Parallel Evolution of Cortical Areas Involved in Skilled Hand Use

Abstract

Dexterous hands, used to manipulate food, tools, and other objects, are one of the hallmarks of primate evolution. However, the neural substrate of fine manual control necessary for these behaviors remains unclear. Here, we describe the functional organization of parietal cortical areas 2 and 5 in the cebus monkey. Whereas other New World monkeys can be quite dexterous, and possess a poorly developed area 5, cebus monkeys are the only New World primate known to use a precision grip, and thus have an extended repertoire of manual behaviors. Unlike other New World Monkeys, but much like the macaque monkey, cebus monkeys possess a proprioceptive cortical area 2 and a well developed area 5, which is associated with motor planning and the generation of internal body coordinates necessary for visually guided reaching, grasping, and manipulation. The similarity of these fields in cebus monkeys and distantly related macaque monkeys with similar manual abilities indicates that the range of cortical organizations that can emerge in primates is constrained, and those that emerge are the result of highly conserved developmental mechanisms that shape the boundaries and topographic organizations of cortical areas.

Mapping multisensory parietal face and body areas in humans

“…Based on the locations of movements evoked, action zones are approximately organized into overlapping areas of upper, middle, and lower spaces near the body. Other studies using tactile stimulation or retrograde neuronal tracing on nonhuman primates have also suggested that topographic organizations of multiple body parts exist in the PPC, and some subdivisions also receive visual input (19)(20)(21)(22). A few human neuroimaging studies have shown evidence of distinct body-part representations in the superior parietal lobule (23)(24)(25).…”

Cortical evolution in mammals: The bane and beauty of phenotypic variability

“…In terms of neural organization, cebus monkeys have independently evolved a relatively larger cortical sheet, such that their encephalization (28,29) resembles that of distantly related Old World monkeys rather than their closely related sister groups, New World monkeys. In addition, they have independently evolved direct corticospinal projections to the ventral horn motor neurons that project to muscles of the digits (30) and have also independently evolved a cortical field, area 2, associated with processing proprioceptive inputs (31). This example illustrates two important points.…”

Convergent Evolution of Vocal Cooperation without Convergent Evolution of Brain Size

“…Natural selection acts on the behavior of an individual, not on isolated neural structures [Padberg et al, 2007]. Evolutionary changes to organisms occur as parallel processes, involving the interplay between the environment, an individual's musculoskeletal structures and the brain [Krubitzer and Kaas, 2005].…”

“…Evolutionary changes to organisms occur as parallel processes, involving the interplay between the environment, an individual's musculoskeletal structures and the brain [Krubitzer and Kaas, 2005]. In fact, given similar pressures, identical cortical areas can emerge by virtue of the constrained nature of cortical development [Krubitzer and Kaas, 2005;Padberg et al, 2007]. For instance, cortical areas 2 and 5, associated with motor planning and coordination, are very well developed in macaques, an Old World monkey, as well as in Cebus monkeys, a New World monkey.…”

“…In other New World primates, however, areas 2 and 5 are either absent or poorly developed [Padberg et al, 2005]. The reason for this becomes apparent when we consider that, of the New World primates, Cebus monkeys are the only species known to use a precision grip -a grip that is common among Old World monkeys and apes (including humans) [Padberg et al, 2007]. Such a grip alters the mechanics of the hand, facilitating object manipulation through an increase in the possible number of digit configurations.…”