Body Mass Estimates and Encephalization Quotients: A Fresh Look at the Australopithecines and Homo habilis

The australopithecines and Homo habilis have been publicized for years as examples of evolutionary transitional forms that launched our own human lineage. Dogmatic evolutionists have rationalized these claims on the basis of brain expansion, encephalization quotients, and bipedalism. However, any evolutionary justification for brain expansion in these extinct creatures must rest in a precise model for the determination of body mass. To insure an accurate body mass model, one must take into account whether the animal is quadruped, facultative biped, or obligatory biped. Past body mass estimates for the australopithecines and Homo habilis were based on assumptions about their bipedalism that have proven to be erroneous. When a body mass model is used accounting for the facultative bipedalism of the australopithecines and Homo habilis, the data shows that they are not highly encephalized, and hence nothing more than a microevolutionary adaptation of the pan-troglodytes. * Patrick H. Young, Ph.D., P.O. Box 27001, Richmond, Virginia 23234, www.creationists.org/patrickyoung.html [email protected] Accepted for publication: October 10, 2005 Introduction One of the classic examples of alleged evidence for human origins that evolutionists have proposed to promulgate the evolutionary transitional status of the australopithecines and Homo habilis is a comparison of increasing cranial capacity versus their perceived evolutionary timescale (Falk, 1980; 1987; 1998; Kirkwood, 1997; Lee and Wolpoff, 2003; McHenry, 1994a). In this context, various fossilized crania are plotted against time as an attempt to demonstrate a gradual increase in brain volume from the time of our perceived most recent common ancestor to the large-brained humans we observe today (Figure 1). The goal of this type of demonstration is to provide visual evidence (however weak) that some evolutionary advancement in intelligence over time has occurred. Realistically, the scientific validity of using cranial capacity alone as a justification for brain expansion through any perceived evolutionary timescale is suspect at best, because there is no correction for body size (McHenry and Coffing, 2000; Kappelman, 1996; Holloway, 1988; Conroy et al., 1990). It is also in significant dispute as to whether the relative brain size of the australopithecines actually did increase over perceived evolutionary time (Falk, 1987; 1998; Martin, 1990). However, the majority view in evolutionary thinking appears to indicate that the australopithecines did possess a larger relative brain size than the apes (Pilbeam and Gould, 1974). Moreover, assuming it is a valid taxon (Brace, 1979; Wood and Collard, 1999), Homo habilis is believed to be the first creature to demonstrate a measurable increase in relative brain size (Falk, 1987; Lovejoy, 1981; Hawks et al., 2000; Pilbeam and Gould, 1974; Haeusler and McHenry, 2004; Ruff et al., 1997). While a larger brain is not necessarily a predictor of higher intelligence (Beals et al., 1984; McLeod, 1983), 218 Creation Research Society Quarterly brain size is known to have a strong positive correlation with body size (Jerison, 1970). In other words, a larger body size usually requires more neurons and thus a correspondingly larger brain to handle the increase in total structural mass. However, neither suggests any true evolutionary adaptation (Pilbeam and Gould, 1974; Jerison, 1976), nor evolution to a higher taxonomic group (Cheek, 1981; Custance, 1968; Hummer, 1977; 1978; Cuozzo, 1977). Also, Jerison (1976) noted: ...large brains do not, in general, confer an evolutionary advantage over smaller brains. Instead large and small brains represent different but equally good evolutionary outcomes (p. 90). Currently, plots using cranial capacity alone as perceived evidence for some evolutionary advancement have fallen out of favor (Duffett, 1983). Instead, evolutionists seem to have converged on the concept of encephalization which was originally proposed by Dubois (1897). The Dubois allometric factor is based on an impromptu association of brain size to body size/body function which results in a somewhat predictable logarithmic relationship (Jerison, 1970). Dubois’ initial work on encephalization has been significantly