The earliest list of the Seven Rishis is given by Jaiminiya Brahmana 2.218 - 221:-
6. Visvamitra, and
followed by Brihadaranyaka Upanishad 2.2.6 with a slightly different list:
1. Gautama and
3. Viśvāmitra and
5. Vashiṣṭha and
6. Kaśyapa, and
the late Gopatha Brāhmana 1.2.8 has
8. Vrighu and Kaśyapa.
In other texts, different lists appear; some of these rishis were recognized as the 'mind born sons' (Sanskrit: manasa putra) of Brahma, the representation of the Supreme Being as Creator. Other representations are Mahesha or Shiva as the laya karaka and Vishnu as the sthiti karaka. Since these seven rishis were also among the primary eight rishis, who were considered to be the ancestors of the Gotras of Brahmins.
Sapta Rishis are the Hierarchy working under the guidance of the Highest Creative Intelligence, Parmatma. The Saptarishis keep changing for every Manvantara. They bring down to the earth the required Knowledge and Energies to strengthen the processes of Transition (Pralaya). They are naturally the most evolved Light Beings in the Creation and the guardians of the Divine Laws.
The reason for the differences in the mentioned list is that there are seven Rishis who govern the functioning of the Cosmos in batches and all the above-mentioned Rishis have had the post of a 'Saptarishi' at some point. These Rishis had different names in different places. Saptarishis are seven Rishis and nobody else is given the title of the saptarishis. Hence, clearly they have different names. But, many rishis were known as maharishis (great sages.)
As per our sashtras there are four yugas: Krita Yuga, Treta Yuga, Dvapara Yuga, and Kali Yuga. We are presently in the Kali yuga, lifespan being 432,000 years [we are in 5105 year now in 2014]; Dvapara Yuga is twice Kali Yuga, Treta Yuga is thrice Kali yuga and Krita Yuga is four times Kali yuga. Over all, 4,320,000 years termed as 1 Maha yuga. 1000 Maha yugas make the day of 12 hours for Brahma (Creator) and during another 12 hours, Brahma takes rest and there is no creation during this period. Thus 1 day for Brahma constitutes 1000 Maha yugas 4,320,000,000 years). Thus 1 year constitutes 360 x 4,320,000,000 = 1,555,200,000,000 years;
lifespan of Brahma is 100 years = 100 x 1,555,200,000,000 = 155,520,000,000,000 years.
Second Manvantara — the interval of Swarochisha Manu
Urja, Stambha, Prańa, nanda , Rishabha, Nischara, and Arvarívat.
Third Manvantara — the interval of Auttami Manu
Sons of Vashista: Kaukundihi, Kurundi, Dalaya, Śankha, Praváhita, Mita, and Sammita.
Fourth Manvantara — the interval of Támasa Manu
Jyotirdhama, Prithu, Kavya, Chaitra, Agni, Vanaka, and Pivara.
Fifth Manvantara — the interval of Raivata Manu
Hirannyaroma, Vedasrí, Urddhabahu, Vedabahu, Sudhaman, Parjanya, and Mahámuni.
Sixth Manvantara - the interval of Chakshusha Manu
Sumedhas, Virajas, Havishmat, Uttama, Madhu, Abhináman, and Sahishnnu.
The present, seventh Manvantara — the interval of Vaivasvata Manu
Kashyapa, Atri, Vashista, Vishvamitra, Gautama, Jamadagni and Bharadvaja
1. The Shatapatha Brahmana and Brihadaranyaka Upanishad (2.2.4) acknowledge the names of sevenrishis (or Saptarshis) as:
2. Krishna Yajurveda in the Sandhya-Vandana Mantras has it as:
3. Mahabharata gives the Seven Rishis’ names:
4. Brihat Samhita gives the Seven Rishis’ names as: