Why are some people left-handed, whereas the majority are right handed? Rutherford and Fry revisit The Sinister Hand episodes to further investigate handedness in humans and animals. They considered cockatoos, chimpanzees and Hannah's dog, Molly, to discover that humans are unique, with just one in ten of us being left-handed.
They ask if there is an evolutionary reason for just 10% of the human population being southpaws
Hannah talks to primatologist Prof Linda Marchant from Miami University about Neanderthal teeth and termite fishing.
Adam consults handedness expert Prof Chris McManus from University College London. He's been trying to track down the genes responsible for whether we're right or left handed.
And what about left-handed brains or eyes or molecules?
Prof Andrea Sella explains handedness, or chirality, at the molecular scale and why when we consider Thalidomide, something seemingly so trivial can be extremely important.
They also explore the left-handed brain. Some researchers point to a link between left-handedness and impairments like autism or dyslexia. Others claim that lefties are more creative and artistic.
So what's the truth? The team consults Professors Sophie Scott, Chris McManus and Dorothy Bishop to find out.
This episode is an updated version of two earlier broadcast episodes.
If you have any Curious Cases for the team to investigate please email curiouscases@bbc.co.uk
Producers: Fiona Roberts & Michelle Martin
Presenter: Adam Rutherford & Hannah Fry
A BBC Audio Science Unit production for BBC Radio 4, first broadcast in 2021.