The term dharma is derived from the root dhri which mean to sustain, uphold, hold together. Its predecessor seems to be the early Vedic concept of rita. Rita is cosmic order and with its opposite anrita ‘expresses also moral order’, it forbids and also commands positive action wherein it assumes the form of something more than mere truth, satya.
Rita is a cosmic force and to its laws even the gods are subject. Rita and yajnya are the twin concepts which run like warp and woof through the whole fabric of early Hindu social and moral thought. Rita means as much the physical order of the universe, as the due order of sacrifice and the moral law in the world.
The concept of Rita did not survive for a long period of time.
Rita did not entirely disappear for it was transformed the concept of dharma which held such decisive sway over Hindu thinking through the ages.
Alongside of Rita the idea of dharma also evolves in the Vedic literature. Dharma then comes to mean custom, moral laws, laws or duties in general, what is right etc. In the Brahmanas the concept of dharma largely supersedes the old concept of Rita and for obvious reasons. As the complexity of social organization increased the old concept of Rita was found to be inadequate whereas the concept of dharma was able to answer practically all the needs of the evolving situation. Dharma, then, gradually emerges as a code of duties towards gods, sages, manes, men and the lower orders of creation.
Source – Indian Thought Through The Ages by B. G. Gokhale