Bats (Order Chiroptera) make up an astonishing one-fifth of living mammals. They are one of the few mammalian groups to use echolocation, and the only group that has achieved powered flight, yet we know relatively little about their evolution. So, what group of mammals did bats evolve from, and how did they develop the ability to fly?
For years it was unclear what organisms bats evolved from and when they did so. Some theories suggested flying lemurs (Order Dermoptera) were the most closely related to bats due to similarities in their morphologies. But thanks to molecular evidence, we now know that these similarities are most likely convergent. Instead, it suggests that they diverged from other mammals some 64 million years ago (Ma), making them a surprisingly old group of mammals, and that they are more closely related to mammals like cats and dogs than primates like flying lemurs and humans.
This also means that there must be a lot of missing fossil evidence for the evolution of bats. The first definitive bats that have been found date back to 52.5 Ma during the Early Eocene, such as Onychonycteris finneyi and Icaronycteris index. This means that there is over a 10 million year gap in the fossil record of bat evolution, in which time they must have fully developed powered flight. Bat fossils are so rare due to their fragile, light skeletons, which make it easier to fly but don’t preserve well. Thus, most bat fossils are from lagerstätten (sites of exceptional preservation), so when we do find these fossils, they can tell us a lot about that animal.
So we know when bats evolved, but how did they develop flight? Like with many of the other flying vertebrates, there were two main competing hypotheses: the trees-down and the ground-up hypothesis, but the former is accepted as being much more likely. This is because for the ground-up hypothesis to be correct, the ancestors of bats would have to run in order to take off, which is unlikely to be possible based on the morphology of modern bats. Therefore, the trees-down hypothesis is more often favoured, which states that the bat ancestors were arboreal and first developed gliding to move between trees before developing powered flight. In order to glide, they evolved elongated finger bones with a membrane of skin between them to create an aerodynamic surface. This membrane eventually evolved into their wings.
What was more of a question to palaeontologists was when bats evolved echolocation, which many bats use to navigate at night and find food. For many years, it was unclear if bats evolved to fly or to echolocate first, or if both developed in tandem. This changed with the discovery of Onychonycteris finneyi. Analysis of its inner ear showed that it wasn’t capable of echolocation, but could fly, and so it would seem flying evolved in bats first.
So, after decades of research, we have finally managed to piece together the evolution of flight in bats. It is a true testament to luck, because finding such incredible lagerstätten has let us figure out so much about such an elusive group of incredible animals.
 A Vampire Bat. These animals are in the subfamily Desmodontinae, and feed on the blood of other organisms, called hematophagy. Image Available at: https://www.smithsonianmag.com/smithsonian-institution/what-vampire-bat-teach-economics-friendship-180963492/.
 Teeling, E.C., Scally, M., Kao, D.J., Romagnoli, M.L., Springer, M.S., Stanhope, M.J. (2000) ‘Molecular evidence regarding the origin of echolocation and flight in bats’, Nature, 403, pp. 188-192. Available at: https://www.nature.com/articles/35003188.
 A: dorsal view of Onychonycteris finneyi skeleton. B: ventral view of Onychonycteris finneyi skull. C: Icaronycteris index skeleton. A & B taken from Simmons et al. (2008); C taken from https://commons.wikimedia.org/wiki/File:Icaronycteris_index.jpg.
Information References and Further Sources
 Bishop, K. L. (2008). ‘The Evolution of Flight in Bats: Narrowing the Field of Plausible Hypotheses’, The Quarterly Review of Biology, 83 (2), pp. 153-169. Click Here.
 Jepsen, G. L. (1996). ‘Early Eocene Bat from Wyoming’, Science, 154 (3754), pp. 1333-1339. Click Here.
 Shen, Y.-Y., Liang, L., Zhu, Z.-H., Zhou, W.-P., Irwin, D. M., and Zhang, Y.-P. (2010). ‘Adaptive evolution of energy metabolism genes and the origin of flight in bats’, Proceedings of the National Academy of Sciences of the United States of America, 107 (19), pp. 8666-8671. Click Here.
 Simmons, N. B., Seymour, K. L., Habersetzer, J., and Gunnell, G. F. (2008). ‘Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation’, Nature, 451, pp. 818-821. Click Here.
 Teeling, E. C., Scally, M., Kao, D. J., Romagnoli, M. L., Springer, M. S., and Stanhope, M. J. (2000). ‘Molecular evidence regarding the origin of echolocation and flight in bats’, Nature, 403, pp. 188-192. Click Here.
 Teeling, E. C., Springer, M. S., Madsen, O., Bates, P., O’Brien, S. J., and Murphy, W. J. (2005). ‘A Molecular Phylogeny for Bats Illuminates Biogeography and the Fossil Record’, Science, 307 (5709), pp. 580-584. Click Here.
 Zhang, G., Cowled, C., Shi, Z., Huang, Z., Bishop-Lily, K. A., Fang, X., Wynne, J. W., Xiong, Z., Baker, M. L., Zhao, W., Tachedjian, M., Zhu, Y., Zhou, P., Jiang, X., Ng, J., Yang, L., Wu, L., Xiao, J., Feng, Y., Chen, Y., Sun, X., Zhang, Y., Marsh, G. A., Crameri, G., Broder, C. C., Frey, K. G., Wang, L.-F., and Wang, J. (2013). ‘Comparative Analysis of Bat Genomes Provides Insight into the Evolution of Flight and Immunity’, Science, 339 (6118), pp. 456-460. Click Here.