refined over the years with the extensive experimental research on primate brain size by Jerison (1970) and Martin (1990). The result of their work was the proposal of an encephalization quotient which is defined as the ratio of the true mass of a brain to the anticipated mass of the brain for a given body size (Jerison, 1970; Martin, 1990). It is essentially nothing more than an index of brain volume that can be compared to different organisms. However, the relationship fails to scale properly across different taxonomic groups of mammals (Pagel and Harvey, 1989). The allometric relationship is traditionally derived from a bivariate regression analysis of brain volume and body weight to determine the degree of encephalization of the animal in question (Sacher, 1970; Martin, 1990). Since a majority of mammals are expected to have an encephalization quotient of 1, a higher encephalization quotient implies the organism is more complex (Jerison, 1970; Martin, 1990; McHenry, 1988). The australopithecines are an extinct group of creatures alleged to be an early evolutionary branch of divergence from chimpanzees (Ayala and Cela-Conde, 2003). Some believe they are transitional forms that are forerunners to the genus Homo (Wood and Collard, 1999; Wood, 1992; Tobias, 1991; Skelton and McHenry, 1992). Others believe they are nothing more than an extinct evolutionary branch (Pilbeam and Gould, 1974), and others believe they are simply an extinct form of ape (Cuozzo, 1977; Custance, 1968; Hummer, 1977; Lubenow, 1992; Mcleod, 1983; Oxnard, 1975; 1984). More importantly however, Pilbeam and Gould (1974) have stated that at least three principal species of Australopithecus are all just different adaptations of the same animal and their brains are all equally expanded beyond the ape grade. In this context, evolutionists have elevated the australopithecines to a pseudo-transitional status higher than the pan-troglodytes for no other reason than a larger uncorrected average brain mass. Louis Leakey et al. (1964) were the first to propose Homo habilis as the earliest member of the genus that evolutionists also place modern humans. Homo habilis is primarily distinguished from the australopithecines by its larger brain size (Haeusler and McHenry, 2004; Pilbeam and Gould, 1974; Susman, 1994; Blumenschine et al., 2003). Homo habilis has been proposed by evolutionists as the earliest hominid to exhibit the increased brain size required to evolve human intelligence. However, the validity of placing this extinct animal in the genus Homo has been in significant dispute ever since Leakey’s discovery was first published (Wood and Collard, 1999; Kramer et al., 1995; Lieberman et al., Figure 1. Extinct hominid brain mass versus perceived evolutionary date. Volume 42, March 2006 219 1988; Miller, 2000; Tobias, 2003; Hummer, 1977; Hummer, 1978; Cheek, 1981; Custance, 1968). The primary goal of the present study is to review different body mass models and determine which is a more accurate predictor of the physical size of Australopithecus afarensis, Australopithecus africanus, Australopithecus robustus, Australopithecus boisei, and Homo habilis. Encephalization quotients are then recalculated using the accepted body mass model to determine if the evolutionary transitional status for the australopithecines and Homo habilis has any scientific validity. Additionally, this paper will evaluate the accuracy of awarding transitional status to the australopithecines over pan-troglodytes. Methods of Estimating Brain Mass and Encephalization Quotient Brain Mass The brain mass calculation below is taken from the formula used by Ruff et al. (1997). The equation was first derived by Martin (1981) employing a least squares regression analysis to determine a bivariate relationship of brain mass and cranial capacity using the data from 27 primate species. Ruff’s regression equation (below) has a correlation coefficient of 0.995. The brain masses derived in this paper originate from the individual crania of each hominid fossil presented in Table I. Brain mass = 1.147 x cranial capacity 0.976 (1) Encephalization Quotient The encephalization quotient in the form presented below is taken from Ruff et al. (1997). This equation relates brain mass and body weight, and is derived from a regression analysis of 309 extant placental mammal species with a