[Taxacom] Ardipithecus not a hominid?

[John Grehan]

This note is for those interested in the Ardipithecus evidence and may wish to comment.

 

I’m doubtful that there is a strong case, if any real case at al, for Ardipithecus being a hominid. If any on this list are convinced of the hominid status for Ardipithecus I will be very interested in a counter-critique of the evidence.

 

 In looking over the evidence one of the major problems is that the presentation of evidence is so astonishingly superficial and that much of the argument rests on synthetic reconstructions. Overall there is a complete lack of detail that would allow independent critical assessment of their claims. Its almost as if the authors have avoided that capability by omitting detailed, high resolution images in their supplementary material.

 

It seems to me that the standards of documentation for hominid claims are less than is usually required for phylogenetic reconstructions. 

 

The most glaring omission is, of course, the total absence of evidence for a shared last common ancestor with humans and chimps, the repeated assertion that the fossil is not like chimps while at the same time restricting all comparisons to the chimp. 

 

Dentally Ardipithecus does not even conform to specialized features found in other hominids (australopiths, Homo) as well as orangutans -  protoconid is more mesially situated,  anterior fovea (trigonid basin) smaller than talonid,  trigonid basin less vertical and lingually facing  trigonid basin enclosed by distinct paracristid coursing mesially then down to lingual side,   talonid basin more horizontally oriented and enclosed by distinct hypocristid

 

As for the hominid evidence, it resides around interpretations of the position of the foramen magnum and the structure of the pelvis.

 

The foramen argument is that the anterior edge of the foramen is only 1.3 mm posterior to the center of the carotid foramen. They compared this to 6.4 in bonobos and 7.3 in chimps, but they also recognized that the position overlaps (minimally) with Australopithecus which range 0-5 mm posterior. They get around this by arguing that the distance as a percentage of bicarotid breadth is less than 10% in hominids compared with greater than 10% in Pan and that Ardipithecus is clear different from Pan.

 

They note that in humans there is considerable overlap with chimpanzees, but they dismiss the human  condition because the jugular and carotid sizes are much larger that “almost certainly” (one sees this phrase here and there) directly affects measures of basion position and that the human condition has little consequence in evaluating and interpreting Ardipithecus.

 

In the juveniles of extant anthropoids, including humans,  the basion, bicarotid and biporionic cords are in alignment, but with growth the basion may come to be posterior to both chords which may become dissociated if the petrosal bones become elongated resulting in a more anterior position of the carotid foramina. The basion may come to be anterior to both cords remaining in alignment. Or the basion may be displaced forward with the anterior margin between the bioporionic and bicarotid cord. Thus the position of the biocarotid cord is changeable.

 

In gorillas the basion tents to be posterior to the bioporionic chord, but in orangutans and some monkeys the basion remains close to the bioporionic cord. 

 

At the very least the basion position would appear to be problematic and the bipedal interpretation rests heavily on an uncorroborated assumption.

 

With all comparisons in Table 1 being limited to two australopiths, Sahelanthropus (assumed to be a hominid in the Ardi papers) and chimps, the hominid inferences get a bit tricky to say the least. The details of dentition are rather poorly illustrated, but perhaps some of the features can be tested against the current illustrations – so something for me to look into further.

 

Below is a list of hominid features used in Grehan and Schwartz (2009) in relation to the current Ardipithecus information

 

pubic ramus long – not applicable to Ardipithecus

 

ilium superoinferiorly short – inferred in the reconstruction but not well substantiated in the original material

 

ilium expanded posteriorly – shown in reconstruction. Not clear in the original material.

 

iliac crest (tubercle) thickened – section of ilium missing

 

anterior inferior iliac spine knoblike – inferred. Said to be present as shown in Fig. 1. Looks much the same as a chimp

 

anterior inferior iliac spine near acetabulum – if indeed present

 

linea aspera present –P. 71e4 4th paragraph- said to be “broad proto-linea aspera”. Cannot se spiral line in Fig. 4b. Cannot see linea aspera in Fig. 4b.

 

spiral line connects to linea aspera – P. 71e4 para 2 “medial border of an obvious hypotrochanteric fossa homology converges with the spiral line to form a markedly rugose, elvated plane on the posterior femora surface” Cannot see spiral line in Fig. 4b.

 

femur condyles unequal – no information



femoral neck thickened inferiorly – no information



femoral shaft outwardly angled – no information

 

two tibial tubercles – no information

 

lateral tibial facet concave – no information

 

latearl prox. tibial facet level with medial facet – no information

 

distinct angle at L5-S1 – no information

 

Pelvis – iliac isthmus is said to be short – same as in afarensis. Not sure that they are the same. 

 

 

Dr. John R. Grehan

Director of Science

Buffalo Museum of Science1020 Humboldt Parkway

Buffalo, NY 14211-1193

email: jgrehan at sciencebuff.org

Phone: (716) 896-5200 ext 372

 

Panbiogeography

http://www.sciencebuff.org/research/current-research-activities/john-grehan/evolutionary-biography <http://www.sciencebuff.org/biogeography_and_evolutionary_biology.php> 

Ghost moth research

http://www.sciencebuff.org/research/current-research-activities/john-grehan/ghost-moths <http://www.sciencebuff.org/systematics_and_evolution_of_hepialdiae.php> 

Human evolution and the great apes

http://www.sciencebuff.org/research/current-research-activities/john-grehan/human-origins