Flying primates theory
Encyclopedia
The flying primates theory conjectures that megabat
s, a sub-group of Chiroptera (also known as flying foxes), form an evolutionary sister group of Primate
s. This theory started with Carl Linnaeus, and was again advanced by J.D Smith in 1980. It was proposed in its modern form by Australian neuroscientist Jack Pettigrew
in 1986 after he discovered 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 mammal
s. This was followed up by a longer study published in 1989, in which this was supported by the analysis of many other brain
and body characteristics. Pettigrew suggested that flying foxes, colugo
s 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 bat
s under a same taxonomic group, the Superorder Archonta
). Rather, many biologists resisted the implication that megabats and microbat
s (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.
) disputed the existence of an advanced pattern of connections between the retina
and the superior colliculus
. However, this conclusion was later criticised on methodological grounds. 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. 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.
s would have evolved twice in mammals, but also on biochemical studies of molecular evolution, which indicate that bats are monophyletic. 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"
Megabat
Megabats constitute the suborder Megachiroptera, family Pteropodidae of the order Chiroptera . They are also called fruit bats, old world fruit bats, or flying foxes.-Description:...
s, a sub-group of Chiroptera (also known as flying foxes), form an evolutionary sister group of Primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s. This theory started with Carl Linnaeus, and was again advanced by J.D Smith in 1980. It was proposed in its modern form by Australian neuroscientist Jack Pettigrew
Jack Pettigrew
John Douglas Pettigrew is Emeritus Professor of Physiology and Director of the Vision, Touch and Hearing Research Centre at the University of Queensland in Australia.Professor Pettigrew's research interest is in comparative neuroscience...
in 1986 after he discovered that the connections between the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
and the superior colliculus
Superior colliculus
The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
(a region of the midbrain) in the megabat Pteropus
Pteropus
Bats of the genus Pteropus, belonging to the megabat or Megachiroptera sub-order, are the largest bats in the world. They are commonly known as the fruit bats or flying foxes among other colloquial names...
were organized in the same way found in primates, and different from all other mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s. This was followed up by a longer study published in 1989, in which this was supported by the analysis of many other brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
and body characteristics. Pettigrew suggested that flying foxes, colugo
Colugo
Colugos are arboreal gliding mammals found in South-east Asia. There are just two extant species, which make up the entire family Cynocephalidae and order Dermoptera. They are the most capable of all gliding mammals, using flaps of extra skin between their legs to glide from higher to lower...
s and primates were all descendants of a same group of early arboreal mammals. The megabat flight and the colugo gliding
Gliding
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word soaring is also used for the sport.Gliding as a sport began in the 1920s...
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 bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s under a same taxonomic group, the Superorder Archonta
Archonta
The Archonta are a group of mammals considered a superorder in some classifications.The Archonta consist of the following orders:*Primates*Plesiadapiformes *Scandentia *Dermoptera *Chiroptera...
). Rather, many biologists resisted the implication that megabats and microbat
Microbat
The microbats constitute the suborder Microchiroptera within the order Chiroptera . They are most often referred to by their scientific name...
s (or echolocating bats) formed distinct branches of mammalian evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, with flight having evolved twice
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
. 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
Insectivora
The order Insectivora is a now-abandoned biological grouping within the class of mammals...
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.
Neurological studies
Soon after Pettigrew's study, work on another genus of megabat (RousettusRousettus
Rousettus is a genus of Old World fruit bats or megabats. They are sometimes referred to as dog-faced fruit bats, or flying foxes. The genus is a member of the suborder Megachiroptera...
) disputed the existence of an advanced pattern of connections between the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
and the superior colliculus
Superior colliculus
The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
. However, this conclusion was later criticised on methodological grounds. 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. 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.
Biochemical studies
The implication that bats are diphyletic has been fiercely disputed by many zoologists, not only based on the unlikelihood that wingWing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
s would have evolved twice in mammals, but also on biochemical studies of molecular evolution, which indicate that bats are monophyletic. 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"