Re: Orangs as Closest

Matthew O. Fraser (mof@prophet.pharm.pitt.edu)
Tue, 06 Aug 1996 23:19:00 -0400

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.
>
> 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.

> 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.

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.

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).

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.

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.

Thanks for Your Thoughts,

Matt (who is pleased to see that this has developed into a very
interesting discussion, at least for me)

-- 

Matthew O. Fraser                    "If you can't answer a man's

Department of Pharmacology            arguments, all is not lost.

School of Medicine                      You may still call him

University of Pittsburgh                       vile names"

Pittsburgh, PA 15261

mof@prophet.pharm.pitt.edu                     John Homans