Big brain (intelligence) comes at a huge cost, and is only useful if you are a generalist. I'd assume that being a generalist drove intelligence. It may have s

Big brain (intelligence) comes at a huge cost, and is only useful if you are a generalist. I'd assume that being a generalist drove intelligence. It may have started with warm bloodedness and feathers/fur and further boosted in mammals with milk production (& similar caring for young by birds) - all features that reduce dependence on specific environmental conditions and therefore expose the species to more diverse environments where intelligence becomes valuable.