Nothing is static in life. Changes take place to evolve into something new and that's what we call Evolution.
Scientists have postulated many hypothetical theories as well as practical evidence to prove that species keep on changing or evolving to adapt to their environment. Since nature is never stagnant, the species that live in it also have to keep changing to survive and reproduce.
The first theory of human evolution was postulated by Charles Darwin in his book "Origin of Species" in which he stated that today's modern man is possibly the descendant of the apes. The last common ancestor of the human is said to be the 'Gibbon' from which it diverted through a number of morphological, physiological and behavioral changes.
Some of the most significant anatomical changes that took place were- bipedal-ism, increase in the size of the brain, lengthy gestation and infancy and decreased sexual dimorphism.
Bipedal-ism was favored in order to make the hands free for reaching out for food and it also saved energy while walking. This adaptation enabled the early human to walk, run and hunt for long distance and time that also increased their duration of survival. The increase in the size of the brain led to many improvements in human behavior like- social improvement and language learning.
Sexual dimorphism brought about many changes in the male stature that resulted in pair bonding which was essential for parental care during the term of long infancy.
The modern human being, biologically known as 'Homo Sapiens' had its birth in Africa, where the human fossils were found. They migrated as apes from the tropical forest to Savannah forest that had more open space, shrubs and grass where they started walking on two legs and used their hands to carry things and food. Today's modern homo sapiens is the only species that came through a number of intermediate stages that became extinct very soon.