occurred by the time of the LCA of chimpanzees and humans. Thus, an assessment of the genetic underpinnings of being human should not just focus on the terminal human lineage but should also encompass earlier periods of human ancestry (Fig. 3.2). Moreover, there are other mammals with aspects of their phenotypes (e.g., enlarged brains) that are similar to aspects of the distinctive modern human phenotype. Examining the ancestries of these mammals is a further way to assess the genetic underpinnings of distinctive modern human phenotypic features and suggests that all such features are not necessarily unique to modern humans.
Expanded cognitive abilities are hallmarks of modern humans. Why such abilities were selected for in modern humans and in the human lineage, and how they are maintained, is of great interest. As noted by Darwin more than 135 years ago, differences observed between the modern human mind and the mind of our closest living relatives can be more appropriately characterized as differences in degree, not differences in absolute kind. As such, we expect that the roots of the adaptive evolution that led to the modern human mind trace back to ancient stem lineages in primate and mammalian phylogeny. For example, humans have a phenomenal ability to design and use complex tools, but this ability depends on the opposable thumb, which had evolved in the early primates, as attested to by its presence in slow loris and other primate species.