Thumbs, Tools, and Early Humans

Abstract

senting cells. Contributions from Gleichmann's laboratory have provided another dimension for this approach by examining lymphoproliferative responses to different xenobiotics in vivo. This work complements our studies because it demonstrated that immune reactivity was not against the administered compound, but a metabolite of higher oxidation state; we showed how such products could be produced within a lymphoid compartment.In order to account for the autoimmune side effects of drugs, Gleichmann and colleagues adopt a variant of the conventional explanation of drug-altered self-proteins as the initiating event. Other recent proposals include direct activation of lymphocytes through redox cycling or inhibition of DNA methylation reactions. However, the By comparative functional analysis of thumb morphology, the report by Randall L. Susman (1) tackles the key question of which extinct hominids used tools. He contrasts living apes and their short, thin, weak thumbs and resulting power grasps with living humans and their long, stout, strong thumbs and refined precision grasps. The latter is said in the report to be correlated with tool behavior of extinct hominids; it is proposed, for example, that Australopithecus afarensis with its ape-like thumb was not a toolmaker, but Paranthropus robustus with its human-like thumb was.Even with their inferior thumbs, however, apes engage in dextrous manipulation of objects by precision grasping and make and use a variety of tools (2) metacarpophalangeal joint by these muscles, Susman (1) explains the relatively expanded human metacarpal head as an adaptation to reduce joint stress. He concludes that "humans have broader metacarpal heads than apes" (1, p. 1571) and, using a ratio of metacarpal head breadth to metacarpal length, shows that "values for African apes and modem humans do not overlap" (1, p. 1571). Susman (1) then uses this ratio as a criterion to separate modern toolmakers from non-toolmakers and to infer toolmaking capabilities in fossil hominids.We tested this hypothesis by measuring pollical metacarpal head breadth and metacarpal length in adult humans, bonobos, and chimpanzees, and expanded the sample to include gorillas (3). When these data are plotted along with those of adult humans, following Susman (1), metacarpal head breadth of gorillas exceeds that of chimpanzees and overlaps markedly the range of variation shown for humans (Fig. 1A). Applying the ratio used by Susman (1), the range of values for mountain gorillas overlaps that of humans more than it does that of chimpanzees (Fig. iB). The majority of fossil taxa for which tool use is implied by Susman (1) fall well within the range of variation observed for gorillas (Fig. 1B) (4). Susman also noted "great apes have relatively shorter thumbs than do humans, with metacarpals that are reduced in relative length" (1, p. 1571). Plots of thumb metacarpal length against body size (5) in adult apes and humans demonstrate that, at comparable body sizes, adult gorillas have first metacarpals that do not differ s...

Shape variation of the human pollical distal phalanx and metacarpal

“…Gorillas' distal phalanges show some shape similarity to those of humans; they are tapered distally but are "capped by a horse-shoe rim of bone which approaches in relative breadth the arrangement in man" (Marzke, 1964: 151). The radioulnarly wide first metacarpal head of gorillas (Hamrick and Inouye, 1995;Ohman et al, 1995) also suggests that other factors besides selection in favor of the sorts of manipulation involved in tool use can produce convergent results. The expanded first metacarpal head of gorillas is all the more telling as an example because their relatively short thumbs are not the digits primarily stressed in their knuckle-walking mode of locomotion.…”

“…Regression analyses of distal phalanx 1 radioulnar tuft width on base width allow comparison with the results of Susman (1988); regression analyses of metacarpal 1 radioulnar head width on metacarpal 1 maximum length likewise extend research on the value of this metacarpal ratio (Susman, 1994;Hamrick and Inouye, 1995;Ohman et al, 1995). Due to the use of the regressions for comparison and functional interpretation, reduced major axis (RMA) regression is the technique of choice (Aiello, 1992).…”

“…In another paper, Susman (1994) further proposed that the wide head of the hominid first metacarpal is a character of value in separating tool-dependent hominids from both pongids and hominids who cannot demonstratively be recognized as tool-reliant. This conclusion has been criticized for neglecting behavioral reports of pongid tool use and because gorillas share with hominids a wide first metacarpal head (McGrew, 1995;Hamrick and Inouye, 1995;Ohman et al, 1995). Susman (1995Susman ( , 1998 thinks the former are paleontologically irrelevant because tool use of the nature and degree of extant pongids is insufficient to leave morphological markers, and while he acknowledges that some gorillas have wide metacarpal heads, he questions the relevance of these comparative data.…”

Skeletal evidence for precision gripping inCebus apella

“…The relatively narrow thumbs of these apes may reflect a morphological adaptation to facilitate the powerful hook grip required for arm-swinging (Napier, 1960), which could explain why gorillas, the most terrestrial of the apes, have thumbs that are more robust than those of Pan, Pongo or Hylobates (Hamrick and Inouye, 1995;Ohman et al, 1995). Our findings underscore the need to consider ecological factors and associated behavioral and artifactual evidence when applying a single-trait anatomical test to distinguish primate species that use tools from those that do not (Hamrick and Inouye, 1995;McGrew, 1995;Ohman et al, 1995;Susman, 1994Susman, , 1995. Our findings underscore the need to consider ecological factors and associated behavioral and artifactual evidence when applying a single-trait anatomical test to distinguish primate species that use tools from those that do not (Hamrick and Inouye, 1995;McGrew, 1995;Ohman et al, 1995;Susman, 1994Susman, , 1995.…”

“…McGrew (1995) noted that even with relatively weak thumbs great apes use tools in a variety of contexts and that to deny tool behavior to early Australopithecines on the basis of having ape-like thumbs was therefore unfounded. Hamrick and Inouye (1995) and Ohman et al (1995) provided anatomical data for gorillas, orangutans, and gibbons which indicated that thumb robusticity for many gorillas extends into the range of Homo and the robust Australopithecines, thereby challenging the value of Susman's test to distinguish between species that use tools and those that do not. Susman (1995) responded by arguing that even though early Australopithecines may have used tools in an ape-like manner there are neither tools nor evidence of tool-use to support such a claim.…”