Knuckle-walking hominid ancestor: a reply to Corruccini & McHenry

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1 News and Views Brian G. Richmond David S. Strait Knuckle-walking hominid ancestor: a reply to Corruccini & McHenry Department of Anthropology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, U.S.A. brich@uiuc.edu Department of Anatomy, New York College of Osteopathic Medicine, Old Westbury, New York 11568, U.S.A. dstrait@iris.nyit.edu 2001 Academic Press Journal of Human Evolution (2001) 40, doi: /jhev Available online at on We are pleased that Corruccini & McHenry (2001), using their own dataset, independently arrive at the conclusion recently reached by us (Richmond & Strait, 2000), and long advocated by the late Sherwood Washburn (1967, 1968), that there is evidence suggesting that hominids evolved from knuckle-walking ancestors. Corruccini & McHenry use 11 variables of the distal radius derived from their earlier work (Corruccini, 1978; McHenry & Corruccini, 1983) to show that the distal radii of humans and some early hominids resemble African ape radii in morpholgy they believe to be related to knuckle-walking. However, there are some notable differences between their results and ours that deserve comment, and Corruccini & McHenry raise several concerns about our analysis. They (Corruccini & McHenry, 2001) question whether our morphometric analysis adequately distinguishes knuckle-walkers from other taxa, and whether the earliest hominid radii are convincingly shown to resemble African apes. They contend that existing research has been neglected and needs to be more fully addressed. Finally, they argue that their data, and previous work (Corruccini, 1978), provide clearer evidence of knuckle-walking retentions than does our recent work. Corruccini & McHenry (2001) are disturbed by the fact that, in our analysis, there is a slight overlap between Pongo and Gorilla, and that AL 288-1, although more similar to gorillas, falls in the slight region of overlap. Corruccini & McHenry point out that the orang-utan is not a knuckle-walker and should in many ways represent the opposite morphological extreme. Orang-utans are indeed morphologically different from African apes in many ways, particularly in the orang-utan s anatomical specializations for suspensory quadrumanus climbing. However, in our analysis, we did not select features based on their functional relevance to suspension. We carefully selected anatomical features that have been related to a unique function involved in knucklewalking, namely limited wrist extension combined with pronograde weight support (Tuttle, 1967, 1969; Jenkins & Fleagle, 1975). In fact, African apes are fairly distinctive from Asian apes, humans, and other anthropoids in this morphology, but a few (three out of 17) orang-utans overlap into the gorilla range. The slight region of overlap between Pongo and Gorilla should not be over interpreted the distal radii of these species are significantly different from one another (P<0 01, F statistic). The radii of AL fall in this range of overlap, but much more closely resemble typical gorilla radii than orang-utan radii. Hence, the cluster analysis [Figure 2(c) in Richmond & Strait, 2000], the posterior probabilities of /01/ $35.00/ Academic Press

