More on Australopithecus 'knuckle-walking' characters

[Richard Jensen]

John Grehan wrote:

> So here it would be a case of a character that is
> present (if it is accepted as valid - and I note that in one specimen
> the feature is a reconstruction added in) it appears to not be a
> necessary indicator of knuckle-walking since can be absent in
> knuckle-walkers.

The authors clearly inform the reader that the reconstruction played no role
in their data analysis.

>
>
> In a final note, the authors use a linkage distance measurements (Fig.
> 2c) to produce a UPGMA cluster diagram that groups together A. anamensis
> with Gorilla, then A afarensis as the sister group of that clade, and
> then Pan as the next outgroup. The next outgroup comprises a
> sister-group clade for Pongo and Hylobates.
>
>
>
> The sister clade to all these groups includes a sister group
> relationship between A. africanus and Homo, but other members of the
> group include two monkey genera.
>
>
>
> The figure is used to point out that the radius is remarkably
> human-like, and that the earliest fossils are the most African ape-like
> while later fossils more closely resemble modern humans. It seems to me
> that this is a situation where overall similarity of a feature is not
> necessarily correlated with phylogeny. Their reconstruction, if taken to
> be phylogenetic, identifies two australopithecines as Gorilla relatives,
> with chimps being more distant and the human lineage broke away before
> orangutans and gibbons etc. If the phylogenetic connection is not
> intended, then the similarity claimed between these taxa would not seem
> to be evolutionarily informative.
>
>

This is an example of not understanding what is illustrated in Richmond and
Strait's figure 2c.  This fugure is a UPGMA phenogram, based on Mahalanobis
distances, and should not be (and the authors do not do this) interpreted as
an indication of phylogeny.  The authors refer to "clades" only in the
context of other studies, not in reference to their Fig. 2c.



>
> In their figure 2b bivariate plot of canonical scores A afarensis
> actually fits within the range for both orangutans and Gorilla, and not
> chimpanzees (interesting in that orangutans are not knuckle-walkers
> while gorillas are).
>
>
>
> All that aside, the paper is terribly written (and I've written poorly
> enough myself at times) and to me epitomizes the problematic nature of
> systematic quality in hominid-hominoid phylogenetics.

Using this logic, some humans fit within the range of baboons!  What Fig. 2b
illustrates are group relationships in the first two of the three dimensions
of the CVA.  The authors note that all group centroids are, in the full
multivariate context, significantly different from one another (except for
baboons and Patas monkeys; this probably because only three Patas monkey
radii were included in the analysis).  Without access to the third dimension
(which may or may not be significant), we don't know the degree of overlap
between Pongo and the two A. afarensis specimens.  For example, the Pongo
specimens may be in a plane above or below those of Gorilla and the two
afarensis specimens may be in the Gorilla plane.  Besides, the a posteriori
probabilities for group placement indicate that both anamensis and afarensis
are closer to Gorilla than to Pongo (not reported here; an oversight
corrected in a response to a critique).

John's objections appear to me to be based, to some extent, on
misinterpretation of Figures 2b and 2c, and perhaps a misreading of the
authors' comments (I find the paper rather easy reading, although some
editing would have helped make some points more clear).

Dick
--
Richard J. Jensen              | tel: 574-284-4674
Department of Biology      | fax: 574-284-4716
Saint Mary's College         | e-mail: rjensen at saintmarys.edu
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