NFourresearch

Homind Brain Evolution: A new look at old brains

Reanalysis of the endocasts from specimen L338y-6

Behavioral & cerebral asymmetries in siamangs

Evolution of brain size sexual dimorphism

Working memory & the length of the principal sulcus

Abstract Based on an analysis of its endocast, Holloway(1981 Am J Phys Anthropol 53 :109-118) attributed the juvenile Omo L338y-6 specimen to Australopithecus africanus (i.e.., gracile australopithecines) rather than to Paranthropus (Australopithecus ) boisei (robust australopithecines) favored by other workers (Rak and Howell [1978] Am J Phys Anthropol 48 :345-366). Holloway's attribution was based on the specimen's (1) low cranial capacity, (2) gracile-like meningeal vessels, (3) gracile-like cerebellar hemispheres, and (4) absence of an enlarged occipital/marginal (O/M) sinus system. Recent work, however, has shown that criteria 1 and 2 are not useful for sorting gracile from robust australopithecines (Culotta [1999] Science 248 :1109-1111; Falk [1993] Am J Phys Anthropol 92 :81-98). In this paper, we test criterion 3 by quantifying the endocranial cerebellar and occipital morphology reproduced on the Omo L338y-6 endocasts, and comparing it to seven endocasts from South and East African early hominids. Our preliminary results show that metric analysis of this specimen cannot be used to sort it preferentially with either robust or gracile australopithecines. Finally, we demonstrate that, contrary to previous reports, the Omo L338y-6 endocast reproduces an enlarged left occipital sinus (criterion 4). This observation is consistent with the original attribution of the Omo specimen to robust australopithecines (Rak and Howell [1978] Am J Phys Anthropol 48 :345-366). Furthermore, if Omo L338y-6 was a robust australopithecine, this discovery extends the occurrence of an enlarged O/M sinus system to one of the earliest known paranthropines. Am J Phys Anthropol 110 339-406, 1999.

Figure 1. Occipital illustration of Omo L338y-6 partial endocast (White and Falk, 1999)

A broad objective of this research is to investigate the evolutionary foundations of singing and its relationship to the emergence of handedness and language. The neurological processing for singing and emotions is differentially undertaken by the right hemisphere in humans. If the same is true for Hylobates syndactylus (siamangs), then specific behavioral consequences may be predicted. For example, a switch between leading limb preference might be observed between brachiation while animals are singing compared to when they are silent since emotionally laden vocalizations may affect leading limb preference (Stafford et al., 1990). Moreover, the existence of similarities in the neurological organization of song between siamangs and humans may be the result of extensive reorganization of, and building upon, existing structures underlying vocal communication that were inherited from an early common ancestor of both species. Partial support for this hypothesis is illustrated by the homologous asymmetry in the planum temporale of the brain (an area that in humans is known for its association with language function) of chimpanzees and humans (Gannon et al, 1998). Because human language is known to be related to specific brain asymmetries (petalias) and handedness, the extent to which brain asymmetries and handedness are related in nonhuman primates will provide clues to the evolutionary emergence of species-specific vocal communications (in this case singing) and more specifically, language in hominids (Bradshaw & Rogers, 1993; Le May, 1982).

This research explores the brains and behaviors of the melodiously tuneful lesser ape, the siamang. The investigation includes a behavioral component that identifies leading limb preferences during vocal and non-vocal activities, a cerebral component that examines specific asymmetries in the endocasts of siamang and the effect that sex has on each component. Because siamangs produce some of the most elaborate vocalizations among all primates, and rely on a form of locomotion that is dependent on hand use (brachiation), the existence or non-existence of behavioral and cerebral asymmetries related to vocalization in this species may provide clues to the evolutionary history of handedness and its relationship to the emergence of language in hominids. this study is one of the first to concurrently investigate behavioral and cerebral asymmetries in a single species of nonhuman primate.

Preliminary results indicate that siamangs show individual, but not population level lateralization for leading limb preference during brachiation. They also possess a number of cerebral asymmetries including a high occurrence of left occipital and right frontal petalias. Further, males appear to be more lateralized than females in both leading limb preference during brachiation as well as many cerebral aspects.

Abstract Recent reports in the neurosciences underscore the differences between the brains of men and women in gross volume adjusted for body size. Few studies, however, have explored these relationships in nonhuman primates. This study quantifies sex differences for collected and published data in gross brain size and body mass in several genera of sexually and non-sexually dimorphic New and Old World monkeys and apes.

Least squares and multiple regressions were performed for Cebus (n=77; m=45, f=32), Aotus (n=71; m=40, f=31), Presbytis (n=69; m=28, f=41), Papio (n=117; m=53, f=64), Hylobates (n=100; m=51, f=49), and Pan (n=120; m=59, f=61). Least square regressions of body weight to brain size were significant within each sex for Presbytis, Papio, Hylobates, and Pan. Slopes for male and female regressions for each of these genera did not differ significantly. Multiple regression analyses indicated that in addition to body weight, sex was a significant predictor of brain size in Aotus, Presbytis, Papio, and Pan. Therefore, a model positing a common slope and different intercepts for the two sexes was the best fit for these genera. However, for Hylobates, sex was not a significant predictor of brain size and body weight alone provided a better model fit.

Our results indicate that while brain volume is greater on average in adult males than adult females of the same body mass in several genera that are sexually dimorphic in body mass (i.e., Presbytis, Papio and Pan), brain size sexual dimorphism occurred in only one of our non-sexually dimorphic (for body size) genera (i.e., Aotus). Therefore, body size dimorphism may not account for the absolute brain size sex differences observed in some genera of primates.

Abstract During the process of evolution, a selective advantage may have been gained by organisms that had the ability to utilize mentally stored information of a stimulus rather than the stimulus itself. The ability to temporarily store and mentally operate on stimulus information is often termed "working memory". Within the neocortex of primates, the functional anatomic subdivision surrounding the principal (rectus) sulcus plays an important role in modulating the performance of delay-response tasks in monkeys (representing working memory). However, it appears that no study has investigated the direct relationship between the length of the principal sulcus and performance on a delay-response task. Therefore, this paper investigates the relationships between principal sulcus length and performance on delay-response tasks. However, to control for the effect of overall brain size on this relationship, cranial capacity is analyzed with both principal sulcus length and delay-response performance. Results support a consistent and significant correlation between principal sulcus length and performance on delayed-response tasks in a variety of Old World and New World monkeys. Principal sulcus length is also significantly correlated with cranial capacity; however, cranial capacity is not significantly correlated with performance on delayed-response tasks. The results of this investigation provide a method for analyzing cranial capacity and working-memory abilities in select primates based on principal sulcus length, and may prove useful for interpreting endocasts in the primate fossil record. Am J. Phys. Anthropol. 109:33-40.

Figure 1: The pricipal sulcus in the monkey cerebral cortex (Modified after Walker1940)

More recently, we (Dan White and John Redmond) have begun to investigate the possible role of the cerebellum in working memory by examining its relationship to the size of the principal sulcus and performance on delay-response tasks. In this research, we will utilize 3-D digitizing to reexamine the endocasts of the initial study. Stay tuned...