2 514 B. G. RICHMOND AND D. S. STRAIT group assignment, and the respective Mahalanobis D 2 distances (Table 1) and F-test significance values (Table 2) between group centroids show that Australopithecus afarensis (also known as Praeanthropus afarensis) and A. anamensis radii are more similar to Gorilla and Pan than either are to Pongo. Corruccini & McHenry s results differ from ours in several ways. For example, Homo sapiens clusters with knuckle-walkers in Corruccini & McHenry s Figure 3, which they interpret as evidence of knucklewalking retentions in modern humans as well as in some hominids (Corruccini, 1978). Based on the distribution of taxa in our canonical variates analysis (CVA), Corruccini & McHenry believe our analysis serves more to distinguish nonhuman apes from humans and quadrupedal monkeys. In particular, they question the knucklewalking functional significance of our features (despite using similar variables) because humans and the later hominids (SKX 3602, Stw 46) are distinct from African and Asian apes despite sharing a postulated knuckle-walking ancestor. We believe their interpretation confounds function and phylogeny. We do not necessarily expect our variables to cluster taxa in terms of phylogeny. Indeed, if the cluster matched phylogenetic relationships, we would be concerned that our measurements lacked functional significance and merely reflected phylogenetic affinity. Our measurements were chosen for their likely functional significance based on existing work (Tuttle, 1967; Jenkins & Fleagle, 1975). The fact that modern humans do not cluster with African apes supports the notion that these features are functionally relevant because modern humans have a greater range of extension at the radiocarpal joint than do African apes (Heinrich et al., 1993). Therefore, in contrast to Corruccini & McHenry s analysis, our results highlight derived aspects of modern human and later fossil hominid radii (e.g., Stw 46 and, to a lesser extent, SKX 3602), which appear to lack the extension-limiting mechanism characteristic of knuckle-walkers. Although the adaptive significance of increased mobility in wrist extension in modern humans and A. africanus is not known, it has been proposed that wrist extension is important in throwing and hammering activities (Marzke, 1971). Thus, the more humanlike aspects of wrist structure in Stw 46, and perhaps SKX 3602, may represent evidence for improved manual dexterity in A. africanus and possibly Paranthropus robustus (Wood & Richmond, 2000). This would support existing evidence for anatomical adaptations for manual manipulation and possibly tool use in A. africanus (Ricklan, 1987) despite the absence of stone tools. Depending on how securely SKX 3602 is attributed taxonomically, its morphology would be consistent with arguments that P. robustus was capable of making and using stone tools (Susman, 1988, 1991, 1998; Marzke, 1997, but see Trinkaus & Long, 1990). The differences between Corruccini & McHenry s analysis and ours result from different variables and comparative samples. We limited our measurements to a few traits that appear to have a direct functional relationship with extension-limiting and pronograde weight support during knucklewalking, and were preserved on hominid fossils. Corruccini & McHenry include several similar features (their variables 1, 3, 4, and 5), but also incorporate a number of traits for which the functional relationships with knuckle-walking are not apparent, and have not been explicitly described here or in the original study (Corruccini, 1978). These features include the proximodistal height of the ulnar facet, the mediolateral breadth of the entire radiocarpal articular surface, and a small ulnar-carpal angle. They argue that the last feature expresses more forced adduction, but it is not made clear how this postural ability relates to knuckle-walking.

3 KNUCKLE-WALKING HOMINID ANCESTOR 515 Table 1 Mahalanobis D 2 distances between group centroids Taxon A. anamensis A. afarensis A. africanus P. robustus Pan Gorilla Homo Pongo Hylobates P. heseloni Alouatta Papio A. afarensis A. africanus P. robustus Pan Gorilla Homo Pongo Hylobates P. heseloni Alouatta Papio Erythrocebus

4 516 B. G. RICHMOND AND D. S. STRAIT Table 2 P-values based on F-tests indicating the approximate significance of Mahalanobis D 2 distances between centroids Taxon Pan Gorilla Homo Pongo Hylobates Alouatta Papio Erythocebus A. anamensis ** 0 01* 0 00** 0 00** 0 00** 0 00** A. afarensis * * 0 00** 0 00** 0 00** A. africanus 0 00** 0 00** ** 0 00** ** 0 00** P. robustus 0 03* 0 04* * 0 00** 0 01* 0 00** 0 00** Pan 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** Gorilla 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** Homo 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** Pongo 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** Hylobates 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** P. heseloni 0 00** 0 00** 0 00** 0 00** 0 00** ** Alouatta 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** Papio 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 01 Erythrocebus 0 00** 0 00** 0 00** 0 00** 0 00** 0 00** 0 01 Similar results are obtained when all specimens are size-adjusted based on joint area instead of maximum radial breadth (Richmond & Strait, 2000). ** Demarcates P<0 01, * demarcates P<0 05. Four of the remaining eight traits pertain to relative scaphoid lunate facet size, and thus are unlikely to be independent (especially when analyzed as shape variables). Asian apes are known to differ from African apes and other anthropoids in having relatively large lunate facets. Presumably, this morphology allows Asian apes to effectively resist stresses in adducted hand postures. As noted by the original describers (Heinrich et al., 1993), KNM-ER (attributed to A. anemensis) and AL resemble Asian apes while SKX 3602 resembles African apes and humans in lunate to scaphoid proportions. The relatively large lunate facets in the gracile australopithecine specimens suggests two possibilities: (1) a large lunate facet is primitive for great apes, with the convergent evolution of scaphoid facet enlargement in Gorilla, Pan, and Homo, or (2) a large scaphoid facet is primitive in African apes and humans, and the enlarged lunate facet in early australopithecines converged on the Asian ape condition. In either case, the combination of African and Asian apelike features in these early hominids underscores the mosaic nature of human evolution (McHenry, 1975) and the uniqueness of fossil hominids compared to extant species (McHenry, 1984; Lague & Jungers, 1996). It is important to note that while this feature is associated with adducted wrist postures like those employed in arboreal climbing, it is consistent with a heritage comprised of knuckle-walking and vertical climbing (Richmond & Strait, 2000). In sum, we agree that Corruccini & McHenry s analysis is compatible with the knuckle-walking hypothesis, but believe that further research is needed to test the functional significance of many of these traits. Corrruccini & McHenry are disappointed that we did not cite Corruccini s (1978) study on hominoid wrist morphology in our original paper (Richmond & Strait, 2000). Corruccini s study (1978) was directly relevant to our own work (Richmond & Strait, 2000), and we regret that journal-imposed space limitations did not permit us to discuss adequately this important contribution. This exchange provides us with the opportunity to do so. However, we note at the outset that Corruccini s quantitative results were largely driven by morphological similarities that had been previously described in

