In the तीर्थयात्रा पर्व within वनपर्व of श्रीमहाभारत, there is an encounter between भीम‌ & हनुमान. This is a glorious encounter where हनुमान shows His विराट रूप at the request of भीम‌. They also have a long conversation on various matters. One of the question put by भीम‌ to हनुमान is about चतुर्युग ‌as to how human conduct changes from युग to युग. This response of हनुमान is elaborated below. 

भीम उवाच

युगसंख्यां समाचक्ष्व आचारं च युगे युगे। धर्मकामार्थभावांश्च कर्मवीर्ये भवाभवौ।

Asks भीम – Tell me what is the duration of different युग and what are the manners and customs with respect to धर्म,‌काम‌ & अर्थ as well as how aspects like कर्म, वीर्य,‌ life and death are viewed across each of the युग. 

हनूमानुवाच

कृतं नाम युगं तात यत्रधर्मः सनातनः। कृतमेव न कर्तव्यं तस्मिन् काले युगोत्तमे॥

Says हनूमान – that युग‌‌ is called as कृत when only one धर्म is universally followed viz the सनातन धर्म. In that best of युग‌‌, everyone had an उत्तम conduct and there are no need to perform any special कर्तव्य (like यज्ञ).

न तत्रधर्माः सीदन्ति क्षीयन्ते न च‌ वै प्रजाः। ततः कृतयुगं नाम कालेन गुणतां गतम्।।

धर्म knew no decrease and it was for this reason that this युग‌‌ is called as कृत (viz perfect). In the course of time, it lost its गुण

देव दानव गंधर्व यक्ष राक्षस पन्न्गाः। नासन् कृतयुगे तात तदा न क्रय विक्रयः॥

In the कृत युग‌‌, there were no देव, दानव,‌ गंधर्व, यक्ष, नाग & राक्षस. There was also no acts of buying and selling in that युग. 

न सामऋग्यजुर्वर्णाः क्रिया नासीच्च मानवी। अभिध्याय फलं तत्रधर्म: सन्यास एव च॥

The साम,‌ ऋग्‌ & यजस् (three types of मंत्र) did not exist. There was no manual labour. The necessities of life were procured only by thinking about them. The only धर्म  praticed during this time was सन्यास धर्म. 

न तस्मिन् युगसंसर्गे व्याधयो न इन्द्रयक्षयः। न असूया नापि रुदितं न दर्पो नापि वैकृतम्।।

न विग्रहः कतस्तन्द्री न द्वेषो न च‌ पैशुनम्। न भयं नापि संतापो न च‌ ईर्श्या न च‌ मत्सरः।।

In that युग‌‌, there was neither disease nor decay of the senses. There was neither malice nor pride nor hypocrisy nor discord nor ill-will nor cunning-ness nor fear nor misery or envy nor covetousness..

ततः परमकं ब्रह्म सा गतिर्योगिनां परा। आत्मा च‌ सर्वभूतानां शुक्लो नारायणस्तदा।।

For this reason, ब्रह्म was attainable to all in that युग. नारायण as the आत्मा of all life forms was expressed by white colour.

ब्रह्मणा: क्षत्रिया वैश्याः‌ शूद्राश्च कृतलक्षणाः। कृते युगे समभवन् स्वकर्मनिरताः प्रजाः॥

In the कृत युग‌‌, the specific लक्षण of ब्रह्मण, क्षत्रिय, वैश्य & शूद्र were directed to आत्मज्ञान and each used to perform their स्वकर्म

समाश्रयं समाचारं समज्ञानं च केवलम् । तदा हि समकर्माणो वर्णाधर्मानवाप्नुवन्।।

ब्रह्म was the sole goal of all human beings and all manners, customs, knowledge and actions were directed towards attainment of ब्रह्म. And by performance of their own वर्णधर्म, all of them obtained equal merit. 

एकदेवसदायुक्ता एकमंत्रविधिक्रियाः। पृथग्धर्मास्त्वमेक वेदाधर्ममेक मनुव्रताः॥

One देव was always the focus for all and one मंत्र was the क्रिया that all connected with. And though each of the persons had distinct characteristics, all of them followed वेद and धर्म.

चातुराश्रमेययुक्तेम कर्मणा कालयोगिना। अकामफलसंयोगात् प्राप्नुवन्ति परां गतिम्।।

All of them aligned their life to four आश्रम (ब्रह्मचर्य, गृहस्थ, वानप्रस्थ & मोक्ष) and they lived all these modes without any focus on कर्मफल. And thus, since all of them performed निष्काम कर्म, all of them attained the highest गति‌ (or goal viz मोक्ष)

आत्मयोगसमायुक्तो धर्मोsयं कृतलक्षणः। कृते युगे चतुष्पादश्चातुर्वर्णस्य शाश्वतः।।

Key feature of  कृत युग‌‌ is that each and every person considered identification of आत्मा with ब्रह्म as the sole धर्म. Thus, all four वर्ण had eternal fourfold measure.

