The earth spins on an axis that is tilted 23.3 degrees off of its orbit around the sun. This causes the gradual change in the length of daylight between days from june to december, which in turn is the cause of the seasons in the temperate areas of the planet.

Creating the annual lifecycle of organisms here, the tilt gives our environment dynamicism that it would not have if the earth were "straight up", but is not so extreme as to be deadly. The dynamic atmosphere and change in seasons has required additional adaptability in developing life forms, and created secondary contextual stimulus for the developing brains of those organisms.