• Question: Hi, why do we have a thumb? Why not have another finger instead?

    Asked by Kate and Peckasso :) to Chris, Josh, Rebecca, Rob, Susan on 22 Jun 2015.
    • Photo: Chris Armstrong

      Chris Armstrong answered on 22 Jun 2015:

      Natural selection at its finest.

      Somewhere up our evolutionary chain the gene that gave us the thumb as we know it survived “better” than the others. Maybe because it helped us wield tools, or defend ourselves better, meaning that gene was more likely to be passed on.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 22 Jun 2015:

      We inherited thumbs from our ancestors: most of the Old World monkey and apes have thumbs. If you live in trees, a thumb is useful for grasping branches. There is definitely an association between living in trees and having an opposable digit: for example, nearly all passerines (perching birds) have an opposable claw, some tree-dwelling marsupials such as possums have an opposable thumb, and some members of the mouse/rat family (Muridae) that live in trees also have opposable thumbs.

      However, having inherited the thumb, we more or less gave up using it to climb trees and took to using it to grasp things. This led to selective pressure favouring people who could manipulate objects more precisely and/or with more power than others. The result is that the human thumb is considerably more versatile than the thumbs of other primates, though they all descend from a common ancestral thumb.

      Evolution in action.

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