5 KNUCKLE-WALKING HOMINID ANCESTOR 517 qualitative terms by other authors (e.g., Marzke, 1971; Jenkins & Fleagle, 1975). Given the limitations mentioned above, we chose to cite the earlier works. In his study, Corruccini (1978) demonstrated a morphometric affinity between humans and African apes, relative to Asian apes and macaques, in features selected for their potential functional relevance to knuckle-walking (primarily based on work by Tuttle, 1967, and Jenkins & Fleagle, 1975). The two carpal variables most influential in distinguishing African apes and humans from Asian apes and macaques were (1) a measure of the presence or absence of fusion between the os centrale and scaphoid, and (2) the proximo-distal length of the ridge of the capitate head separating the scaphoid and lunate facets. It has long been known that, unlike other apes, the os centrale fuses to the scaphoid early in development in African apes and humans (Mivart, 1866). Early fusion of the os centrale may logically strengthen the carpus during quadrupedal weight support (Marzke, 1971; Sarmiento, 1994; Gebo, 1996). However, os centrale fusion is homoplastic in primates, and appears to have evolved independently multiple times for different reasons. For example, fused os centrale have been observed in aged Asian apes (Schultz, 1950). Among Malagasy strepsirhines, Indri and Babakotia scaphoids have fused os centrale, as do most Palaeopropithecus, while the os centrale is free in Mesopropithecus, Megaladapis, and Archeolemur (Hamrick et al., 2000). The functional significance of the second highly influential trait (length of scaphoid lunate ridge on the capitate head) in Corruccini s (1978) analysis is not entirely clear. Corruccini (1978) cites Jenkins & Fleagle (1975) in reference to the presence of a boundary demarcating the lunate and scaphoid facets on the distal portion of the capitate head in African apes. This demarcation is also well-developed on baboon capitates (personal observation). Thus, it may be the case that Asian apes are derived in having more smoothly confluent capitate heads for carpal mobility (Jenkins & Fleagle, 1975). Interestingly, Ward et al. (1999) report that a keel separates the lunate and scaphoid facets on the capitate (KNM-WT 22944H) attributed to A. afarensis. At the very least, then, more research is needed to determine whether or not a long lunate scaphoid ridge on the capitate head is an adaptation for pronograde weight support, for knuckle-walking specifically, or for some other function. Other features that weighed heavily in Corruccini s (1978) analysis (e.g., lunate facet angulation, ulno-carpal articulation of distal radius) also deserve further research into their function and morphology to determine their potential roles in knuckle-walking. Thus, while it is unlikely that all of the osteometric features can be indisputably attributed to knuckle-walking, Corruccini s study remains an important and provocative contribution, and deserves further attention. Despite Corruccini s (1978) results, researchers over the next decade focused on climbing rather than knuckle-walking as the locomotor mode that preceded and was preadaptive for bipedalism (e.g., Stern, 1975; Stern & Susman, 1981; Prost, 1980; Fleagle et al., 1981; Ishida et al., 1985; Senut, 1988). The popularity of the arboreal and vertical climbing hypotheses in the last several decades indicates that the knucklewalking hypothesis has not been well established (Gebo, 1996). Studies on early fossil hominid carpals have argued that knucklewalking adaptations are not present in the relevant fossils (McHenry, 1983; Ward et al., 1999). These studies did not directly address Corruccini s (1978) study, and did not incorporate variables of the capitate that Corruccini (1978) found to be important in distinguishing knuckle-walkers, perhaps due to the functional ambiguities outlined above.

