One verse

Srimad Bhagavatam 11.17.10

11.17.10

आदौ कृतयुगे वर्णो नृणां हंस इति स्मृत: । कृतकृत्या: प्रजा जात्या तस्मात् कृतयुगं विदु: ॥ १० ॥

ādau kṛta-yuge varṇo nṛṇāṁ haṁsa iti smṛtaḥ kṛta-kṛtyāḥ prajā jātyā tasmāt kṛta-yugaṁ viduḥ

ādau
in the beginning (of the millennium)
kṛta-yuge
in the Satya-yuga, or age of truth
varṇaḥ
the social class
nṛṇām
of human beings
haṁsaḥ
named haṁsa
iti
thus
smṛtaḥ
well known
kṛta-kṛtyāḥ
perfect in the execution of duties by complete surrender to the Supreme Lord
prajāḥ
the citizens
jātyā
automatically by birth
tasmāt
therefore
kṛta-yugam
Kṛta-yuga, or the age in which all duties are fulfilled
viduḥ
was thus known by the learned

In the beginning, in Satya-yuga, there is only one social class, called haṁsa, to which all human beings belong. In that age all people are unalloyed devotees of the Lord from birth, and thus learned scholars call this first age Kṛta-yuga, or the age in which all religious duties are perfectly fulfilled.