Evolution occurs when a species adapts and changes over time through descent with modification. This is based on selective pressures within their environment. Human evolution is no different. Early humans walked on four legs, ate mostly plants, and had small brains. Over millions of years, they adapted to take advantage of the world around them. For example, they began to walk upright on two legs, became predators and skilled hunters, and developed large brains that allowed for sophisticated communication, a higher degree of forethought and planning, complex social interactions, and other traits that define them as a species. Often these adaptations were in response to a change in their environment, such as temperature change in the beginning or ending of an ice age or when a new species of predator moved into the area. Like all species, modern humans continue to change today and further adapt to their environment.