एतत् कृतयुगं नाम त्रैगुण्यपरिवर्जितम्। त्रेतामपि निबोधत्वं यस्मिन् खत्रं प्रवर्तते।।

Thwrefore, कृत युग‌‌ was free from influence of त्रिगुणम् (impacting the lives of people). Now hear from me all about त्रेतायुग in which यज्ञकर्म got introduced (in the world)

पादेन ह्रसतेधर्मो रक्ततां याति च अच्युतः। सत्यप्रवृत्ताश्च ‌नराः क्रिया़धर्मपरायणाः।

धर्म decreased by a quarter in this age and नारायण assumed red colour here. Human beings practiced adherence to सत्य in this age and devoted themselves to actions (in the form of यज्ञकर्म). 

ततो यज्ञाः प्रवर्तन्ते धर्माश्च विविधाः क्रियाः। त्रेतायां भावसंकल्पाः क्रियादानफलोपगाः॥

Thus, यज्ञकर्म were introduced and various other धर्म क्रिया were performed. In  त्रेतायुग, human beings began to devise various goals for themselves and they used to perform धर्म क्रिया and indulge in दान to attain their chosen goals.

प्रचलन्ति न वैधर्मात् तपोदानपरायणाः। स्वधर्मस्थाः क्रियावन्तो नराः त्रेतायुगेsभवन्।।

Human beings did not deviate from धर्म and always engaged in तप‌ & दान. Humans from all वर्ण were devoted to their स्वधर्म and they also performed धर्म क्रिया – such were people in त्रेतायुग.

द्वापरे च युगेधर्मो द्विभागेनः प्रवर्तते। विष्णुर्वै पीततां याति चतुर्धा वेद एव च।।

In the द्वापरयुग, धर्म decreased by half. विष्णु assumed yellow colour and वेद came to be divided into four divisions.

ततोsन्ये च चतुर्वेदाः त्रिवेदाश्च तथापरे। द्विवेदाश्चैक वेदाश्चाप्यनृचश्च तथापरे।।

Then, some learnt all the four वेद, some only three, some only two and some did not know even a single ऋक्. 

एवं शास्त्रेषु भिन्नेषु बहुधा नीयते क्रिया। तपोदानप्रवृत्ता च‌ राजसी भवति प्रजा।।

With शास्त्र having been duly divided and expanded, the number of धर्म क्रिया also multiplied. And while तप & दान were being indulged in by people, these were being done with the रजोगुण pervading their minds.

एकवेदस्य चाज्ञानाद् वेदास्ते बहवः कृताः। सत्त्वस्य चेह विभ्रंशात् सत्ये कश्चिदवस्थितः।।

As men did not have the capacity to study the entire वेद, it came to be divided in several parts. And as the intellect of human beings degraded, few were devoted to सत्य.  

सत्यात् प्रच्यवमानां व्याधयो बहवोsभवन्। कामाश्चोपद्रवाश्चैव तदा वै दैवकारिताः॥

And when human beings fall off from adherence to सत्य, they become exposed to diseases. काम takes over their persona and natural calamities fall on them. 

यैरद्यमानाः सुभृशं तपस्तप्यन्ति मानवाः। कामकामाः स्वर्गकामा यज्ञास्तन्वन्ति चापरे।।

Being affected by these, some then take up to indulging in तपकर्म. Others perform यज्ञकर्म to satisfy their worldly desires or to attain to स्वर्गलोक. 

एवं द्वापरमासाद्य प्रजाः क्षीयन्त्यधर्मतः । पादेनैकेन कौन्तेयधर्मः कलियुगे स्थितः।।

O कौन्तेय! Thus in the द्वापरयुग, human beings become degenerated on account of their adherence to अधर्म. In the कलियुगं, only one quarter of धर्म remains. 

तामसं युगमासाद्य कृष्णो भवति केशवः। वेदाचारा: प्रशाम्यन्ति धर्मयज्ञक्रियास्तथा।।

When this युग appears, विष्णु assumes a black colour. The वेद, धर्म, यज्ञ and other धर्म क्रिया all fall into disuse..

ईतयो व्याधयस्तन्द्री‌ दोषाः क्रोधादयस्तथा। उपद्रवाः ‌प्रवर्तन्ते आधयः क्षुद्भयं तथा।।

Then excessive rains, drought, rats, locusts, birds, diseases, fatigue or tired-ness, anger, deformities, natural calamities, anguish & fear of famine take possession of the world.

युगेष्वावर्तमानेषुधर्मो व्यावर्तते पुनः। धर्मे व्यावर्तमाने तु लोको व्यावर्तते पुनः।।

लोके क्षीणे क्षयं यान्ति भावा लोकप्रवर्तकाः। युगक्षयकृताधर्मा़ः प्रार्थनानि विकुर्वते।।

एतत् कलियुगं नाम अचिराद् यत् प्रवर्तते। युगानुवर्तनं त्वेतत् कुर्वन्ति चिरजीविनः॥

As this युग‌ passes on, धर्म keeps weakening on a daily basis. As धर्म becomes weak, all creatures degenerate. And as the creatures degenerate, their character also degerates. The यज्ञ क्रिया performed at this state of युग often produces contrary outcomes. Even those who live for several युग‌ must also align themselves to these changes (last line applies only to a few चिरंजीव like हनुमान, विभीषण, etc)

Source:

Translation of Mahabharata by MN Dutt