Thoughts on the Sahelanthropus reconstruction
I am at the AAPA meeting in Milwaukee this week, and so posting is by necessity very light. However, the news of the new Sahelanthropus remains and CT reconstruction have come out this week. I have been thinking about them since I got a hold of the proofs last week, so I can post some comments about them. There are some thoughts I'm holding on to for now, however, since I have a manuscript that covers some of them. It's bad enough to be scooped by other people; I surely don't want to scoop myself!
BBC News story, with artist rendition from Nature cover.
The lead story seems to be the reconstruction, probably because it was intended to sort out many of the problems with the distortion in the original fossil. To some extent it succeeds in simplifying the interpretation. For example, the reconstruction clearly places the foramen magnum in a more anterior position than the original. It is not clear to me how the anatomy of the original could conform to the reconstructed base, but doubtless working with a CT is better than working with photographs.
Actually, the article does not place a great emphasis on the anterioposterior position of the foramen magnum. This is sensible, because chimpanzees and australopithecines overlap considerably in this position compared to other basicranial landmarks like the bicarotid line. TM 266 is within the region of overlap, both in the original distorted version and in the reconstructed version.
Instead, Zollikofer and colleagues make two complementary arguments for why the skull is hominid. The first concerns the angulation of the foramen magnum (characterized by the basion-opisthion line) compared to a line tangent to the upper and lower orbital margins.
Despite substantial differences in neck orientation, humans and non-human primates tend to locomote with their orbital planes (the line joining the superior and inferior margins of the orbits) approximately perpendicular to the ground. In addition, primates orient the upper cervical vertebrae approximately perpendicular to the plane of the foramen magnum, and with only a limited range (about 10 degrees) of flexion and extension possible at the cranio-cervical joint. The combined effect of these angular constraints is that the angle between the foramen magnum and the orbital plane is nearly perpendicular in Homo sapiens (103.2 +- 6.9 degrees, n = 23) but more acutely angled in Pan troglodytes (63.7 +- 6.2 degrees, n = 20), and other species with more pronograde postures. The foramen magnum angle relative to the orbital plane in the TM 266 reconstruction is 95 degrees, similar to that in humans and later bipedal hominids such as Australopithecus afarensis (AL 444-2) and A. africanus (Sts 5). TM 266-01-060-1 as a quadruped would requier an unusually extended angle of the neck relative to the plane of the foramen magnum (Zollikofer et al. 2005:757).
A weakness in this argument is that this angle is exquisitely sensitive to the reconstruction. That is, a small difference in the vertical position of either basion or opisthion (the front and rear points on the foramen magnum border, respectively) will have a large effect on the angle of the line passing through these points. But assuming the reconstruction is correct, it is fairly compelling evidence that the habitual posture of the head in Sahelanthropus was not like chimpanzees or gorillas.
The second argument concerns the downward lip of the nuchal crest, which they argue indicates the directionality of the nuchal muscles. It is true that some other hominids have a downward lip on this crest, but I would like to go through a large ape sample to see the range of variation in this trait. In any event, this feature cannot be isolated from the exceedingly unique nuchal morphology in this specimen; the orientation and function of the nuchal musculature cannot be assumed to be like that of other apes whether it had vertical posture or not.
So was it a biped? From the reconstruction alone it may not be possible to confirm or deny the hypothesis. A more vertical habitual posture might or might not imply facultative bipedality. One possibility that would not imply bipedality is that Sahelanthropus had long arms, on the scale of Dryopithecus or longer. In this case, a quadrupedal stance would involve a more vertical trunk position. The distinction between this adaptation and that of gibbons or dryopithecines would be the larger body size and consequent greater degree of terrestriality. This hypothesis might also explain other Miocene hominoids that have been suggested to be like bipeds in certain characters, including Ouranopithecus and Oreopithecus. A test of the relationship of trunk position, limb length, and cranial base morphology might be informative.
Setting aside the question of whether it was a biped, was it a hominid? These are different questions if we assume that the advent of hominid bipedalism followed after some significant time the divergence of hominids from chimpanzees. Aside from Sahelanthropus, the earliest comparably complete hominid cranial remains are less than half its age. The closest is the as-yet-undescribed StW 573 skull. Then is KNM-WT 40000, followed by the cranial remains from Hadar, including the AL 444-2 specimen. A. afarensis and later A. africanus both have extensive adaptations to masticatory force. The extensive nuchal plane of TM 266 is long, narrow, and flat, and it is unlike any early hominid. The browridges are larger (especially in proportion to its relatively small overall cranial size) than in any australopithecine. Thus, it is a challenge to explain exactly what this skull represents in adaptive terms. I think an explanation of its anatomy is in order before it is accepted as being phylogenetically close to the australopithecines.
