Introduction

Key adaptations of living and extinct primates are central to any understanding of the origins and early evolution of our order. One such attribute of the primate order is the tendency toward relatively high levels of encephalization (e.g., Gould, 1975a,b; Jerison, 1973, 1979; LeGros Clark, 1971). Although there is of course considerable overlap in relative brain size with other mammalian species and groups, primates represent the most diverse mammalian order generally characterized by such high levels of encephalization. Hominoids as a group—and our own excessively encephalized species in particular—may garner much of the focus in discussions of primate brain evolution, but I argue here that it is an understanding of early primate brain evolution, which

Brian T. Shea • Department of Cell and Molecular Biology, Northwestern University, 303 E. Chicago Avenue, Chicago, IL 60611

is perhaps most central to explicating subsequent patterns of encephalization in our order. Jerison (1979) was among the first to stress that large relative brain size was probably a characteristic of the early primates, compiling data that could be reliably assembled for fragmentary early fossil primates and other contemporary mammals. Gould (1975b) explicitly stressed that any true understanding of anthropoid and human encephalization required an explanation for the large relative brain size in early primates. He stated (Gould, 1975b: 26):

Primates have been ahead right from the start; our large brain is only an exaggeration of a pattern set at the beginning of the age of mammals. But why did such a large brain evolve in a group of small, primitive, tree-dwelling mammals, more similar to rats and shrews than to mammals conventionally judged as more advanced? ... we simply do not know the answer to one of the most important questions we can ask.

Here I will address this central and unanswered question, and consider some ramifications for the subsequent evolution of brain size in primates. My focus will be on brain size within the context of body size, life history adaptations and reproductive strategies. In line with a theoretical emphasis on how an understanding of development and ontogeny can elucidate primate evolutionary morphology (e.g., Shea, 1988, 1990, 1992a), this paper will complement traditional foci on adult morphology and selective scenarios by incorporating discussion of neonatal and subadult adaptations in early and subsequent primate evolution.

I will argue that primatologists have not previously appreciated how the consequences of body size variations are key to explicating the evolution of relative brain size in both early primates, and their subsequent and long-term adaptive diversifications. The input of body size here is not predominantly viewed in the direct causal or allometric (Gould, 1966) sense; the orientation is rather one in which size is but a single component within a complex and synergistic adaptive network of features. Aboreality and life history features, such as reproductive strategy, are other key elements. The central argument may be abstracted as two sequential elements. First, the high encephalization of early primates relative to their mammalian contemporaries is seen as linked to the evolution of strongly precocial reproductive strategies at relatively small body sizes. In originating at small body size and "living slowly," the early primates were quite unusual mammals since most small species "live fast and die young" (Eisenberg, 1981; Promislow and Harvey, 1990; Read and Harvey,

1989). Second, the characteristic ordinal feature of high relative brain size is viewed as, in large measure, a result of subsequent diversification in both body size and other adaptive strategies among the major groups of primates. In essence, the initial primate head-start is "translated up" to subsequent larger body sizes and adaptive configurations.

Primates are the only mammalian order characterized by this combination of marked size and adaptive diversification from a foundation of small size, precociality, and high relative brain size. I argue that generally high levels of encephalization perhaps comparable to what is seen in modern primates would likely have been evolved if other small and precocial mammals—such as bats, dermopterans, and elephant shrews—had undergone comparable evolutionary diversification in size and adaptive strategy.

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