6 518 B. G. RICHMOND AND D. S. STRAIT As noted by Corruccini & McHenry (2001), McCrossin and colleagues (1998) have recently reported on knuckle-walking features in the metacarpus and distal radius of the Miocene hominoid Kenyapithecus. This report is intriguing, but McCrossin & Benefit (1997) note that large terrestrial digitigrade quadrupeds such as Mandrillus also exhibit extension-limiting morphology at the metacarpophalangeal joint. Thus, in light of earlier descriptions of more cercopithecine-like terrestrial adaptations in Kenyapithecus (McCrossin & Benefit, 1997), it remains possible that the morphology of Kenyapithecus is not homologous with that of African apes. We look forward to further description and analysis of these important fossils. A growing number of researchers (Marzke et al., 1994; Begun, 1993, 1994; Sarmiento, 1994; Gebo, 1996; Pilbeam, 1996; Richmond & Strait, 2000; Corruccini & McHenry, 2001) are seriously considering the possibility that humans evolved from knuckle-walking ancestors. Corruccini & McHenry have uncovered many aspects of wrist morphology that are shared between African apes and humans, and now add support to our recent finding (Richmond & Strait, 2000) that fossil hominids also retain knuckle-walking features. Additional support for the knuckle-walking hypothesis comes from parsimony (Washburn, 1967), and other anatomical aspects of the wrist and hand (Begun, 1993). The identification of these features, and the lack of consensus regarding their meaning, calls for renewed efforts to better understand the biomechanics of knuckle-walking and its morphological correlates. Data on the curvature and contour of carpal articular surfaces for their contribution to stability and mobility (Hamrick, 1996a, b) would be welcome improvements to our understanding of anthropoid wrist functional anatomy. The answer to the debate over what mode of locomotion gave rise to the earliest bipeds will require both new fossils and a clearer understanding of knuckle-walking functional morphology. The discovery and description of relevant fossils nearer to last common ancestor of Pan and Homo may shed light on this debate (White et al., 1994; Senut et al., 2001). However, it is also critical that we fully understand the functional anatomy of knuckle-walking so that, when the appropriate fossils are recovered, there is a clear basis for their interpretation. It is particularly important that we understand how knuckle-walking is kinematically and morphologically distinct from other forms of locomotion, especially other forms of pronograde quadrupedalism. In summary, our study (Richmond & Strait, 2000) differs from that of Corruccini & McHenry (2001) in that we limited our measurements to features linked a priori to knuckle-walking, in order to avoid weakly supported functional inferences or measurements reflecting phylogeny. However, our studies agree in showing that early hominid wrist structure resembles that of African apes. Both studies conclude that evidence exists for the hypothesis that hominid bipedalism evolved from a knuckle-walking ancestor. In addition to new relevant fossils, we need further research on anthropoid wrist function and its morphological correlates to reliably interpret hominid functional anatomy. References Begun, D. R. (1993). Knuckle-walking ancestors. Science 259, 294. Begun, D. R. (1994). Relations among the great apes and humans: new interpretations based on the fossil great ape Dryopithecus. Yearb. phys. Anthrop. 37, Corruccini, R. S. (1978). Comparative osteometrics of the hominoid wrist joint, with special reference to knuckle-walking. J. hum. Evol. 7, Corruccini, R. S. & McHenry, H. M. (2001). Knucklewalking hominid ancestors. J. hum. Evol. Fleagle, J. G., Stern, J. T., Jungers, W. L., Susman, R. L., Vangor, A. K. & Wells, J. P. (1981). Climbing: A

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