The paper by Brunet et al. (2005) presents new mandibular and dental remains of Sahelanthropus, including a lower canine with apical wear.
The new material presented here is important for several reasons. . . . The S. tchadensis hypodigm now includes a minimum of six individuals (a maximum of nine) from three sites in a small area of the Anthracotheriid Unit. Second, these new fossils now present a more complete and reliable understanding of this earliest known hominid taxon. S. tchadensis shares major derived features with other recognized hominids that are consistent with its position in the hominid clade, close to the last common ancestor of chimpanzees and humans. In the dentition these anatomical characters are a non-honing C/P3 complex; no diastema between C and P3; a vertical symphysis with weak transverse tori; canines with a small crown and long root; a lower canine crown with a large distal tubercle, both shoulders being very low; an upper P3 with a steeply sloping buccal surface; postcanine teeth with maximum radial enamel thickness intermediate between chimpanzees and australopithecines; and bulbous, slightly crenulated postcanine occlusal morphology. All the hominid mandubular premolar specimens from Toros-Menalla have the same root pattern, with two roots and three separate pulp canales in each premolar (one mesial and two distal) retaining the presumed primitive condition for the Pan/Homo clade (Brunet et al. 2005: 754).
This is a bit of a confused list, since very few of these characters are actually both derived and shared with later hominids. For example, a character that retains "the presumed primitive condition for the Pan/Homo clade" clearly is not a "major derived feature" shared with "other recognized hominids."
The most persuasive similarity with hominids is the reduced canine. But to my eyes, the Sahelanthropus lower canine is distinct from later hominids, especially considering the prominent ridge, or shoulder, around the base of the crown. This feature is found among dryopithecines, and it may simply be a primitive feature retained in an otherwise reduced canine. So the idea that this is intermediate between a larger, ape-like canine and the canines of later hominids is possible, but not demonstrated.
So in my view, the hypothesis that Sahelanthropus is in fact an early hominid has not been strongly substantiated. In many of its features it is basically plesiomorphic, and shares the morphology of a number of Miocene apes. In a few features, it shares a derived (or partially derived) morphology with australopithecines. It also has cranial features such as its long flat nuchal torus and hulking browridge that are derived, not shared with later hominids, and would therefore tend to indicate a separate evolution for this taxon. In my opinion, we probably have entered a time period early enough that the relationships of early hominids, early chimpanzees, gorillas and their ancestors may not be readily resolved with morphological comparisons.
References:
Brunet M et al. 2005. New material of the earliest hominid from the Upper Miocene of Chad. Nature 434:752-755.
Zollikofer CPE et al. 2005. Virtual cranial reconstruction of Sahelanthropus tchadensis. Nature 434:755-759.
A challenge to Sahelanthropus
And it comes from me! My paper with Milford Wolpoff, Brigitte Senut, Martin Pickford, and Jim Ahern is now available online from PaleoAnthropology! The PDF is freely accessible -- a big advantage with this journal. So go download!
Here's the abstract:
The Toumaï cranium TM 266 is the first known from any Late Miocene African hominoid clade, and is one of the best preserved crania of any Miocene hominoid. Since its publication there has been debate in the scientific literature and discussion in the popular press over the assertion that this cranium is significant because it is the earliest known hominid. The basis of the hominid assessment rests on two interpretations of the anatomy: a hominid-like, small, flat-wearing canine; and, cranial features reflecting an upright stance and bipedal locomotion. In fact, it is widely reported that the specimen is an upright hominid biped (Haile-Selassie et al., 2004; Kimbel, 2004; Lieberman, 2002), although this is yet to be verified by independent observations and study. The history of paleoanthropology may be relevant to this assessment, because there have been similar claims for other extinct primate species. Here, we evaluate the hypothesis that Sahelanthropus (the genus TM 266 is attributed to) is a hominid by examining features of the canine and of the cranial base that are said to reflect canine reduction and change of function, and upright posture and bipedal locomotion. These are hominid autapomorphies and their presence or absence in late Miocene hominoids has fundamental importance for identifying the hominid clade (Wolpoff et al. 2006:36).
