Standing outside at dusk last evening, I watched as squadrons of little brown bats strafed the tree tops and pastures of our Littleton farm. It was easy to understand how many persons, uneducated in the natural sciences, might think that bats are more closely related to swallows and swifts than to terrestrial mammals.
Of course, bats are mammals, represented by more than 1200 species across the globe; about 70% are insectivores while the rest feed primarily on fruit. Though the specifics of their evolutionary history continue to unfold, it appears that bats likely evolved from tree shrews during the Paleocene, the earliest Period of the Cenozoic Era (the Age of Mammals), some 60 million years ago; the process likely began in the late Cretaceous Period, when Tyrannosaurus rex dominated the fauna of Earth. Current fossil evidence, augmented over the past decade, suggests that flight developed before echolocation in the insectivore group and mammalogists suspect that flight initially evolved as a means of escape from predators (especially from ancestral raptors). The earliest bats likely fed during the day (as some fruit bats continue to do today); echolocation eventually permitted crepuscular or nocturnal activity, further enhancing their ability to escape predation. While the first bats graced the planet by 60 million years ago, their major diversification occurred during the Eocene (about 50 million years ago) as another mammalian group, the cetaceans, were returning to the sea.
In essence, bats, little changed from the early Cenozoic, colonized Earth long before most modern mammals appeared. As I watched them last evening, I was looking into the evolutionary past, knowing that my own species, barely 140,000 years old, has now become the major threat to all other creatures on our planet, bats included.
