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Tuesday, 3 February 2015

Human / chimpanzee common ancestry - the current fossil evidence - Part 2

Sahelanthropus tchadensis

Sahelanthropus tchadensis is a late Miocene possible hominin located in the deserts of Chad in 2002 and has been dated to around seven million years ago. The date alone puts it on the early side of the possible time of the common ancestor as determined by genetic studies. It is fair to say that there is no firm consensus about whether it predates the Homo / Pan divergence, post-dates the divergence and is on the Homo side of the divergence, or may even be one of the earliest fossil non-human apes. Whatever it is, (remembering the analogy of trying to work out what your grandfather looked like by looking at photographs of his cousins, siblings and uncles) it is a valuable find.

The paper announcing its discovery states that:
Sahelanthropus has several derived hominid features, including small, apically worn canines—which indicate a probable non-honing C–P3 complex—and intermediate postcanine enamel thickness. Several aspects of the basicranium (length, horizontal orientation, anterior position of the foramen magnum) and face (markedly reduced subnasal prognathism with no canine diastema, large continuous supraorbital torus) are similar to later hominids including Kenyanthropus and Homo. All these anatomical features indicate that Sahelanthropus belongs to the hominid clade.

In many other respects, however, Sahelanthropus exhibits a suite of primitive features including small brain size, a truncated triangular basioccipital bone, and the petrous portion of the temporal bone oriented 60° to the bicarotid chord. The observed mosaic of primitive and derived characters evident in Sahelanthropus indicates its phylogenetic position as a hominid close to the last common ancestor of humans and chimpanzees. [1]
The fossil remains are shown below:


nature00879-f1.2.jpg

One problem palaeontologists face is that the fossils are not infrequently deformed. This was the case with Sahelanthropus, which makes interpretation difficult. Five years ago, the fossil was scanned, and digitally reconstructed [2] in an attempt to correct this deformation. The results appeared to support the theory that it may have been very close to the common ancestor of humans and chimps:
The reconstruction in Fig. 2 is therefore a robust estimate of the cranial form of TM 266-01-60-1 that supports most of the details originally described. However, several features differ notably from those of the original specimen: the cranium as a whole is wider, the occipital contour is rounder sagittally, the nuchal plane is oriented more horizontally, the orbits are larger and more circular, and the face is superoinferiorly taller...The changes evident in the TM 266 reconstruction highlight its unique morphology and confirm several derived features shared with later hominids such as a relatively vertical face with an anteroposteriorly short premaxilla; an anteriorly-positioned foramen magnum linked to a relatively short basioccipital; a relatively flat, large, and horizontally-oriented nuchal plane; and downward lipping of the nuchal crest. These features, together with other dental features (see refs 1, 2), support the conclusion that Sahelanthropus is a hominid (contra Wolpoff et al). [3]
Of course, without post-cranial data such as lower limbs, it is impossible to determine confidently that a species known only by a cranium is bipedal or not. It is possible to infer it however, using details in the skulls of bipeds such as the position of the foramen magnum (the hole at the skull base through which the spinal cord exits) as well as the orbital plane (the line from the top of the eye socket to the bottom of the socket). In bipeds, the foramen magnum is located more towards the bottom, while the orbital plane is perpendicular to the ground. The authors of the paper detailing this reconstruction stated:
Finally, the TM 266 reconstruction permits an assessment of the hypothesis that Sahelanthropus was a biped, an important feature of Pliocene hominids and possibly several Late Miocene hominids. Unequivocal evidence for bipedalism is difficult to obtain from the cranium, but several lines of evidence suggest that TM 266-01-60-1 might have been bipedal. 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 10o) 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 (Fig. 4) is nearly perpendicular in Homo sapiens (103.2+/- 6.9o, n=23) but more acutely angled in Pan troglodytes (63.7+/-6.2o, n=20), and other species with more pronograde postures. The foramen magnum angle relative to the orbital plane in the TM 266 reconstruction is 95o, similar to that in humans and later bipedal hominids such as Australopithecus afarensis (AL 444-2) and A. africanus (Sts 5)13,17. TM 266-01-60-1 as a quadruped would require an unusually extended angle of the neck relative to the plane of the foramen magnum. [4] (Emphasis mine)
This reconstruction helped firm support for the idea that it was a hominid. Building on this data, another paper stated:
The hypothesis that TM 266-01-60-1 is an ape has been once again refuted by the recent virtual reconstruction , which confirms the presence of many hominid features in S. tchadensis and indicates that it was likely some kind of biped. However, questions remain about the species' systematic relationship to extant apes, other known Late Miocene hominids, and later hominids from the Pliocene. Although we do not believe it yet possible to resolve reliably the phylogenetic position of S. tchadensis and other early hominids, a first step is to consider in what ways the TM 266-01-60-1 cranium shares primitive features with extant apes and derived features with various known hominids. Thus, we assess here the morphological similarities and differences of the reconstructed TM 266-01-60-1 cranium with ape and other hominid taxa by using geometric morphometric methods along with comparative quantitative data on variables that reflect key similarities and differences in hominid craniofacial morphology.

