Research

The evolution of primate carpals

One of the most exciting aspects of studying primates is that to understand primates is to understand ourselves. Historically, when anthropologists have asserted and applauded human uniqueness, they have pointed to what humans do and do not do with their hands. Humans are uniquely adept tool users. Humans do not use their hands at all to move through the world, instead balancing exclusively on our hind limbs. It is telling that humans have the most uneven population distribution of handedness of any primate (approximately 9 right-handed people for every one left-handed person). Our hands have shaped our evolution and our evolution has shaped our hands.

As the mediator between our strong, adept digits and the rest of our upper limb, the wrist must strike a balance between mobile enough to permit a capacity for object manipulation but stable enough to withstand the forces that tool use or other movements impose. As such, carpal morphology and how these bones relate to each other has a substantial impact on how an animal is able to use its hands. Despite the promise that these bones hold in helping us understand the evolution of hand use, however, their overall morphology, relationships, and pattern of morphological variation across primates are relatively understudied. A reason for this is because their shapes defy simple quantification the way that a long bone like the femur can be quantified by its length. Another reason is that imaging techniques have only recently become good enough and accessible enough to create high quality scans of these tiny bones in smaller animals. My dissertation work involves amassing a sample of primate carpals and quantifying their shape, relationships, and how they have evolved. The aim of this research is to understand how we have evolved the dexterity required for tool use and how our locomotor behavior has changed as our lineage has evolved.

Dexterity

Humans have long been defined by our advanced ability to make tools. When naming the oldest member of our genus Homo, paleoanthropologists assigned it the species name habilis, translating to "handy" and chosen to reflect the unique tool using ability of Homo. Although it's since been observed that other primates (e.g. chimps, macaques, capuchins) also use stone tools, identifying and understanding the extent to which the earliest humans used stone tools remains a topic crucial to understanding human evolution. Did early hominins like Australopithecus make stone tools or simply use them, as chimpanzees do today? What grips were hominins capable of and can we find morphological correlates in the hand?

Form, Function, & Phylogeny

Phenotypic evolution is influenced by selection towards optimal phenotypes for particular functions or behaviors, but it is also influenced by shared evolutionary history. This is the reason why more closely related species tend to resemble each other phenotypically. Disentangling the influences of function and phylogeny on the morphology of species living today can help us better understand what aspects of an animal's morphology is relevant to behaviors it shares with other species and what aspects are simply the result of its place in the tree of life.

Locomotor Behavior

From the vertical clinging and leaping of tarsiers to the five-limbed below-branch suspension of spider monkeys to the knuckle-walking of chimpanzees and gorillas, primates have an unparalleled diversity in hand use during their habitual locomotor behaviors. When and how primates evolved the many ways they use their hands as they move through their environments largely remains a mystery. How did apes evolve to hang and swing from tree branches? Did they evolve this ability multiple times or just once? How does this ability differ between apes and the branch swinging monkeys? How did humans evolve to stop using their hands at all?

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