PerspectiveEvolution

An Eocene Big Bang for Bats

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Science  28 Jan 2005:
Vol. 307, Issue 5709, pp. 527-528
DOI: 10.1126/science.1108871

Bats, the only mammals capable of powered flight, constitute more than 20% of living mammal species (1). [HN1] Unlike birds and other terrestrial vertebrates, most bats use echolocation [HN2]—a biological form of sonar—to locate and track their prey (2). Bats are found on every continent except Antarctica, and they exploit a wide variety of food sources including insects, small vertebrates, fruit, nectar, pollen, and even blood (13). More than 110 bat species may coexist in some ecological communities, a number that far exceeds that of any other mammalian group (1, 3). Despite their prominent position among mammals, the evolutionary history of bats is largely unknown because of a limited fossil record and incomplete phylogenies. On page 580 of this issue, Teeling et al. [HN3] (4) provide a new evolutionary tree for bats that helps to explain how, when, and where this extraordinary diversity may have originated.

Living bats are classified [HN4] into 18 families on the basis of shared anatomical specializations and echolocation habits (see the figure), and another six families are known from fossils (1, 5, 6). Although biologists have long agreed that these groups represent distinct evolutionary lineages, there has been no consensus concerning relationships among them. The lack of a well-resolved phylogeny (evolutionary tree) [HN5] for bats has hindered attempts to understand the origins of major specializations in these mammals, and has complicated efforts to untangle the temporal and biogeographic history of the group.

One extant family (Pteropodidae, or Old World fruit bats [HN6]) lacks the sophisticated echolocation abilities of other bats. Because bat echolocation is a complex system involving specialization of the respiratory system, ear, and brain (2), it has generally been assumed that echolocation evolved only once in bats. This hypothesis has been supported by phylogenetic analyses of morphological data by a number of groups [e.g., (5)]. These analyses revealed a basal split among bats between a single lineage leading to all echolocating bats (Microchiroptera) and another lineage leading to non-echolocating pteropodids (Megachiroptera)[HN7]. However, recent analyses of DNA sequence data have challenged this hypothesis, instead suggesting that some echolocating bats (rhinolophoids [HN8]) are more closely related to pteropodids than to other echolocating bats [e.g., (7, 8)]. These relationships imply that echolocation either evolved twice in bats or evolved once but was later lost in pteropodids. Either scenario would require a complete rethinking of our understanding of the evolutionary history of bats, including new evolutionary explanations for more than 20 different anatomical specializations shared by living echolocating bats [but not pteropodids (5)].

Temporal pattern of bat diversification.

Teeling et al. (4) used their molecular tree and a series of fossil calibration points to reconstruct the timing of bat radiation. They calculate that living bats last shared a common ancestor at or just after the Cretaceous-Tertiary boundary. The four major lineages of living echolocating bats (A to D) appear to have subsequently originated within a narrow time frame in the Early Eocene [52 to 50 million years ago (4)]. Although divergence dates estimated from models of DNA sequence evolution are notoriously subject to biases, the Eocene dates in this study are reasonably consistent with the results of morphology-based reconstructions [e.g. (3, 6)]. The results of Teeling et al. suggest that all 18 of the living families of bats were distinct by the end of the Eocene (about 34 million years ago). [Adapted from (4)]

Weaknesses in prior molecular studies have left some doubt about their interpretation. For example, analyses of different genes yielded incompatible phylogenetic trees, and sampling of living bat families was incomplete, leaving open the possibility that sampling biases were responsible for the surprising molecular results. The new study by Teeling et al. (4) overcomes these difficulties by simultaneously analyzing portions of 17 nuclear genes sampled in all extant families. The resulting phylogenetic tree, which is strongly supported by the data, confirms the results of earlier molecular studies (see the figure). Non-echolocating pteropodids nest among lineages of echolocating bats, implying a dual origin for echolocation or its loss in pteropodids. The authors do not attempt to choose between these alternatives, but a second analysis including several extinct families (4) suggests that a single origin for echolocation (followed by its loss in pteropodids) is most likely.

