Evolution refers to gradual and orderly biological changes that result in a species from their pre-existing forms in response to the changing adaptional demands of their environment. Physiological as well as behavioural changes that occur due to the evolution process are so slow that they become visible after hundreds of generations. Modern human beings possess some features for several thousand years. Three such important features which differentiate them from their ancestors are - (i) A bigger and developed brain with increased capacity for cognitive behaviours (ii) Ability to walk upright on two legs (iii) A free hand with a workable opposing thumb.

Our behaviours are highly complex and more developed than those of other species because we have got a large and highly developed brain. Human brain development is evidenced by two facts. Firstly, the weight of the brain is about 2.35 per cent of the total body weight, and it is the highest among all species. Secondly, the human cerebrum is more evolved than other parts of the brain. These evolutions have resulted due to the influence of environmental demands.

Some behaviours play an obvious role in evolution. For example, the ability to find food, avoid predators, and defend one’s young are the objectives related to the survival of the organisms as well as their species. The biological and behavioural qualities, which are helpful in meeting these objectives, increase an organism’s ability to pass it on to the future generation through its genes. The environmental demands lead to biological and behavioural changes over a long period of time.