Evolution is a word saturated with meanings. To some it connotes a progressive diversification of life culminating with Homo sapiens, to some the development of a system or an idea. Charles Darwin described the process as “descent with modification”, and modern biologists define evolution as change with continuity in successive generations of organisms. We know that all animal populations change as time progresses, and that this change is often dictated by the environment. Each newborn generation sees a certain percentage of it’s members live to reproduce. Most of the time the individuals who survive to reproduce are simply a matter of chance, but sometimes an individual wins the genetic lottery and is born with a combination of traits that give it an advantage. If these traits lead to the production of a greater than average number of offspring these advantageous traits will be passed on, and over a number of generations will become common in the population. This process is know as natural selection, and it drives evolutionary change.
Today there are hundreds of species of bats, belonging to the single order Chiroptera. As mammals they came from a long lineage of animals that arose from the Therapsid reptiles in the Triassic, some 120 million years ago (mya). They were nocturnal, insectivorous, and had highly developed senses of smell and hearing. Most of all they were small, and occupied the fringe niches of a planet dominated by dinosaurs.
The earliest fossil bat is Icaronyteris index, and dates from 50 mya. I. index appears to be fairly modern, and we believe that it could echolocate based on the size and shape of its cochlea. There are a few other fossils, but most of our knowledge of bat evolution relies on other types of evidence. We know that since the time of I. index bats have diversified to fill a wide variety of niches, and we can imagine how bats like I. index changed over time to become the species of today.
The more interesting problem is determining the evolutionary history of bats before I. index. Molecular studies that compare the percentile difference in the DNA of different species of bats estimate that the difference between the two suborders of bats is the same as the difference between most orders of mammals. Orders like rodents and primates began to speciate 80-65 mya, and we can speculate that bats got a head start on these. At this time we can not know for sure, but a safe guess is that bats were around to watch the extinction of the dinosaurs 65 million years ago.
Evolving the ability to fly required a complex change in anatomy of the animals that became bats. Most researchers believe that bats and primates evolved from arboreal insect eaters, and that bats went through a gliding stage. The wings of bats can be thought of as two sections, the membranes supported by the arms and the membrane supported by the fingers provides the thrust and maneuverability. We can speculate that the membrane in the arms evolved first, and this would have been sufficient to glide from tree to tree with. For arboreal animals gliding uses less energy and is safer than climbing down the tree, crossing the ground between trees, and climbing back up another tree. Types of lizards, marsupials, and squirrels have all evolved gliding membranes. Bats would have preyed on insects, and animals that could control their direction would have had a selective advantage so the hand wings that define the Chiropterans would have evolved rapidly. Flight probably evolved before sophisticated echolocation, but it is clear that the latter was evolving early on in the history of bats and that this ability facilitated the diversification of bats into the hundreds of niches that they occupy today.
Written by Bryan Carstens, an OBC volunteer,and a MSU graduate who focused heavily on evolutionary biology. He is now in graduate school continuing his studies on bats.