Brachiation, a significant moment of development for us humans 15 million years ago.

 ‘brachiation’, is the ability that some monkeys acquired about 15 million years ago to swing from branch to branch by their arms. 

This must have been a crucial development (‘All the ape lines that have survived have been brachiators: i.e. all non-brachiating apes have become extinct.), 

Also that it is much more complicated than it first appears. On the one hand, brachiation was a very unintelligent process (‘more like the acquisition of amazing gymnastic skills that will put off the dreadful day when we have to sit down and think about things’), but on the other, it required quite a lot of brainpower simply to carry out

 (‘And why is brachiation brainy? Because, as a trip to the zoo will suggest, it is very difficult accurately to assess the stability and measure the “distance off” of branches being used to support and propel four limbs moving at speed’).

 More to the point, though, brachiation changed the monkey’s view of the world, straightening his torso and so opening ‘a protected space in front of the eyes, where objects could be delicately presented for intimate consideration’.