The TM 266-01-60-1 reconstruction exhibits several widely recognized hominid synapomorphies, including a long, flat nuchal plane that is more horizontally oriented than in African apes (contra ref. 4); a shortened basioccipital; and a more anteriorly positioned foramen magnum... In addition, as discussed in Brunet et al., the dentition of TM 266-01-60-1 and referred specimens resembles those of Ardipithecus and later hominids compared with Pan in several respects such as reduced canines, a C/P3 nonhoning complex, and postcanine teeth with thicker enamel. TM 266-01-60-1 also shares several derived cranial features with later hominids, especially in the posterior cranial vault and the cranial base... However, as noted above, some aspects of the overall facial structure of TM 266-01-60-1 are unlike those of any known published Australopithecus, including AL 444-2 (A. afarensis). Several of these features, such as subnasal height, palatal protrusion, and relative upper facial projection...may be derived differently than those in Australopithecus. Others are probably either primitive or convergent with Homo. For example, the upper and middle portions of the face in TM 266-01-60-1 are similar in width, as in Pan and Homo, but unlike the hexagonal-shaped configuration in Australopithecus, in which the midface is wider than the upper face because of masticatory-related expansion of the zygomatic region.

Although Sahelanthropus tchadensis is clearly a hominid, its complex mosaic of features poses some interesting systematic questions about early hominid evolution that can be resolved only with more data of four types. First, more information is needed to compare Orrorin, Ardipithecus, and Sahelanthropus in detail. Second, additional hominid fossil material is needed from Late Miocene deposits in different parts of Africa. Third, we need a better understanding of the developmental and functional bases of cranial morphology that are necessary to hypothesize phylogenetically informative characters. Fourth, we particularly need a better understanding of the vertebrate and hominid biogeographical relationships through Africa during the upper Miocene. [5]
The authors placed Sahelanthropus tchadensis on a family tree:

F3.medium.gif

Note that it is placed before the Australopithecus / Homo split, placing this fossil very close to the human / chimp common ancestor. However, this position (as I mentioned earlier) is opposed by some fairly respected anthropologists. In a 2006 paper, Milford Wolpoff et al [6] re-examined the data, and came up with a different conclusion.
This enigmatic Miocene species mixes characteristics of some australopithecines and some portions of the Homo clade such as “Homo erectus.” Yet, the hominid interpretation was not without objection (Wood, 2002) because TM 266 has also been described as a hominoid ape (Cela-Conde & Ayala, 2003; Wolpoff et al., 2002). The ape interpretation could mean that Sahelanthropus was uniquely ancestral to a living ape, or that the species was an extinct related lineage that diverged before the hominids, or that it is close to or actually the last common ancestor of hominids and chimpanzees. [7]
What is not in doubt is that this species is transitional. However, as Wolpoff notes, it could be ancestral to living apes, close to the Homo / Chimp common ancestor or even an extinct line. After a fairly rigorous analysis of the data, they concluded:
Sahelanthropus was an ape living in an environment later abandoned by apes but subsequently inhabited by australopithecine species. Both TM 266 and the later australopithecines reflect the masticatory adaptations of earlier Miocene apes, and differ from later African apes in many of these adaptations. This creates the potential for significant homoplasies, and combined with the consequences of a limited fossil record, makes assessment of phylogeny problematic. Yet, it is a highly significant discovery; both because it evidences a unique pattern of homoplasy, and perhaps mostly because of the insight it might give for understanding the ancestral condition before the hominid-chimpanzee split. [8]  (Emphasis mine)
However  the authors of this paper did not dismiss this fossil as having no bearing on evolution:
So every discovery counts. Sahelanthropus may be an early member of the gorilla clade, as two of us (B.S., M.P.) have suggested, or it may be closely related to the chimpanzee clade, or to the human-chimpanzee common ancestor (Wood, 2002), or perhaps most likely a member of an extinct closely related clade. In any of these cases, Sahelanthropus tends to confirm the notion (Wolpoff, 1982) that the ancestral condition for hominids, and for extant African apes, is much more like the Miocene ape condition than like living chimpanzees. Only if it turns out that Sahelanthropus represents a more distantly related hominoid lineage would it fail to be informative about the ancestral condition for these lineages, and address the issue of whether the chimpanzee is a reasonable model for this last common ancestor. [9]  (Emphasis mine)
In short, the jury is out (though for what it is worth I suspect Wolpoff may be correct, unless we can find post-cranial data that definitely support bipedality). However, as we can see, those who argue for and against it being ancestral to our line agree that it is transitional, and of significance to the issue of human evolution.

References

1. Brunet M et al "A new hominid from the Upper Miocene of Chad, Central Africa" Nature (2002) 418:145-151
2. Zollikofer CPE et al "Virtual cranial reconstruction of Sahelanthropus tchadensis" Nature (2005) 434:755-759
3. ibid p 756
4. ibid, p 757-758
5. Guy F et al  "Morphological affinities of the Sahelanthropus tchadensis (Late Miocene hominid from Chad) cranium" Proc Natl Acad Sci USA (2005) 102:18836-18841
6. Wolpoff MH et al "An Ape or the Ape: Is the Toumaï Cranium TM 266 a Hominid?" PaleoAnthropology 2006: 36−50
7. ibid, p 37
8. ibid, p47
9. ibid, p 48