There are two important differences between our analysis and earlier analyses of the skull. First, we provide more comparative data from Miocene primates. Some of the apparent similarities with hominids -- especially considering the morphology, size, and wear on the canines -- are clearly present in other Miocene ape lineages. This is of course the primary difficulty in defining hominids on dental remains alone, since several lineages of Miocene apes appear to have been convergent on some hominid dental features. These similarities don't preclude Sahelanthropus as a hominid, but they remove a major support for that hypothesis.
And second, we provide a biomechanical assessment of the reconstructed skull and its relevance for bipedality and posture. Rather than simply looking for similarities with hominids or chimpanzees, we actually developed a model for how the skull and neck musculature must have functioned. One angle in reconstruction makes the skull look like modern humans -- the foramen magnum - orbital plane angle. But regardless of this angle, the skull actually cannot have functioned in a vertical posture because of the length of the nuchal plane and vertical height of inion. Also, this angle in Toumaï doesn't look anything like early hominids -- australopithecine skulls have FMOP angles similar to chimpanzees!
I think this line captures the point:
The point is not that the TM 266 cranial rear and posterior portion of the cranial base was unlike hominids because the region looks like apes, but that TM 266 had a posture that is not upright because the region reflects nuchal functions similar to those of apes (Wolpoff et al. 2006:46).
There's lots of other interesting stuff in the paper also -- including a section about the hominid-chimpanzee divergence date that I never thought we would say, but is looking very prescient considering recent data. It was a real pleasure to explore several different topics and bring them together, and we'll have much more to do!
References:
Wolpoff MH, Hawks J, Senut B, Pickford M, Ahern J. 2006. An ape or the ape: is the Toumaï TM 266 cranium a hominid? PaleoAnthropology 2006:36-50.
Sahelanthropus :: introduction
In 2002, French paleoanthropologist Michel Brunet and his team announced the discovery of a fossil skull and unassociated jaws and teeth from Chad, in North-Central Africa, which may be the oldest hominid yet found (Brunet 2002). At around 7 million years, the remains from Toros-Menalla are not only older than any East African hominid fossil, they are twice the age of any other hominids known from outside East Africa. The team called the discovery Sahelanthropus tchadensis, a name that emphasizes the hypothesis that the remains represent the earliest known hominids.
Several features of the skull are similar to early hominids. The canine teeth of the skull and mandibles are relatively small, and may have wear on their tips that indicates that they were not used as cutting teeth, as they are in apes. Also, the foramen magnum is placed more anteriorly than in most great ape skulls, which may reflect a habitual upright posture with the spine positioned below the head. Facial characteristics, including a thick supraorbital torus and a relatively vertical face, especially below the nose, are similar to later hominids and are not found in living apes. And the teeth have relatively thickened enamel, more similar to early hominids than to living chimpanzees and gorillas.
Some scientists do not accept these features as strong evidence that Sahelanthropus is a hominid. Pointing out that small canines are sometimes observed in Miocene apes, especially female remains, Milford Wolpoff and colleagues (Wolpoff et al. 2002) argue that this and other features provide no evidence for a close relationship with hominids. The foramen magnum position of the specimen is not outside the range of chimpanzee variation, while the long and flat nuchal plane is inclined to a degree seen only in gorillas. And while the facial features somewhat resemble much later hominids, they do not resemble other early hominids, so they apparently are parallel developments rather than homologies, and thus do not show a close relationship between Sahelanthropus and early hominids.
Of course, even if the Toros-Menalla remains are not hominids, they are immensely important nonetheless. At around 7 million years , they are the only known ape fossils from the critical time period when hominids, chimpanzees, and gorillas all began to differentiate from each other. If they are not hominids themselves, they may well represent the gorilla lineage, a line with no fossil record at all. Even if they belong to an extinct Miocene ape lineage, the remains provide essential evidence for Late Miocene ape diversity in Africa, and may be the nearest fossil relative to the chimpanzee-human-gorilla common ancestor. The fossils therefore potentially tell us much about the ancestral condition for hominids, and help to clarify the confusion caused by the contrast between the chimpanzee-like Aramis dental remains and the dental morphology preserved at other early hominid sites, discussed in this chapter. And perhaps most importantly, the fossils focus attention on a broader range of geographic locations and ecological conditions within Africa as possible contexts of early hominid evolution. Like much later remains from Chad, these fossils remind paleoanthropologists that East Africa is not the only place where early hominid evolution occurred, and may not even be an especially important place, despite the great preservation of hominid fossils in this area.
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks