A fossil found in Wyoming has resolved a long-standing question about when bats gained their sonar-like ability to navigate and locate food.
They found that flight came first, and only then did bats develop echolocation to track and trap their prey.
A large number of experts had previously thought this happened the other way around.
Details of the work by an international team of researchers is published in the prestigious journal Nature.
Echolocation - the ability to emit high-pitched squeaks and hear, for example, the echo bouncing off flying insects as small as a mosquito - is one of the defining features of bats as a group.
There are over 1,000 species of bats in the world today, and a great many of them can echolocate to navigate and find food.
But some, especially larger fruit bats, depend on their sense of smell and sight to find food, showing that the winged mammals can survive without their capacity to gauge the location, direction and speed of flying creatures in the dark.
The new fossil, named Onychonycteris finneyi, was found in the 52-million-year-old Green River Formation in Wyoming, US, in 2003. It is in a category all on its own, giving rise to a new genus and family.
Its large claws, primitive wings, broad tail and especially its underdeveloped cochlea - the part of the inner ear that makes echolocation possible - all set it apart from existing species. It is also drastically different from another bat fossil unearthed in 1960, Icaronycteris index, that lived during the same Early Eocene epoch.
Many experts had favored an "echolocation first" theory because this earlier find, also from the Green River geological formation in Wyoming, was so close in its anatomy to modern species.
But the new fossil suggests this wasn't the case.
"Its teeth seem to show that it was an insect eater," said co-author Kevin Seymour, a palaeontologist at the Royal Ontario Museum in Canada.
"And if it wasn't echolocating then it had to be using other methods to find food."
The next big question to be answered, he added, was when and how bats made the transition from being terrestrial to flying animals .