Flying primates theory

The flying primates theory conjectures that megabats, a sub-group of Chiroptera (also known as flying foxes), form an evolutionary sister group of Primates. This theory was proposed by Australian neuroscientist Jack Pettigrew in 1986 [cite journal | author=Pettigrew JD |year=1986 |title=Flying primates? Megabats have the advanced pathway from eye to midbrain |journal=Science |volume=231 |issue=4743 |pages=1304–1346 |doi=10.1126/science.3945827 | pmid=3945827] after discovering that the connections between the retina and the superior colliculus (a region of the midbrain) in the megabat Pteropus were organized in the same way found in primates, and different from all other mammals. This was followed up by a longer study published in 1989, [cite journal | author=Pettigrew JD, Jamieson BG, Robson SK, Hall LS, McAnally KI, Cooper HM |year=1989 |title=Phylogenetic relations between microbats, megabats and primates (Mammalia: Chiroptera and Primates) |journal=Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences |volume=325|issues=1229 |pages=489–559 |doi=10.1098/rstb.1989.0102] in which this was supported by the analysis of many other brain and body characteristics. Pettigrew suggested that flying foxes, colugos and primates were all descendants of a same group of early arboreal mammals. The megabat flight and the colugo gliding could be both seen as locomotory adaptations to a life high above the ground.

The flying primate theory met resistance from many zoologists. Its biggest challenges were not centered on the argument that megabats and primates are evolutionarily related, which reflects earlier ideas (such as the grouping of primates, tree shrews, colugos and bats under a same taxonomic group, the Superorder Archonta). Rather, many biologists resisted the implication that megabats and microbats (or echolocating bats) formed distinct branches of mammalian evolution, with flight having evolved twice. This implication was borne out of the fact that microbats do not resemble primates in any of the neural characteristics studied by Pettigrew, instead resembling primitive mammals such as Insectivora in these respects. The advanced brain characters demonstrated in Pteropus could not, therefore, be generalized to imply that all bats are similar to primates. More recently, the flying primates hypothesis was rejected when scientists compared the DNA of bats to that of primates. These genetic studies support the monophyly of bats. [cite journal | doi=10.1073/pnas.111551998 | year=2001 | month=May | author=Springer, Ms; Teeling, Ec; Madsen, O; Stanhope, Mj; De, Jong, Ww | title=Integrated fossil and molecular data reconstruct bat echolocation | volume=98 | issue=11 | pages=6241–6 | pmid=11353869 | pmc=33452 | journal=Proceedings of the National Academy of Sciences of the United States of America | url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=11353869 | format=Free full text] [cite web | title=Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation | doi=10.1038/nature06549 | url=http://www.nature.com/nature/journal/v451/n7180/abs/nature06549.html | accessdate=2008-07-03 | date=2008-02-14 | work=Nature] [cite journal | doi=10.1126/science.1105113 | year=2005 | month=Jan | author=Teeling, Ec; Springer, Ms; Madsen, O; Bates, P; O'Brien, Sj; Murphy, Wj | title=A molecular phylogeny for bats illuminates biogeography and the fossil record | volume=307 | issue=5709 | pages=580–4 | pmid=15681385 | journal=Science (New York, N.Y.)] [cite journal | doi10.1093/molbev/msi180 | year=2005 | month=Sep | author=Eick, Gn; Jacobs, Ds; Matthee, Ca | title=A nuclear DNA phylogenetic perspective on the evolution of echolocation and historical biogeography of extant bats (chiroptera) | volume=22 | issue=9 | pages=1869–86 | pmid=15930153 | journal=Molecular biology and evolution | url=http://mbe.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=15930153 | format=Free full text]

Neurological studies

Soon after Pettigrew's study, work on another genus of megabat (Rousettus) disputed the existence of an advanced pattern of connections between the retina and the superior colliculus. [cite journal | author=Thiele A, Vogelsang M, Hoffmann KP |year=1991 |title=Pattern of retinotectal projection in the megachiropteran bat "Rousettus aegyptiacus" |journal= Journal of Comparative Neurology |doi=10.1002/cne.903140404 |volume=314 |pages=671–683] However, this conclusion was later criticised on methodological grounds. [cite journal | author=Rosa MG, Schmid LM |year=1994 |title=Topography and extent of visual-field representation in the superior colliculus of the megachiropteran Pteropus |journal=Visual Neuroscience |volume=11 |issue=6 |pages=1037–1057] Later studies have sought further evidence of unique characteristics linking the megabat and primate brains. These studies have had limited success in identifying unique links between megabats and present-day primates, instead concluding that the megabat brain has characteristics that may resemble those likely to have existed in primitive primate brains. [cite journal | author=Ichida JM, Rosa MG, Casagrande VA |year=2000 |title=Does the visual system of the flying fox resemble that of primates? The distribution of calcium-binding proteins in the primary visual pathway of "Pteropus poliocephalus" |journal=Journal of Comparative Neurology |doi=10.1002/(SICI)1096-9861(20000131)417:1<73::AID-CNE6>3.0.CO;2-C |volume=417 |pages=73–87] Nonetheless, modern neuroanatomical studies have repeatedly supported the existence of very significant differences between the brains of megabats and microbats, which is one of the anchors of the "Flying primates" theory. [cite journal | author=Maseko BC, Manger PR |year=2007 |title=Distribution and morphology of cholinergic, catecholaminergic and serotonergic neurons in the brain of Schreiber's long-fingered bat, "Miniopterus schreibersii" |journal= Journal of Chemical Neuroanatomy |doi=10.1016/j.jchemneu.2007.05.004 |volume=34 |pages=80–94 ] [cite journal | author=Maseko BC, Bourne JA, Manger PR |year=2007 |title=Distribution and morphology of cholinergic, putative catecholaminergic and serotonergic neurons in the brain of the Egyptian rousette flying fox, "Rousettus aegyptiacus" |journal= Journal of Chemical Neuroanatomy |issue=3-4 |doi=10.1016/j.jchemneu.2007.05.006 |volume=34 |pages=108–127]

Biochemical studies

The implication that bats are diphyletic has been fiercely disputed by many zoologists, not only based on the unlikelihood that wings would have evolved twice in mammals, but also on biochemical studies of molecular evolution, which indicate that bats are monophyletic. [cite journal | author = Mindell DP, Dick CW, Baker RJ |year=1991 |title=Phylogenetic relationships among megabats, microbats, and primates |journal=Proceedings of the National Academy of Sciences of the USA |volume=88 |issue=22 |pages=10322–10326 |doi=10.1073/pnas.88.22.10322] [cite journal| author=Stanhope MJ, Czelusniak J, Si JS, Nickerson J, Goodman M |year=1992 | title=A molecular perspective on mammalian evolution from the gene encoding interphotoreceptor retinoid binding protein, with convincing evidence for bat monophyly |journal=Molecular Phylogenetics and Evolution |volume=1 |issue=2 |pages=148–146 | doi=10.1016/1055-7903(92)90026-D] However, other studies have disputed the validity of these conclusions. In particular, it has been argued that phylogenies that are based solely on DNA data can be subject to an artifact named the "base-compositional bias" [cite journal | author=Hutcheon JM, Kirsch JA, Pettigrew JD | year=1998 |title=Base-compositional biases and the bat problem. III. The questions of microchiropteran monophyly |journal=Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences |volume=353 |issue=1368 |pages=607–617 |doi=10.1098/rstb.1998.0229]

References

External links

* [http://www.uq.edu.au/nuq/jack/consensus.htm Jack Pettigrew's criticism of the molecular evidence]