Yugas back before Darwin, the Indian scientific heritage had already explained the human evolution theory. Lord Vishnu has taken ten avatars (Dashavatar) to save and clean this earth, spanning across the yugas. The first of those is Matsyavatar (fish) indicates the life started from underwater, which is also admitted by the so-called scientists. ‘Kurmavatar’ (tortoise), the second avatar, depicts moving of life from underwater to earth surface. Varahavatar (Wild Boar), the third one, indicates the life has moved completely out of water and adapted to land. The fourth incarnation, Narasimhaavatar (Half man-Half animal), indicates transformation from animal to human form. Fifth incarnation of Vamanaavatar (Dwarf) indicated pure human form and the development of intelligence. The sixth incarnation, Parasuram, (Forest dweller with axe as his weapon), developed weapons with axe as the first one, indicating the first settlement of humans in forests. The next is Ramavatar, a complete transformation into a man living in a society, ruling kingdoms and people, with Dharma in place; most civilized culture of life with communities and houses for villagers and advanced weaponry were developed during this avatar. The eighth one, Balarama, often referred to as Buddhavatar according to some sources (Plough in hand), formed base for agriculture and agrarian economy. And, the nineth one is Krishnavatar, which showed domestication of animals and developing economies based on cattle, etc., while also teaching all aspects of human life, from birth to death. The tenth avatar, yet to come, is Kalki, which indicates the mass destruction of humans as a punishment to deeds done on earth.
Each of these avatars saved earth from demons, created peace and resulted in formation of yugas one after the other, finally leading to Kaliyuga, the one we are in now.