From Paws to Hands: The Evolution of the Forelimb and Cortical Areas Involved in Complex Hand Use

Leah Krubitzer, Center for Neuroscience, Department of Psychology, University of California, Davis

Forelimb morphology and use in mammals is extraordinarily diverse.  Evolution has produced wings, flippers, hooves, paws and hands which are specialized for a variety of behaviors such as flying, swimming and grasping to name a few.  While there is a wealth of data in human and non-human primates on the role of motor cortex and posterior parietal cortical areas in reaching and grasping with the hand, these cortical networks did not arise de novo in primates, but likely arose early in mammalian evolution since most mammals use the forelimbs for reaching and grasping as well as other behaviors.  Yet, we know relatively little about how frontoparietal networks that control the forelimb have evolved in mammals.  Our laboratory has previously described the organization of somatosensory cortical areas in a variety of mammals and find that both morphology of the limb and how the limb is used are reflected in the organization of cortical fields that represent both mechanosensory receptors and proprioreceptors.  In recent studies we examine the organization of movement maps using intracortical microstimulation techniques in a range of mammals to determine the extent of cortex from which movements can be evoked, and how behavioral specializations of the limb are represented in movement maps in the cortex.  While there are some features of organization that are similar across species, such as gross topography, most of the details of map organization are species specific.  Thus, movement maps are much more variable across species than are sensory maps, and are variable across individuals within a species, suggesting that these maps are, in large part, a product of experience.  We propose that motor cortex co-evolved with modifications to the hand and forelimb, and is built during development based on commonly used muscle synergies.