Which Mahabharata verse does Swami Vivekananda refer to show earliest humans were Brahmins and they denigrated to lower varnas as time progressed?

Question

In the Kumbakonam lecture of 1897, Swami Vivekananda said the following:

We read in the Mahâbhârata that the whole world was in the beginning peopled with Brahmins, and that as they began to degenerate, they became divided into different castes, and that when the cycle turns round, they will all go back to that Brahminical origin. This cycle is turning round now, and I draw your attention to this fact. Therefore our solution of the caste question is not degrading those who are already high up, is not running amuck through food and drink, is not jumping out of our own limits in order to have more enjoyment, but it comes by every one of us, fulfilling the dictates of our Vedantic religion, by our attaining spirituality, and by our becoming the ideal Brahmin. There is a law laid on each one of you in this land by your ancestors, whether you are Aryans or non-Aryans, Rishis or Brahmins, or the very lowest outcasts. The command is the same to you all, that you must make progress without stopping, and that from the highest man to the lowest Pariah, every one in this country has to try and become the ideal Brahmin. This Vedantic idea is applicable not only here but over the whole world. Such is our ideal of caste as meant for raising all humanity slowly and gently towards the realisation of that great ideal of the spiritual man who is non-resisting, calm, steady, worshipful, pure, and meditative. In that ideal there is God.

Which Mahabharata verse does Swamiji refer to here?

Answer 1

Sage Bhrigu is quoted as saying it.

The sage Bhrigu said, ‘There is really no distinction between the different orders. The whole world at first consisted of Brahmanas. Created (equal) by Brahman, men have, in consequence of their acts, become distributed into different orders. They that became fond of indulging in desire and enjoying pleasures, possessed of the attributes of severity and wrath, endued with courage, and unmindful of the duties of piety and worship, - these Brahmanas possessing the attributes of passion, - became Kshatriyas. Those Brahmanas again who, without attending to the duties laid down for them, became possessed of both the attributes of Goodness and Passion, and took to the professions of cattle-rearing and agriculture, became Vaisyas. Those Brahmanas again that became fond of untruth and injuring other creatures, possessed of cupidity, - engaged in all kinds of acts for a living, and fallen away from the purity of behaviour, and thus wedded to the attributes of Darkness, became Sudras. Separated by these occupations, Brahmanas, falling away from their order, became members of the other three orders. All the four orders, therefore, have always the right to the performance of all pious duties and of sacrifices. Even thus were the four orders at first created equal by Brahman who ordained for all of them (the observances disclosed in) the words of Brahma (in the Vedas). Through cupidity alone, many fell away, and became possessed by ignorance. The Brahmanas are always devoted to the scripture on Brahma; and mindful of vows and restraints, are capable of grasping the conception of Brahma. Their penances therefore never go for nothing. They amongst them are not Brahmanas that are incapable of understanding that every created thing is Supreme Brahma.

(Mahabharata Santi Parva Section CLXXXVIII )

Answer 2

Vivekananda on Mahabharata

Stories of Karna and Rama has been told since ages by brahmins.My mother also used to tell me the tales of these heroes and how their brothers respected and loved them despite so much tragedy in their lives. Mahabharata and Ramayana have been composed much after the happening of events. Rama has more followers than any other person in India

-Swami Vivekananda