In The Origin of the Species by Charles Darwin, the basic driving force behind origin and continuation of specific traits is natural selection or the survival of the fittest. Natural selection means that weaker members or a species or those with limited reproductive success do not pass genes to future generations, thereby ensuring that only genes which promote the long term survival of the species come to dominate the gene pool. The gradual dominance of survival-oriented traits causes the nature of a species to gradually evolve.
In the case of intelligence, it allows technological innovation and cooperation, both of which promote survival. The tool-using intelligent hominid can gather or hunt food more effectively than the non-tool using one. Language and cooperation can lead to survival in enabling individuals to band together and triumph in territorial disputes and to develop division of labour, leading to greater efficiency on food and tool production.