Teeling et al. reanalyzed a previously published morphological data set that included four extinct families (5), using their DNA tree as a constraint. The resulting trees placed the Eocene fossil families as sister taxa (close relatives) to the lineage leading to all living bats. Previous analyses of fossilized stomach contents, ear anatomy, and limb structure concluded that two of the fossil families were sophisticated echolocators similar to most living bats (5). The other fossil groups are also thought to have been capable of echolocation, although they may not have been able to track flying prey (5). Taking the relationships of these fossils into account, it seems most likely that echolocation evolved only once in bats, before diversification of the group. The lineage leading to bats was thus characterized by two remarkable specializations seen in no other land mammals: powered flight and echolocation. The coincidence of these adaptations—which may have been functionally and evolutionarily linked (9)—was probably the key that started the evolutionary diversification of the group.

The scope of this “big bang” Eocene [HN9] radiation is unprecedented in mammalian history. What may have caused it? Teeling et al. speculate that bats diversified in the Early Eocene in response to an increase in prey diversity, and that the varied echolocation and flight strategies that characterize families may have evolved as a result of differential exploitation of ecological niches available at that time. The origin of the major bat lineages in the Eocene is apparently coincident with a rise in mean annual temperature, a significant increase in plant diversity, and the peak of Tertiary insect diversity (4). The evolutionary success of bats thus may have resulted from ancestral bats being in the right place at the right time. As flying predators capable of capturing prey on the wing, they would have had few competitors for the rich resources of the Eocene night. The only other vertebrates that exploit niches for nocturnal flying predators are the owls and nightjars [HN10]. Interestingly, owls may also have undergone an adaptive radiation in the Eocene (10).

The geographic origin of bats has been a source of debate because Eocene bat fossils have been found on most continents (5, 6). Teeling et al. (4) seemingly have solved this problem with their new phylogeny, which unambiguously indicates a Laurasian [HN11] (Northern Hemisphere) origin for bats. Once the group was established, different lineages of bats probably diversified on different continents, but the scale of the Teeling et al. analysis does not allow them to untangle these patterns. Indeed, it is the scale of bat diversity that makes this study so tantalizing. Their results give us a new framework for understanding bat evolution, but it is only the tip of the iceberg. Although broad in scope, their evolutionary tree includes fewer than 40 out of more than 1100 living bat species—just 3%. Understanding the patterns of evolution of different echolocation strategies, diets, body sizes, flight styles, and reproductive habits of bats will require much more fine-grained phylogenies than are now available. As always, the devil will be in the details.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

Glossaries of natural history, geological, and biological terms are provided by the University of California Museum of Paleontology.

A glossary of mammalogy terms is provided for a mammalogy course at the University of Guelph.

A glossary of vertebrate and paleontology terms is provided by the Palaeos Web site.

An evolution glossary is provided by PBS's Evolution Web site.

Web Collections, References, and Resource Lists

The Google Directory provides links to Internet resources on evolution.

The Yahoo Directory provides links to Internet resources on evolution and phylogeny.

Biology Links: Evolution are provided by the Department of Molecular and Cellular Biology, Harvard University.

EvoNet.org provides links to Internet resources in evolutionary biology.

The Biology Browser from BIOSIS provides links to Internet resources. A collection of links to Chiroptera is included.

K. Holsinger, Department of Ecology and Evolutionary Biology, University of Connecticut, maintains a directory of resources for ecology, evolutionary biology, systematics, and conservation biology.

Bat Conservation International provides links to bat Internet resources.

LookSmart's Directory provides links to bat information resources on the Internet.

Online Texts and Lecture Notes

PBS's Evolution Web site offers presentations, articles, and links to resources on evolutionary biology topics.

Animal Diversity Web is an online database of animal natural history, distribution, classification, and conservation biology provided by the University of Michigan Museum of Zoology. A glossary is provided.

The University of California Museum of Paleontology (UCMP) presents Web exhibits about living and fossil organisms, geology and geologic time, and evolutionary theory. A Journey Into Phylogenetic Systematics is provided.

The Tree of Life Web Project provides information about the evolutionary history and characteristics of organisms.

