Bats, belonging to the order Chiroptera, are the only mammals capable of true, sustained flight, allowing them to colonize nearly every terrestrial environment. With over 1,400 species, bats are the second most diverse order of mammals. Their sudden appearance in the fossil record, already possessing fully formed wings, creates a long-standing mystery regarding the transition from a terrestrial ancestor. Bats play an immense role in ecosystems as pollinators, seed dispersers, and regulators of insect populations.
The search for the bat’s closest living relatives has historically been complicated, with early morphological studies incorrectly placing them near primates and flying lemurs in a group called Archonta. This classification was based on superficial physical similarities, but molecular genetics revolutionized the understanding of mammalian phylogeny. Modern DNA sequencing places bats firmly within the superorder Laurasiatheria, a vast group that also includes mammals like carnivores, pangolins, horses, and true insectivores such as shrews and moles.
This genetic evidence corrected the misconception that bats were closely related to rodents or other small mammals. The most accepted phylogenetic models suggest that bats are closely allied with the Eulipotyphla, the order that contains shrews, hedgehogs, and moles. These molecular studies also provide a timescale for bat evolution using the molecular clock, which estimates divergence times based on the accumulation of genetic mutations over millions of years.
According to molecular clock estimates, the common ancestor of all living bats diverged from its non-flying Laurasiatherian relatives near the Cretaceous-Paleogene boundary, approximately 66 to 62 million years ago. This timing places the origin of the bat lineage shortly after the mass extinction event that eliminated the non-avian dinosaurs, suggesting a rapid evolutionary radiation into newly vacant ecological niches. The divergence between the two major suborders, the Yinpterochiroptera and Yangochiroptera, is estimated to have occurred around 63 million years ago, marking a very early split.
The Oldest Known Bats
The fossil record provides concrete evidence of the first recognizable bats, but it presents a challenge because the earliest specimens are already accomplished fliers. The oldest known complete bat skeletons date back to the early Eocene epoch, around 52 million years ago, discovered in the Green River Formation of Wyoming. One of the most significant finds is the primitive bat Icaronycteris index, which was small, weighing only about 15 to 29 grams with a wingspan just over 30 centimeters.
The anatomy of Icaronycteris confirmed that the fundamental skeletal adaptations for powered flight were fully established early on. Its forelimbs were dramatically elongated, with the wing membrane stretching between the long fingers, a defining characteristic of all modern bats. Unlike most living bats, however, Icaronycteris retained a claw on its second finger, which suggests a slightly different mode of movement or roosting than its modern descendants.
These ancient specimens possessed a full, unspecialized set of teeth, similar to a modern shrew, indicating a diet primarily focused on insects. Crucially, the cranium of Icaronycteris showed an enlarged auditory bulla—a structural feature associated with the specialized hearing required for echolocation. This combination of fully developed flight and a potentially specialized auditory system highlights the rapid, almost simultaneous evolution of the two defining bat characteristics.
Another notable Eocene bat from the same area is Onychonycteris finneyi, which is slightly older than Icaronycteris and provides a window into the sequence of evolutionary events. While Onychonycteris was capable of powered flight, its skeletal structure was more primitive, featuring shorter wings that suggest a less efficient flight style. More importantly, it lacked the specialized cranial features associated with laryngeal echolocation, suggesting that powered flight may have slightly preceded the full development of the bat’s sophisticated sonar system.
The Simultaneous Evolution of Flight and Echolocation
The development of powered flight and laryngeal echolocation represents one of the greatest adaptive leaps in mammalian history, and the sequence of their evolution remains a topic of scientific debate. The fossil evidence, particularly Onychonycteris, supports a “flight-first” hypothesis, where the ability to fly evolved initially, perhaps from a gliding ancestor, before the complete laryngeal echolocation system was in place. This model suggests a transition from an arboreal, leaping ancestor that used its emerging wings to catch insects or glide to safety.
Conversely, some theories propose that a rudimentary form of echolocation, perhaps derived from simple ultrasonic communication calls, began to evolve first to help the nocturnal ancestor navigate and hunt in the dark. This sensory specialization would have then driven the selection for longer digits and membranes to better capture the insects detected by sound, eventually leading to powered flight. The energetic coupling between the two traits supports a tandem evolution, as the muscles used for flight are also synchronized with the exhalation that generates the echolocation pulse, making the system highly energy-efficient.
Molecular studies provide deep insight into the genetic changes that underpinned this sensory transformation. The protein Prestin, encoded by the Prestin gene, is responsible for the rapid contractions of outer hair cells in the cochlea, enabling the high-frequency hearing necessary to process returning ultrasonic echoes. Analysis of the Prestin gene in echolocating bats reveals evidence of strong convergent evolution, meaning the specific amino acid changes that enhance high-frequency hearing appeared independently in different bat lineages.
This molecular convergence, also seen in other auditory genes like KCNQ4 and Tmc1, suggests that the genetic machinery for sophisticated echolocation was highly prone to evolving the required function or that the ability evolved more than once within the bat order. The combination of powered flight and laryngeal echolocation created a unique, highly specialized predator that dominated the nocturnal skies, leading to rapid diversification shortly after the Eocene.
Diversification into Modern Bat Groups
The initial evolutionary leaps of flight and echolocation paved the way for the immense radiation of bats, resulting in their division into two primary suborders: Yinpterochiroptera and Yangochiroptera. This classification, based on molecular data, replaced the older division of Megachiroptera (megabats) and Microchiroptera (microbats), which was found to be inaccurate because molecular evidence showed some microbats are genetically closer to megabats than to other microbats.
The Yinpterochiroptera suborder includes the Old World fruit bats (Pteropodidae), the only bat family that does not use laryngeal echolocation, relying instead on keen eyesight and smell. These bats are primarily frugivorous and nectarivorous, feeding on fruit, pollen, and flowers. However, Yinpterochiroptera also encompasses several families of echolocating microbats, such as the horseshoe bats (Rhinolophidae), which are specialized insectivores using complex calls emitted through their nostrils.
The Yangochiroptera suborder contains the vast majority of the world’s bat species and is characterized by its members’ universal use of laryngeal echolocation and incredible dietary plasticity. While most Yangochiroptera are insectivorous, they have diversified into an array of specialized ecological niches across the globe. This suborder includes bats that feed on fish, frogs, and other small vertebrates, as well as the unique vampire bats, the only mammals that feed exclusively on the blood of other animals.
This post-Eocene diversification, peaking between 30 and 50 million years ago, showcases how the innovations of flight and echolocation allowed bats to exploit resources unavailable to other mammals. The resulting variety of diets, ranging from specialized nectivores to hard-shelled insect crushers, demonstrates the outcome of this powerful evolutionary event.