Re: AAT Theory

Phillip Bigelow (
14 Sep 1995 16:25:10 -0700

>chris brochu <> writes:
>>It's me again - the croc worker.
>>I approached this issue with the same philosophical tool I use in my own
>>work - the principle of parsimony.
>>Hypothesis 1: ...... Tree to land directly.
>>Hypothesis 2: ...... Tree to water to land.
>>At this point, a consideration of the phylogeny of these animals is
>>important. None of the close outgroups to Hominidae are aquatic.
>>If there was an aquatic phase, it doesn't show up on a phylogeny of the known primates.

Tom Clarke responded:
>Careful. You are mixing hominids and primates here. The AAH advocate would
>argue that true bipedalism is just the aquatic characteristic which you seek.

Not to nit-pick too much, but hominids *are* primates. Apes, monkeys, and
hominids are all part of the group Primates.
Given that, it is important to include both hominids and apes in the
cladistic analysis. In that vein, Chris was correctly including hominids
and apes together. Provided that ancestor-descendent relationships can be
approximated, of course.

Chris continues:
>>Hypothesis 1 is clearly the most parsimonious. To accept hypothesis 2,
>>we would have to assume, ad hoc, an important stage in our evolution that
>>is not supported by primate phylogeny.

True Chris; hypothesis 2 has more "steps" (a
phrase often used in cladistics to indicate the number of changes required
to get from morphology "A" to morphology "B"). In cladistics, a large
number of "steps" is considered to be less parsimonious than is a fewer
number of "steps". AAT would therefore fail some of the statistical
criteria in cladistics as well.

Tom Clarke continues:
>Seriously, in an earlier post I asked what the non-primate fossil
>community made of the venue of fossils. There must be reasoning
>behind when a fossil found in an aquatic environment implies an
>aquatic animal.

The paleontological community would probably answer your question the
following ways:
1) It is recognized that most fossils, be they terrestrial animals, or
animals living in water, *get deposited by- and in- water*.

2) When a fossil is found, it's paleo-ecology is determined by the following
criteria: a) comparison with similar taxa living today; and b) if no living
descendent exists to compare with the fossil (such as pterosaurs), then
the morphology of the fossil determines it's habits and ecology.

Paleontologists have always been wary of reading too much into the
depositional environment of where a particular fossil is found...considering
that most land animals are found in river channel (as in the Wankel T. rex),
river floodplain (as are the majority of land animals), near-shore marine
(as is the case with many ankylosaur dinosaurs), and slow-moving to standing
water (?lake) (as is the case with Lucy).

Skeletal morphology is the way to find the answers in these cases.