Why don't more animals have opposable thumbs?

On a recent kayaking trip, CrowdScience listener Lanier sliced through his right thumb, putting it out of action for a while. This resulted in profound inconveniences, such as a compromised smartphone typing technique.

Missing the use of his thumb made him wonder: since opposable thumbs are so advantageous to us humans, why didn’t they evolve in more species?

We unpick the evolution of our own unique thumbs with the help of paleoanthropologist Tracy Kivell. We see how our grip compares to that of various primate relatives like chimpanzees and spider monkeys; and discover why other tetrapods like horses and turtles have no use for thumbs. Meanwhile in Australia, host Marnie Chesterton meets an animal with not one but two ‘thumbs’ on each hand: the koala. Why did koalas develop this anatomical trait when their closest living relative, the wombat, did not? And is it only a matter of time before koalas develop smartphones?

Presented by Marnie Chesterton.