Human evolution

[John Grehan]

I started reading the August issue of Scientific American devoted entirely
to human evolution. As usual I'll stick my neck out and offer comments
(whether they be considered uninformed or otherwise). The full title is "A
new look at human evolution" but for the most part the articles deal with
views and perspectives that have been around for some time - many years in
some cases. I was immediately impressed by the vagueness of systematic
criteria being presented which is perhaps not surprising given the sloppy
nature of character definition I have found in my current readings of
higher primate systematics supporting the African ape/chimpanzee connection.

The first article deals with Sahelanthropus which authors have argued to be
'hominid' even though the article makes what are, to me, quite interesting
statements that researchers disagree over what makes the human lineage
unique and that we 'have trouble defining hominids'. The article then goes
on to describe cladistics as the tool for systematic analysis followed by
the two seminal hominid characters being bipedalism and transformation of
the canine (from honing against the first lower premolar to its incisor
form). The trouble with these is that they do little to understand the
relationships of such 'hominid' taxa with other apes.

It is against this vagueness of the traditional hominid model with its
uncertainty over what constitutes the hominid lineage that I would contrast
with the orangutan model where it is possible to delineate likely
candidates among fossil taxa by their conformity to those Homo-orangutan
synapomorphies while lacking those synapomorphies for the orangutan clade
itself. In this respect there are fossil apes that are going to be
problematic simply because there is so little material available - but this
is a problem in material - not the definition.

In the case of Sahelanthropus, its widely (although not unanimously - some
workers consider it a gorilla relative) hailed representation as the
earliest hominid is problematic. The teeth are described as being
between  chimps and Australopithecus. In my view this suggests basically
'thin' enamel that may lie outside the Homo-orangutan configuration. The
skull has a large continuous supraorbital torus which is 'similar' to later
hominids including Kenyanthropus and Homo. The trouble with this is that,
as posted earlier, this character is lacking in australopithicenes (while
present in chimps and gorilla). The illustration of Kenyanthropus in Nature
is not sufficient for me to see whether a continuous torus is present, but
the authors do not mention one. What would be nice to see is whether the
upper palate of Sahelanthropus has one or two incisive foramina, since it
is the single foramen that is uniquely shared between humans and orangutans
and so far appears to be correlated with the presence of thick dental
enamel in fossil taxa such as Hispanopithecus, and Sivapithecus.
Interestingly, the 10 myr Rudapithecus has a single incisive foramen, but
thin enamel, while Afropithecus has thick enamel, but two incisive
foramina. These may represent sister taxa to the  Homo-orangutan clade. As
for Ardipithecus - it has thin enamel, but the upper palate is unknown. I
doubt it is such a close relative as generally claimed.

Quite a bit of speculation I agree. Just thoughts for the moment. While the
Homo-orangutan connection is clear on the basis of extensive
synapomorphies, many are soft-tissue features absent in fossils, and the
fossils themselves are so incomplete in many taxa for critical areas (such
as the upper palate). I've not delved very deeply into the fossil taxa yet,
so I admit in advance to possible mistakes in my current representation and
I will stand corrected as necessary.

John Grehan



Dr. John Grehan
Director of Science and Collections
Buffalo Museum of Science
1020 Humboldt Parkway
Buffalo, New York 14211-1293
Voice 716-896-5200 x372
Fax 716-897-6723
jgrehan at sciencebuff.org
http://www.sciencebuff.org/biogeography/Panbiogeography/Panbiogeography-Gate.htm
http://www.sciencebuff.org/HepialidaeGate.htm