The oldest known fossilized remains of bats (50-60 million years ago) show that even in the early Eocene, bats looked very much like they do today. Long before human ancestors even resembled apes, bat species similar to species of Microchiroptera (microbats) today were flying and using echolocation to pursue insects!
Bats represent the only known example of flight in mammals. How this trait evolved, or from where is not certain due to a lack of fossilized evidence. The ‘missing link’ between non-flying mammals and bats is still ‘missing’- and an important piece of the puzzle, without which it is impossible to know with certainty the ancestor of modern bats.
Scientists believe, though, that the Microchiroptera most likely evolved from small gliding mammals of the order Insectivora. This order today includes small rodent like mammals that feed on insects such as shrews or moles. Scientists believe these bats may have developed the ability to fly to escape predation, or to pursue an increasingly abundant and diverse supply of flying insects that were evolving about the same time.
The two suborders of bats, the Microchiroptera (microbats) and Megachiroptera (megabats), appear to share many similarities suggesting a common ancestor and similar evolution. While this is generally the accepted theory, there are some significant differences between the two suborders that’s led some scientists to suggest that the Megachiroptera may have evolved differently from Microchiroptera.
Those that support this argument point to the lack of echolocation among Megachiroptera species, the difference in their pulp-grinding teeth, and the shape of their heads as evidence. The earliest Megachiroptera fossil record is from only 35 million years ago (Oligocene). The teeth of these fruit-eating bats are designed to crush pulp and differ significantly from those of insectivorous Microchiroptera. It is generally agreed that the shape of the teeth of Megachiroptera could not have evolved from the shape of teeth found in Microchiroptera. The lack of echolocation among Megachiroptera species also spurs the question of how this adaptation evolved in one suborder, but not the other.
Those scientists who believe the Megachiroptera may not share an ancestor with the Microchiroptera suggest these larger, fruit-eating bats they may have evolved from the Primates.This theory is certainly interesting in that it suggests that flight evolved among mammals not just once but twice and that humans share a common ancestor with fruit bats!
These questions about the bats’ evolution remain to be answered. Bat skeletons are fragile and often did not last long enough to fossilize. Therefore, it is difficult to know with certainty how the micro- and mega- bats evolved. Scientists today are still piecing together the picture!