Washington, May 22 (IANS) An improved sense of smell jump-started brain evolution in the ancestral cousins of present-day mammals, according to paleontologists.

The findings may help explain why mammals evolved such large and complex brains, which in some cases ballooned 10 times larger than relative body size.

By reconstructing fossils of two early Jurassic period mammals - Morganuocodon and Hadrocodium - researchers provide new evidence that the mammalian brain evolved in three major stages.

First by improvements in sense of smell, next by an increase in touch or tactile sensitivity from body hair and third by improved neuromuscular coordination or the ability to produce skilled muscle movement using the senses, reports the journal Science.

"Now we have a much better idea of the historical sequence of events and of the relative importance of the different sensory systems in the early evolution of mammals," said Tim Rowe, who led the study at the University of Texas in Austin.

"It paints a much more vivid picture of what the ancestral mammal was like and how it behaved, and of our own ancestry," said Rowe, also the university's director of the Vertebrate Paleontology Lab, according to a Texas statement.

The study used a medical imaging technique called X-ray computed tomography (CT) to reconstruct brain moulds of the 190 million year old Morganuocodon and Hadrocodium fossils from China.

These tiny, shrew-like critters are thought to be precursors to existing mammals or "pre-mammals."