correlation coefficient of 0.96 (Martin, 1981; Ruff et al., 1997; McHenry and Coffing, 2000). EQ = brain mass / (11.22 x body mass0.76) (2) Discussion Determination of Hominid Body Weight While there have been several papers published attempting to relate various cranial and post-cranial fossils to hominid body weight (Aiello and Wood, 1994; Jungers, 1988; McHenry, 1988; Wolpoff, 1973; Kappelman, 1996), estimations that employ hind-limb joint diameter seems to be the best predictor of total body mass (McHenry, 1992; Jungers, 1988; Ruff and Walker, 1991; Kappelman, 1996). Several proposed regression calculations yield formulae extracted from either human data (McHenry, 1992) or from data that includes all hominoidea (Jungers, 1988; McHenry, 1992; Kappelman, 1996). The body mass formulae evaluated in this paper are based on two models proposed by McHenry (1992) utilizing human data (male and female North Americans, Khoisan, and Pygmy) and hominoid data (extant male and female apes along with the human data). Regressions using the human data assume that the animal in question is an obligatory biped, while regressions based on hominoid data include obligatory bipeds and animals that are facultative biped and quadruped. Table II is a comparison of several hominid body weight estimates using McHenry’s human and hominoid regression formulae. Generally, body mass calculations originating from the human regression formula will bias the data towards lower body weights because obligatory biped animals possess less muscle density in their upper body. Conversely, facultative bipeds or quadrupeds possess a much higher percentage of upper body mass because their primary mode of locomotion involves the use of their upper body extremities. While some evolutionists continue to question using human data to calculate the body weight of extinct hominids (Jungers, 1988), countless articles have appeared in secular journals presenting data on encephalization quotients focused on calculated extinct hominid body masses originating from formulae based on human models (Aiello and Dean, 2002; Ruff et al., 1997; McHenry and Coffing, 2000; McHenry, 1994a; 1994b; Wolpoff, 1973). When McHenry first proposed his human model, it was not without some skepticism. McHenry (1992) stated, “It is difficult to assess whether human or hominoid formulae give the best results” (p. 421). Jungers (1988) said that “Homo sapiens should be omitted from these models because they possess abnormally large hind-limb joints for their body size and this condition does not characterize early hominids” (p.117). It has also been demonstrated that body weight estimations for A. robustus and A. boisei based on formulae derived from hominoid post-cranial remains correlate much better with their robust jaws than estimations based on human formulae (McHenry, 1991a). McHenry (1992) has further stated: “Common sense might favor the human equations simply because all known hominids are bipedal” (p. 421). However, generally stating that all known hominids are bipedal is extremely misleading. The australopithecines (A. afarensis, A. africanus, A. robustus / boisei) and Homo habilis are all facultative bipedal (Wood and Collard, 1999) which is a very different declaration than just saying they are bipedal. A facultative bipedal animal is one whose primary mode of locomotion 220 Creation Research Society Quarterly Table I. Hominid Encephalization Quotient Data Using the Hominoid Body Mass Model SAMPLE CRANIAL CAPACITY BRAIN MASS++ BODY MASS+ ENCEPHALIZATION QUOTIENT+++ cm3 Grams Male (Kg) Female (Kg) Male Female Australopithecus afarensis++++ AL 333-45 500 494 60 36 1.96 2.91 AL-162-28 400 397 60 36 1.57 2.34 AL-333-105 400 397 60 36 1.57 2.34 Australopithecus africanus++++ MLD 1 500 494 53 37 2.16 2.84 MLD 37/38 435 431 53 37 1.88 2.48 STS 5 485 479 53 37 2.09 3.76 STS 19/58 436 432 53 37 1.89 2.49 STS 60 428 424 53 37 1.85 2.44 STS 71 428 424 53 37 1.85 2.44 Taung 405 402 53 37 1.76 2.31 Australopithecus robustus/boisei++++ L388Y-6 448 444 76 40 1.47 2.38 SK 1585 530 523 76 40 1.73 2.81 KMN-ER 406 510 504 76 40 1.67 2.71 KMN-ER 732 500 494 76 40 1.64 2.65 OH 5 530 523 76 40 1.73 2.81 KMN-WT 17000 410 407 76 40 1.35 2.18 KMN-WT 17400 400 397 76 40 1.32 2.13 KMN-ER 13750 475 470 76 40 1.56 2.52 KMN-ER 407 506 500 76 40 1.66 2.68 KMN-ER 732 500 494 76 40 1.64 2.65