Scientists think that bats probably first appeared many years even before the oldest known bat bones, perhaps 65 to 100 million years ago. That would mean that bats lived at the same time as the dinosaurs. The earliest known megabats lived 35 million years ago. Many scientists think that they have evolved 9 (developed) from a different ancestor. They think that the megabats may be more closely related to primates than microbats.

The oldest known fossil microbat, called Icaronycteris index was found near Yellowstone Park in Wyoming. It lived 50 million years ago. Fossil bats that lived 45 million years ago have been found in Germany. Some of them were so well preserved that the fossilized remains of insects were found in their gut. Surprisingly, the fossilized bats that were found very closely resemble the bats that we know today.

As far as the vampire bat goes, there has long been the question of why they feed on blood. Many propose that the vampire bat originated from fruit eating bats. Another theory says that the bats developed a taste for blood from feeding on ticks and other blood feeding insects. It is also a mystery that the vampire bat lives in only the new world tropics 9 central and South America). Another theory states that the bats began to feed on insects and insect larvae found in wounds on large animals.
Throughout the tropics, screwworms lay their eggs in wounds and the larvae develop into large numbers. This theory identifies as to why the vampire bat is only found in the new world. None of these theories has been proved, evidence from proteins say that the vampire bat has been around for 6 to 8 million years.