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| Philippine Tarsier - Roberto Verzo's Photostream |
Primates are mammals possessing a generally accepted group of physical characteristics.
These include opposable thumbs, a relatively large brain, eyes that are
oriented towards the front of the head, two nipples and stereoscopic
(binocular) vision.
Until recently,
taxonomists have generally adopted the similar classification systems proposed
by Simons in 1972
and Fleagle in 1998.
According to these systems, the primate order was divided into two sub-orders:
the prosimians and the anthropoids.
Prosimians
included lemurs, lorises and tarsiers, grouped together because they were
considered morphologically more primitive than other primates. The anthropoids
comprised all other primates – in other words, monkeys, apes and man.
Fleagle’s
system also placed humans in the superfamily Hominoidea, along with apes. The
apes, which included orangutans, gibbons, gorillas, chimpanzees and bonobos,
were then separated into the family Pongidae, while humans were grouped into
the Hominidae.
The Re-Classification of Tarsiers
Recent
anatomical and biochemical studies, however, have thrown some doubt on this
system. According to Ben F. Koop
and fellow researchers at the Wayne State University, for instance, the alpha
and beta globin genes in tarsiers (see fig.1) bear more resemblance to those of
the anthropoids than those of the prosimians. DNA hybridization comparisons and
RNA sequencing have also suggested a similar link.
In addition,
the nostrils of tarsiers and anthropoids are surrounded by dry skin whereas
lemurs and lorises have moist nostrils. Other physical differences, such as the
absence of a tapetum (a layer in the retina that helps in night vision) in
tarsiers and anthropoids, have led many taxonomists to devise two new primate
sub-orders. These are the Strepsirhines (lemurs and lorises) and the
Haplorhines (tarsiers, monkeys, apes and humans).
In
evolutionary terms, this new grouping coincides with the view that tarsiers
represent an evolutionary bridge between the two groups (see fig.2) and are in
fact quite unique in many aspects of their morphology and behaviour. Jason Cowan,
of Washington State University goes so far as to suggest they should be placed
in a sub-order of their own.
New Groupings for Apes and Humans
Moreover,
molecular comparisons of members of the super family Hominoidea have revealed
that gorillas and chimpanzees are more closely related to humans than other
apes. This similarity is so pronounced that they have now been placed together
with humans in the family Hominidae. In fact, 98.77% of the DNA in humans and
chimpanzees is identical, representing an evolutionary divergence that occurred
as recently as 6 million years ago.
Gibbons, on
the other hand, have been grouped with siamangs into the family Hylobatidae.
According to Evans, 2009, this family represents a closer evolutionary link to
Old World Monkeys (a 27 gene difference) than the Hominidae (a 31- 36 gene
difference) and the Pongidae (a 33 gene difference). The Pongidae family now
consists only of the orangutans, a group of South East Asian apes that diverged
from other hominoids around 11 million years ago.
Note that
another system places the orangutans in the Ponginae, a sub-family of the
Hominidae, as a result of their close genetic relationship to this family. The
African apes (gorillas, chimps and bonobos), however, remain the nearest
evolutionary link to humans.
Geographical Distribution and Evolution
Considering
humans are believed to have arisen in Africa, our close relationship to the
African apes appears to make sense. Humans and apes in fact belong to the
infra-order catarrhines because, along with biochemical similarities, they have
downward pointing noses and lack prehensile (branch-gripping) tails. Old World
Monkeys such as baboons and macaques are also included in this group, and can
be found in Africa or Asia.
Similarly, New
World monkeys (infra-order platyrrhines), all found in South America, Central
America and the Caribbean islands, are grouped together because of physical
similarities such as broad noses and prehensile tails. Lemurs and lorises are
found only in Madagascar, while tarsiers are limited to a narrow geographical
range within South East Asia.
These findings
are consistent with the concept that evolution occurs in populations that are
geographically isolated. The Strepshirines (lemurs and lorises) for instance,
are a unique group of primates largely because Madagascar, isolated from India
for over 80 million years and separate from the coast of Africa, has provided
ideal conditions for natural selection to have occurred.
Similarly, the
New World Monkeys may have evolved to develop their particular features after
South America separated from North America. Alternatively, according to McKenna
(1980), they possibly arrived some time later by ‘island’ hopping from North
America via the Caribbean islands. In any case, the original primates to arrive
in this continent had ample time to evolve in relative isolation.
It would
appear, then, that geographical distribution and evolutionary trends, along
with physical and biochemical factors, all tend to support current primate
classification systems. The creation of the new sub-orders, Strepshirines and
Haplorhines, to reflect the unique nature of the Tarsier, is perhaps the best
example of this.
References
•
Cowan, J., Washington State University, 2006, 'Are Tarsiers Really a
Taxonomic Enigma?', soar.wichita.edu
•
Fleagle, 1998, 'Taxonomy of the Primates', theprimata.com
•
McKenna,M (1980), 'Early History and Biogeography of South America's
Extinct Land Mammals', Plenum, New York
•
Simons, E. 1972, 'Primate Evolution', Macmillan, New York
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