Re: Orangs as Closest
HARRY R. ERWIN (herwin@mason2.gmu.edu)
7 Aug 1996 13:21:19 GMT
Matthew O. Fraser (mof@prophet.pharm.pitt.edu) wrote:
: In article <839349014snz@crowleyp.demon.co.uk>, Paul@crowleyp.demon.co.uk wrote:
: Hi Paul,
: > Orangs (and gibbons) are restricted to tropical rain forests without
: > a dry season. Normally they have to get their water without coming
: > down to ground, so a month or so of drought would kill off most of
: > them. And since they need a *continuous* rain forest to travel,
: > it's unlikely they ever got outside South-East Asia.
Pongid remains are found throughout southern Asia. Remains of primitive
gibbons are found in East Africa (Micropithecus about 20 MYr BP), China
(Laccopithecus), India (Krishnapithecus) and generally in southern Asia
(Hylobates). The niche specialization of modern populations of both is
probably a relatively recent development.
: >
: > Did Schwartz deal with this aspect?
: I don't think that he did. But I'm not sure how important it is, as what
: Orangs are like now doesn't really tell us whether they shared a comon
: ancestry with hominids. Let me give you the example of the proboscidians
: and the sirenians (excuse me if the spellings are off, it's been a long
: day). Everyone agrees (I think) that these two groups are more closely
: related to each other than any other groups, and therefore shared a common
: ancestor not shared by any other groups. You certainly couldn't throw an
: American manatee up on the African savanah or the Indian jungles and
: expect it to live.
Probably the last common ancestor of Pongo and Homo dates to the late
Miocene and is somewhere between Kenyapithecus and Dryopithecus. My
character DB puts Ankarapithecus barely on the Sivapithecus side of the
divide and puts Lufengpithecus below both.
: > As I see it, the arboreal specialization is a one-way street; once
: > you're up in the high canopy, there's no way you can get down; if
: > there is a partly terrestrial niche, there will always be a partly
: > terrestrial animal occupying all, or most, of it and your arboreal
: > adaptations will be extremely disadvantageous. If orangs were the
: > closest to hominids then they must have split before the orang
: > became an arboreal specialist, and that's most unlikely.
: Arboreal specialization may well be a one way street, but I'm not sure
: about that one either. Surely ancestrial primates were mostly arboreal,
: and only later certain groups went down to more terrestrial niches. Most
: prosimians and New World Monkeys are entirely arboreal. Presumably they
: shared a common ancestor at some point with Old World Monkeys, Apes and
: Hominids.
That's what the fossil evidence seems to show. The speculation on
catarrhine monkeys is particularly interesting. Cheek pouches are
regarded as the evolutionary innovation that allowed them to forage on
the ground and digest their food in the trees.
: Also, I'm not suggesting that Orangs gave rise to hominids, only that they
: possible shared a common ancestor. Following that, both lineages were
: free to specialize in whatever direction that Natural Selection led them.
: > The LCA of orangs and chimps/gorillas must have been a mostly
: > terrestrial animal to leave descendant populations in both Africa
: > and Asia.
: Hmmm. Not sure about that, either. Depends on the continuity of canopies,
: etc. Of course, terrestrial travel is not out of the question, and the
: two together could have been the most efficient/safest mode of spread.
And we do find highly arboreal colobine monkeys with a discontinuous
distribution, where the gaps have not had a continuous closed canopy
forest since at least the early Miocene, before the evolution of the
colobines.
: The picture may be, as Schwartz would suggest:
: Pan Gorilla Pongo Hominid
: * * * *
: * * * *
: * * * *
: ** * *
: * * *
: * *
: * *
: * *
: * *
: *
: *
: Or it may be, if you consider the Apes to be closest to each other:
: Pan Gorilla Pongo Hominid
: * * * *
: * * * *
: * * *
: * * *
: * * *
: ** *
: * *
: * *
: *
: Or, I think what most favor:
: Hominid Pan Gorilla Pongo
: * * * *
: * * * *
: * * *
: * * *
: * * *
: * *
: * *
: *
: All three assume a thin enamelled common ancestor, at least if you buy
: that that the larger group comprising Apes and Hominids shared common
: ancestry with (or within) Old World Monkeys. I have a difficult time with
: the last two scenarios, however, as it seems that thick enamel, a
: specialization, would have to be lost at some point (twice for the last
: scenario).
Thick enamel is now regarded as probably primitive. The DNA data favor a
trifurcation of Pan, Gorilla, and the hominids.
: I remember someone stating that dental remains are no longer favored by
: paleontologists. Is this true? Sometimes we have so little else. Bone
: is living tissue and responds to environmental (both internal and
: external) stresses throughout life. A sagittal crest is not genetically
: encoded directly, it is the result of muscle attatchment. Strongly
: sexually dimorphic species, where the male has a pronounced saggital crest
: and females do not. I guarrantee that if you shoot up a female with
: dihydrotestosterone, she'll be growing a crest. Weight lifters have
: heavier, thicker bones, yadda, yadda, yadda.
Dental evolution in hominoids tends to be ambiguous, since the underlying
structure is basically proconsulid and the bells and whistles are very
specific to the biome occupied by the animal. Change the biome, and the
detailed characteristics change. All very convergent and confusing.
: Dentition, however, is much less susceptible to environmental stress, and
: is therefore a better genetic marker, I would think. Yes, you can wear
: teeth down, and yes they get carries, but certain features remain.
: Patterns of eruption. Cusp patterns. Enamel thickness. All kinds of
: handy markers. Happily, that which makes it so also preserves it best.
--
Harry Erwin, Internet: herwin@gmu.edu, Web Page: http://osf1.gmu.edu/~herwin
49 year old PhD student in computational neuroscience ("how bats do it" 8)
and lecturer for CS 211 (data structures and advanced C++)
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