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<p>What makes our minds human? How did they evolve to be this way? This dissertation
presents data from two complementary lines of research driven by these orienting questions.
The first of these explores the `what' of human cognitive evolution through comparative
studies with chimpanzees and bonobos. The general aim of these studies is to understand
which aspects of cognition are unique to humans, and which are shared with our closest
living relatives. Chapters 2-3 test the hypothesis that humans have unique cognitive
skills for reasoning about the attention of other individuals (theory of mind), and
unique motivation to use these skills in cooperative contexts with conspecifics.
In Chapter 2 I show that understanding others' attention is unlikely to be the `small
difference that makes the big difference', as some researchers have proposed. However,
my data support the possibility that species differences in the ontogeny of these
skills may have robust consequences for the adult cognitive phenotype. In Chapter
3 I show that (contrary to previous reports) nonhuman apes are also motivated to engage
in some simple triadic social activities, which resemble those characteristic of human
children. Again however, I identify important differences between humans and other
apes in their spontaneous preferences for these types of activities, and their attitudes
toward a partner when cooperative behaviors are interrupted. The second half of
this dissertation (Chapters 4-5) explores the `why' and `how' of cognitive evolution.
Chapter 4 outlines the kind of research questions and methods that comparative psychologists
will need to embrace in order to use the comparative method to its full potential
in the study of cognitive evolution. Chapter 5 provides a proof of principle for
this approach using a dataset including 33 species tested on two cognitive tasks measuring
inhibitory control. Here I show that cognitive skills for inhibitory control are
closely related to phylogeny across species, and strongly predicted by absolute (but
not relative) brain size. Further, I show that two of the other leading hypotheses
put forth to explain primate intelligence, namely sociality and diet, do not predict
cognitive skills on these tasks. These data illustrate the power of the comparative
method for understanding cognitive evolution, and provide a starting point for future
studies embracing this approach. Collectively, this research refines our understanding
of how human cognition differs from that of other primates, and illustrates the utility
of studying cognitive evolution from an explicitly phylogenetic comparative framework.</p>
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