Recent work has shown that general intelligence is not limited to humans, but also found in many animals. Between-species variation in general intelligence is closely linked to brain size. Brains have two peculiarities that affect the evolution of intelligence. Firstly, they are energetically expensive, so the ecological ability to sustain larger brains constrained the evolution of intelligence. Secondly, brains must be trained before they are useful. Therefore, species that can rely during development on social inputs that make skill-learning more efficient are more likely to evolve larger brains. These two principles also apply to human evolution.