier, commonly-held hypothesis that there was a diverse crew of placental mammals before the event that led to the disappearance of dinosaurs and 70% of the planet's species.
That theory had been based exclusively on genetic data. But scientists said combining the genetic evidence with anatomical and fossil evidence helped create a clearer picture of the history.
"Discovering the tree of life is like piecing together a crime scene - it is a story that happened in the past that you can't repeat," Leary said.
"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."
According to the new theory, some 200 000 to 400 000 years after dinosaurs went extinct, the little placental mammal started evolving along a number of different paths, giving rise to the incredible diversity of species we've seen in the eons since - including more than 5 100 living today.
The study also helped illuminate the evolutionary history that led from this common ancestor through to modern-day animals, and showed, for instance, that one group of African animals, including elephants and aardvarks, first developed in the Americas.
"Determining how these animals first made it to Africa is now an important research question along with many others that can be addressed using MorphoBank and the phylophenomic tree produced in this study," said author Fernando Perini, of Brazil's Minas Gerais Federal University.
Mary Silcox, of the University of Toronto Scarborough, added "this project is not exhaustive, but exposes a way forward to collect data on other phenomic systems and other species."