Shared neurologic mechanisms are the main argument in favor of continuity between nonhuman and human primates’ minds. Several contributors to this Sackler Colloquium have studied these common mechanisms in the field of memory and its brain counterparts. In Chapter 4, Robert Clark and Larry Squire offer a history of the scientific debate provoked by Owen’s (1859) proposal of a lack of evolutionary continuity between human and other primates on the grounds of several brain traits, hippocampus minor (HM) among them (Scoville and Milner, 1957). Since HM was proposed to have a strong role in the organization of memory, the possibility of using animal models appeared, thus supporting evolutionary continuity for the neuroanatomy of human memory. Clark and Squire examine such cross-species similarities in the field of the multiple-memory systems paradigm, offering challenges to the animal model when concurrent discrimination tasks are considered in both humans and monkeys.
Two chapters in this book focus on the evolution of memory. Peter Carruthers points out in Chapter 5 that, despite being fundamental to learning, speech, reading comprehension, prospection and planning, as well as to reflective serial conscious reasoning, working memory (WM) has been scarcely investigated under the across-species perspective. On the grounds of current research, Carruthers holds that WM is a homologous trait shared by humans and nonhuman primates, although our species is unique in aspects like inner speech. Also, humans may be unique in making frequent task-independent use of their WM abilities. However,
in the absence of direct comparative studies, claims on the WM continuity or discontinuity remain somewhat speculative.
Next, in Chapter 6, Timothy Allen and Norbert Fortin offer a complementary analysis of the large body of research on the evolution of episodic memory (EM). The authors propose that protoepisodic memory systems link avian and human phylogenies, supporting the homologous character of traits, such as hippocampal–parahippocampal–prefrontal pathways that would be shared from a common neural ancestry, as opposed to the alternative possibility of evolutionary convergence. Despite this shared capacity, Allen and Fortin discuss eventual divergences, such as with regard to human language, self-consciousness, empathy, and Theory of Mind, holding that these constitute species-specific attributes associated with the expansion in human brains of prefrontal areas.
Differences in social behavior between species rely in part on the neuromodulatory regulation of neural circuits. In Chapter 7, Steve Chang and his coworkers offer clues on how biological specializations for social function transform ancestral mechanisms by means of duplication, repurpose, or differential regulation at multiple levels of organization, from neurons and circuits to hormones and genes. Social behavior shapes the structure and function of these mechanisms in a feedback way. Therefore, the authors hold that a neuroethological approach to the study of human and nonhuman primate social behavior might clarify the phylogeny of interactions between social behavior and neuromodulatory regulation.
The counterpart of phylogeny is ontogenetic development. Comparisons between human and macaque neocortical development show differences that might relate the relatively prolonged neuronal maturation in humans to the enhancement of social learning and transmission of cultural practices, including language. However, few data exist on the ontogenetic neural development of the apes that are more closely related to humans. By means of an experimental analysis, Serena Bianchi and her coworkers, in Chapter 8, show for the first time how Pan paniscus synaptogenesis matches the human case, with a peak of synapse density during the juvenile period (2–5 years of age). Also, chimpanzees and humans share a late development of dendrites of prefrontal pyramidal neurons, compared to sensorimotor areas, offering a common potential for enhanced developmental plasticity. The authors hold that their findings suggest that several key features of human brain ontogeny emerged prior to the divergence of the chimpanzee and human lineages.