Sahelanthropus Or 'Sahelpithecus'?
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brief communications 10.Noonkester, V. R. J. Atmos. Sci. 41, 829–845 (1984). 1.2 claim that there are other facial similarities 1.8 11.Hudson, J. G. & Yum, S. S. J. Atmos. Sci. 58, 915–926 (2001). to Homo. However, the facial similarities are 12.Garrett, T. J. & Hobbs, P. V. J. Atmos. Sci. 52, 2977–2984 (1995). 1.0 13.Hudson, J. G. & Li, H. J. Atmos. Sci. 52, 3031–3040 (1995). mostly not with early hominids but with 1.6 14.Noone, K. J. et al. J. Atmos. Sci. 57, 2729–2747 (2000). Pleistocene Homo, and therefore do not 0.8 15.Noone, K. J. et al. J. Atmos. Sci. 57, 2748–2764 (2000). β provide any phylogenetic information (no Competing financial interests: declared none. 1.4 evidence hints that Homo erectus could be 0.6 6–7 million years old). There is little sub- 0.4 nasal prognathism because the canines are Relative dispersion 1.2 small and the subnasal region is short, 0.2 COMMUNICATIONS ARISING and the closely packed anterior dentition, 0 200 400 600 Palaeoanthropology Droplet number concentration (cm–3) crowded together because of the expanded postcanine teeth, explains the absence of Figure 1 Relation between the relative dispersion of cloud droplet Sahelanthropus or diastemata. The vertical height of the size distribution, ᒎ, and the number concentration of cloud ‘Sahelpithecus’? impressive supraorbitals is greater than in droplets, N. Symbols indicate programs and/or references from any extant ape or australopithecine, and can which the data points were derived. Connected points represent eginning with Ramapithecus, there has only be matched in Homo erectus and in a cases previously identified as evidence for an indirect aerosol been a continued search for an ape- few later humans. effect. The parameter ȋ is defined by equation (2). Green symbols Blike hominid ancestor in the Miocene The supraorbital size is attributed to (from ref. 8): triangle, FIRE, northeastern Pacific; crossed circles, Epoch. Sahelanthropus tchadensis is an enig- strong sexual selection1, which we consider SOCEX, Southern Ocean; filled circle, ACE1, Southern Ocean. Blue matic new Miocene species, whose charac- unlikely. The size and form of the supra- symbols: filled circles, ASTEX8, northeastern Atlantic; diamonds, teristics are a mix of those of apes and orbital structures are probably a mechanical SCMS8, Florida coast; filled triangles, Sounding9, ASTEX; filled Homo erectus and which has been pro- response4 to strain from anterior tooth squares, horizontal9, ASTEX; open inverted triangles, level 1; open claimed by Brunet et al. to be the earliest loading in the region above the orbits, con- upright triangles, level 2; open circles, level 3 — all from south- hominid1. However, we believe that features centrated by the flexed frontofacial angle. west of San Diego10; open diamonds, SCMS11; stars, vertical, of the dentition, face and cranial base that The biomechanical model of the supra- ASTEX12; plus signs, horizontal, ASTEX12; multiplication signs, are said to define unique links between this orbital region5 would predict that an ASTEX13; squares, INDOEX, Indian Ocean (G. M. McFarquhar, per- Toumaï specimen and the hominid clade orthognathic face such as that of Sahelan- sonal communication). Red circles, MAST6,14,15, California coast. are either not diagnostic or are conse- thropus, combined with a low forehead, cre- quences of biomechanical adaptations. To ates the potential for greater strain during polluted cloud compete for water vapour represent a valid clade, hominids must anterior tooth loading than would a prog- and broaden the droplet size distribution share unique defining features2, and Sahel- nathic face with a higher forehead, as in compared with clean clouds that have fewer anthropus does not appear to have been an African apes. Significant force during ante- droplets and less competition. obligate biped. rior tooth use is indicated by the expanded According to equations (1) and (2), an We consider the following features that posterior temporalis musculature –– this increase in ᒎ acts to negate the effect of are proposed by Brunet et al. to constitute muscle forms a sagittal crest that meets the increased N on effective radius and cloud links with the hominid clade. First, they nuchal crest very high on the posterior reflectivity. Because this effect has been note that the canine is small. The canine vault, in a gorilla-like morphology that far largely neglected in estimates of the indirect breadth (the length is not given) is similar exceeds the much weaker-muscled chim- aerosol effect, cooling by an indirect aerosol to the chimpanzee mean, being within the panzee condition. The supraorbital torus is effect is likely to have been overestimated. range of both chimpanzee and gorilla the bony response to strain. From the data presented in Fig. 1, we esti- females and of chimpanzee males. The Third, Brunet