Palaeos, maintained by M. A. Kazlev, is a reference resource on the history of life. Information pages on paleontology and vertebrates are provided. An overview of Mammalia is included.

A collection of presentations on biology and evolutionary theory is provided by the Talk.Origins Archive.

The Rediscovering Biology Web site provides an online textbook with a section on evolution and phylogeny.

D. Rand, Department of Ecology and Evolutionary Biology, Brown University, offers lecture notes for a course on evolutionary biology. Lecture notes on phylogenetic inference and molecular systematics are included.

S. Carr, Department of Biology, Memorial University of Newfoundland, provides lecture notes for a course on evolution and systematics. Lecture notes on phylogenetic systematic analysis are included.

J. Baskin, Department of Biology, Texas A&M University, Kingsville, provides lecture notes for a mammalogy course.

K. Thompson, College of Life Sciences, University of Maryland, offers lecture notes for a mammalogy course. Lecture notes on Chiroptera (bats) are included.

General Reports and Articles

The 2 March 2001 issue of Science had an Enhanced Perspective by M. S. Springer and W. W. de Jong titled “Which mammalian supertree to bark up?” about a report by F.-G. R. Liu et al. titled “Molecular and morphological supertrees for Eutherian (placental) mammals.”

The 13 June 2003 issue of Science was a special issue on the tree of life.

The Bookshelf of the National Center for Biotechnology Information (NCBI) makes available the 2003 textbook by E. Koonin and M. Galperin titled Sequence-Evolution-Function: Computational Approaches in Comparative Genomics. A chapter on comparative genomics and new evolutionary biology is included.

The 4 February 2003 issue of the Proceedings of the National Academy of Sciences had an article by M. S. Springer, W. J. Murphy, E. Eizirik, and S. J. O'Brien titled “Placental mammal diversification and the Cretaceous-Tertiary boundary.”

S. Kumar, Department of Biology, Arizona State University, makes available in PDF format a 2001 encyclopedia article by S. Kumar and A. Filipski titled “Molecular phylogeny reconstruction.”

Numbered Hypernotes

1. Bats. Articles on bats are included in Wikipedia and in the Columbia Encyclopedia. The America Zoo Web site offers an illustrated presentation on Chiroptera. An introduction to bats is provided by the Endangered Species Program of the U.S. Fish and Wildlife Service. An introduction to bats is provided the Smithsonian Institution. An entry on Chiroptera is included in the Tree of Life Web Project. Animal Diversity Web offers information on bats. UCMP offers a presentation on Chiroptera, as well as a presentation on bat flight. A presentation on bats (with a section on their evolution) is provided by the Natural History Collections of the University of Edinburgh. M. L. Thies, Department of Biological Sciences, Sam Houston State University, Huntsville, TX, provides lecture notes on Chiroptera and bat flight for a mammalogy course. The instructor's text of the K8AIT Principles of Aeronautics includes a section on bats.

2. Echolocation. How Bats Work by Tom Harris, on the How Stuff Works Web site, includes a section on seeing with sound. The Oakland Museum of California offers a bat echolocation animation. M. L. Thies offers lecture notes on echolocation in bats for a mammalogy course. C. D. Hopkins, Department of Neurobiology and Behavior, Cornell University, offers lecture slides in PDF format on echolocation (parts one and two) for a neuroethology course; a student project on Microchiroptera is also made available. A student presentation on echolocation in bats is made available on the Topics in Neuroethology Web site provided by M. Nelson, Beckman Institute, University of Illinois. The Auditory Neuroethology Laboratory, University of Maryland, uses the echolocating bat as a model system; a presentation on auditory scene analysis by echolocation in bats is provided. The Bat Ecology and Bioacoustics Laboratory, University of Bristol, UK, provides an interactive biosonar site.

3. Emma C. Teeling, Stephen J. O'Brien, and William J. Murphy are at the Laboratory of Genomic Diversity, National Cancer Institute, Frederick, MD. Mark S. Springer is in the Department of Biology, University of California, Riverside. Ole Madsen is in the Department of Biochemistry, University of Nijmegen, Netherlands. Paul Bates is at the Harrison Institute, Sevenoaks, Kent, UK.

