A series of recent reports have questioned the ability of great apes to comprehend declarative communication and have suggested that this ability is biologically based and may have driven the evolution of human language. We tested three groups of differently reared chimpanzees and bonobos for their ability to understand declarative signals in an object-choice task. The scores of the two groups of apes that were reared in a sociolinguistically complex environment were significantly higher than the scores of the standard-reared group. The results further showed that bonobos did not outperform chimpanzees. Our results demonstrate that environmental factors, particularly access to a sociolinguistically rich environment, directly influence great apes' ability to comprehend declarative signals and suggest that, contrary to recent claims, apes have the biological capacity to utilize purely informative communication.

The cultural intelligence hypothesis (CIH) claims that humans' advanced cognition is a direct result of human culture and that children are uniquely specialized to absorb and utilize this cultural experience (Tomasello, 2000). Comparative data demonstrating that 2.5 year old human children outperform apes on measures of social cognition but not on measures of physical cognition support this claim (E. Herrmann, J. Call, M. V. Hernandez-Lloreda, B. Hare, & M. Tomasello, 2007). However, the previous study failed to control for rearing when comparing these two species. Specifically, the human children were raised in a human culture whereas the apes were raised in standard sanctuary settings. To further explore the CIH, here we compared the performance on multiple measures of social and physical cognition in a group of standard reared apes raised in conditions typical of zoo and biomedical laboratory settings to that of apes reared in an enculturated socio-communicatively rich environment. Overall, the enculturated apes significantly outperformed their standard reared counterparts on the cognitive tasks and this was particularly true for measures of communication. Furthermore, the performance of the enculturated apes was very similar to previously reported data from 2.5 year old children. We conclude that apes who are reared in a human-like socio-communicatively rich environment develop superior communicative abilities compared to apes reared in standard laboratory settings, which supports some assumptions of the cultural intelligence hypothesis.

While numerous publications have shown that apes can learn some aspects of human language, one frequently cited difference between humans and apes is the relative infrequency of declaratives (comments and statements) as opposed to imperatives (requests) in ape symbol use. This paper describes the use of declaratives in three language-competent apes and two children. The apes produced a lower proportion of spontaneous declaratives than did the children. However, both groups used declaratives to name objects, to interact and negotiate, and to make comments about other individuals. Both apes and children also made comments about past and future events. However, showing/offering/giving, attention getting, and comments on possession were declarative types made by the children but rarely by the apes.Recent discussions about the evolution of communication have stressed a perceived distinction between humans and our closest evolutionary relatives, the great apes, wherein human nature is described as uniquely cooperative relative to the more competitive great apes (e.g. Moll and Tomasello, 2007;Tomasello, 2007). Some researchers argue that after splitting from the common ancestor, our evolutionary ancestors formed a social environment that supported the desire and drive to communicate and cooperate with others. According to this hypothesis, this socio-communicative environment resulted in a qualitative difference in the cooperative nature of humans relative to the other apes and this cooperative nature drove the evolution of human language and cognition (Moll and Tomasello, 2007). These researchers further suggest that, due to their uncooperative nature, chimpanzees are not fully capable of taking advantage of human social environments that increase cooperative and communicative abilities.

Brain asymmetries, particularly asymmetries within regions associated with language, have been suggested as a key difference between humans and our nearest ancestors. These regions include the planum temporale (PT) - the bank of tissue that lies posterior to Heschl’s gyrus and encompasses Wernicke’s area, an important brain region involved in language and speech in the human brain. In the human brain, both the surface area and grey matter volume of the PT is larger in the left compared to right hemisphere, particularly among right-handed individuals. Here we compared the grey matter volume and asymmetry of the PT in chimpanzees and three other species of nonhuman primate in two Genera including vervet monkeys (Chlorocebus aethiops sabaeus), rhesus macaques (Macaca mulatta) and bonnet macaques (Macaca radiata). We show that the three monkey species do not show population-level asymmetries in this region whereas the chimpanzees do, suggesting that the evolutionary brain development that gave rise to PT asymmetry occurred after our split with the monkey species, but before our split with the chimpanzees.

Abstract. The ability to comprehend purely informative (declarative) communications has been put forward by several researchers as a possible biological distinction between humans and our nearest evolutionary relatives, the great apes. Most studies that purport to show that great apes cannot comprehend declaratives utilize the object choice task as a basis of measurement and have relatively few subjects. Here we report on a large-scale study of chimpanzees and bonobos as well as a meta-analysis of earlier studies. The data from this large pool of subjects suggest that environmental and methodological differences greatly affect the ability of great apes on the object choice task. Therefore, it is unlikely that an adaptation for declarative comprehension drove the evolution of human language.Keywords: great apes, declarative communication, object choice task, chimpanzee, bonobos, evolution of language PINKER and BLOOM (1990) made the case that human language could have evolved through natural selection mechanisms. To extend this claim as far as possible, they begin their arguments under the assumption that no nonhuman animal has a homologous communicative system to human language, and therefore language evolved de novo in the human evolutionary line. However, to truly determine what environmental pressures may have led to the development of language, we must first delineate which abilities were selected for (were new to our line) and which were likely present in a common ancestor. One method of determining these prob-