et al. infer that the inter- mate that a 15% increase in N at Nǃ100 crown is low and the root is narrow relative mediate thickness of the specimen’s post- cmǁ3 causes a total forcing that ranges to the crown, suggesting that Toumaï might canine enamel is also an important link. between ǁ0.19 and ǁ0.93 W mǁ2, which have been female (canine area is a more However, thickened enamel relative to corresponds to a factor that is 10–80% reliable sex indicator than brow ridges); chimpanzees would be expected whatever lower than the ǁ1.03 W mǁ2 calculated for however, the postcanine teeth are all large the phylogenetic relations of Sahel- the Twomey effect alone2. compared with chimpanzees — as in several anthropus, not only because of the other The effect of the enhancement in ᒎ is Miocene ape females. adaptations to a diet that requires powerful evidently large enough to be considered in The canine shows apical wear and has a mastication, but also because thickened assessing the indirect aerosol effect, and thin strip of wear along the distal edge of enamel is a plesiomorphic condition. understanding the relation between ᒎ and N the crown, which reaches the crown base. Fourth, the authors assign an anterior will help to reduce the large uncertainty Although Brunet et al. conclude that the position to the foramen magnum on the inherent in this effect. tooth was not used in honing, we find this basis of its front edge meeting the bicarotid Yangang Liu, Peter H. Daum difficult to reconcile with the details that and biporion chords, claiming that in chim- Brookhaven National Laboratory, Upton, they provide. Decades ago, when Miocene panzees the foramen magnum is posterior New York 11973, USA primate jaws with small canines and to these chords and that this positioning e-mail: [email protected] enlarged postcanine teeth were found, they reflects their posture. STS 5, an obligate 1. Twomey, S. Atmos. Environ. 8, 1251–1256 (1974). were given distinct names (Kenyapithecus biped, does not differ from some chim- 2. Charlson, R. J. et al. Science 255, 423–430 (1992). and Ramapithecus, for example) and were panzees by these criteria, however. Neither 3. Liu, Y. & Daum, P. H. Geophys. Res. Lett. 27, 1903–1906 (2000). described as the earliest hominids because does this apply to the biporion chord of 4. Liu, Y. & Daum, P. H. Proc. 13th Int. Conf. On Clouds and (it was assumed) the canine honing func- some chimpanzees, according to the pos- Precipitation, Reno, USA 586–589 (2000). 5. Martin, G. M., Johnson, D. W. & Spice, A. J. Atmos. Sci. 51, tion had, by then, been replaced by tools ition of the foramen magnum determined 1823–1842 (1994). (it was also assumed that they were from photographs of 70 adult chimpanzee 6. Ackerman, A. S. et al. J. Atmos. Sci. 57, 2684–2695 (2000). bipeds)3. These specimens turned out to be cranial bases aligned with the Frankfurt 7. McFarquhar, G. M. & Heymsfield, A. J. J. Geophys. Res. D 106, female apes. horizontal (J. Ahern, personal communica- 28675–28698 (2001). 8. Yum, S. S. & Hudson, J. G. Atmos. Res. 57, 81–104 (2001). Second, the specimen has a large, con- tion). This alignment is important in the 9. Hudson, J. G. & Yum, S. S. J. Atmos. Sci. 54, 2642–2654 (1997). tinuous supraorbital torus, and the authors determination, but we have no information NATURE | VOL 419 | 10 OCTOBER 2002 | www.nature.com/nature © 2002 Nature Publishing Group 581 brief communications on how the Sahelanthropus position was Overlooking their flippant taxonomic than a hominid ancestor. They interpret assessed. Moreover, the anterior edge of the proposal (the genus name ‘Sahelpithecus’), our description of distal dentin exposure of foramen is far from the back of the Sahelan- which disregards the requirement for a the upper canine as evidence of honing thropus third molar, in contrast to hominids new genus to have a type species and wear (roughly equivalent to describing an and similar to chimpanzees and female description7, we disagree with their (pre- African millet pestle as a Samurai sword). gorillas. sumably more serious) opinions on the The Toumaï canine is not honing because There are many other features that link morphology and phylogeny of the Toumaï it does not display the sharpened distal the specimen with chimpanzees, gorillas or fossil. edge that is shared by all apes. Rather, this both, to the exclusion of hominids. Most Because the Toumaï fossil is the earliest tooth is similar to those of later hominids significantly, the nuchal plane is long, flat known hominid ancestor8, it is not surpris- in both size and proportion to the post- and angled at about 55ᑻ to the Frankfurt ing that it bears primitive characters. Fol- canine teeth. horizontal: “relatively longer than in Pan lowing modern systematic practice, we used In a modern example of how to miss [and] Gorilla … and with crests as marked newly evolved characters (rather than the morphology between measuring as those of Gorilla”1.