4. Bat classification. UCMP provides an introduction to bat classification. The Tree of Life Web Project provides a listing of the classification of bats. Animal Diversity Web provides lists of Chiroptera families and species. An index with links to bat species is provided on the 42eXplore Web site.

5. Phylogeny and phylogenetic trees. An introduction to systematics is provided by Nearctica. A presentation on systematics and molecular phylogenetics is provided in the NCBI's Science Primer. A. Vierstraete, Department of Biology, University of Ghent, Belgium, offers a presentation on phylogenetics and phylogenetic trees. W. I. Sellers, Department of Human Sciences, Loughborough University, UK, provides lecture notes in PDF format on phylogenetic reconstruction. R. Irwin, Department of Biological Sciences, University of Tennessee at Martin, provides an introduction to phylogeny for a course on organic evolution. K. Thompson offers lecture notes on mammalian systematics and bat evolution and phylogenetics for a mammalogy course. Phylogenetics: Computing Evolution is a unit of a course on using computers in molecular biology offered by the New York University Medical Center.

6. Pteropodidae. The Columbia Encyclopedia includes an entry on fruit bats. An introduction to Pteropodidae is provided by the Wildlife Trust's Wild Ones Web site. Animal Diversity Web provides resources on the Family Pteropodidae. The May 1997 issue of the Proceedings of the National Academy of Sciences had an article by L. J. Hollar and M. S. Springer titled “Old World fruitbat phylogeny: Evidence for convergent evolution and an endemic African clade.”

7. A comparison of Microchiroptera and Megachiroptera is provided by the Warwickshire Bat Group. The Tree of Life Web Project offers a presentation on Microchiroptera. T. Garland Jr., Department of Biology, University of California, Riverside, makes available in PDF format a December 2004 article by J. Hutcheon and T. Garland Jr. titled “Are megabats big?”

8. Rhinolophoids. The Tree of Life Web Project has an entry for the Rhinolophoidea. Animal Diversity Web has a presentation on the Family Rhinolophidae. W. Murphy makes available in PDF format a 2003 article by E. C. Teeling et al. titled “Nuclear gene sequences confirm an ancient link between New Zealand's short-tailed bat and South American noctilionoid bats” (7). The 5 February 2002 issue of the Proceedings of the National Academy of Sciences had an article by E. C. Teeling et al. titled “Microbat paraphyly and the convergent evolution of a key innovation in Old World rhinolophoid microbats.”

9. The Eocene Epoch. The San Diego Natural History Museum provides a geologic time line. Palaeos provides information on the Eocene Epoch. UCMP provides an introduction to the Eocene Epoch and information on the fossil record of Chiroptera. The PBS Evolution Web site offers a presentation on the Eocene. C. Scotese's PALEOMAP Project provides an Eocene map and more information, as well as maps outlining early and late Eocene climate. The May 2004 issue of UCMP News had an article titled “Eocene fossils provide a glimpse of the future.” The Global Change Student Guide includes a section on Cenozoic climates.

10. Owls and nightjars. Entries on owls and on nightjars are included in Wikipedia. Animal Diversity Web provides information about owls and images of nightjars. The Secret Lives of Owls includes a section on owl classification and evolution. The Owl Pages include information on the physiology of owls and lists of owl species. Information on nightjars is provided by D. Roberson. G. Ritchison, Department of Biological Sciences, Eastern Kentucky University, provides lecture notes on bird biogeography for an ornithology course.

11. Laurasia. Laurasia is defined in S. Baum's glossary of oceanography and the related geosciences. Entries on Laurasia are included in the Hutchinson Encyclopedia and in Wikipedia. C. Scotese's PALEOMAP Project provides a map showing Laurasia and a brief description; an animation of the breakup of Pangea is also available. The formation of Laurasia and Gondwana is described in Volcano World's presentation titled “From Pangea to the present.” J. Stein Carter, Biology Department, Clermont College, University of Cincinnati, offers a presentation on continental drift and evolution for an ecology course.

12. Nancy B. Simmons is in the Division of Vertebrate Zoology, American Museum of Natural History, New York.

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