Most people only know about the fact the famous sage Vasishta, guru of Rama, was one of the mind-born sons of Brahma. What's not as well-known, however, is the fact that Vasishta is no longer the son of Brahma! In fact if you look at the Anukramani for Rig Veda Book 7 in my answer here, you'll find Vasishta listed not as Vasishta son of Brahma, but rather "Vasishta Maitravaruni", i.e. Vasishta son of Mitra and Varuna. Let me explain.
Once the ancient king Nimi, ancestor of Janaka, wanted to conduct a Yagna, so he asked the sage Vasishta to be his priest. Vasishta told Nimi that he had already promised to conduct a Yagna for Indra, so he would conduct Nimi's Yagna afterward. But Nimi didn't want to wait, so he hired someone else to be his priest. When Vasishta found out about this, he was furious, and cursed Nimi to have his soul separated from his body. In response, Nimi uttered the same curse back to Vasishta, so Vasishta's body died and Vasishta's soul was left to wander. Finally, when the gods Mitra (god of friendship) and Varuna (the ocean god) produced two children out of attraction to the Apsara Urvashi, Vasishta was reborn as one of those children, the other being the famous sage Agastya. Here is how the Srimad Bhagavatam describes it:
After beginning sacrifices, Mahārāja Nimi, the son of Ikṣvāku, requested the great sage Vasiṣṭha to take the post of chief priest. At that time, Vasiṣṭha replied, “My dear Mahārāja Nimi, I have already accepted the same post in a sacrifice begun by Lord Indra. I shall return here after finishing the yajña for Indra. Kindly wait for me until then.” Mahārāja Nimi remained silent, and Vasiṣṭha began to perform the sacrifice for Lord Indra. Mahārāja Nimi, being a self-realized soul, considered that this life is flickering. Therefore, instead of waiting long for Vasiṣṭha, he began performing the sacrifice with other priests. After completing the sacrificial performance for King Indra, the spiritual master Vasiṣṭha returned and found that his disciple Mahārāja Nimi had disobeyed his instructions. Thus Vasiṣṭha cursed him, saying, “May the material body of Nimi, who considers himself learned, immediately fall.”
For unnecessarily cursing him when he had committed no offense, Mahārāja Nimi countercursed his spiritual master. “For the sake of getting contributions from the King of heaven,” he said, “you have lost your religious intelligence. Therefore I pronounce this curse: your body also will fall. After saying this, Mahārāja Nimi, who was expert in the science of spiritual knowledge, gave up his body. Vasiṣṭha, the great-grandfather, gave up his body also, but through the semen discharged by Mitra and Varuṇa when they saw Urvaśī, he was born again.
This earlier chapter of the Srimad Bhagavatam provides a bit more detail about the birth of Vasishta and Agastya:
Agastya and Vasiṣṭha Ṛṣis were the common sons of Varuṇa and Mitra, the tenth son of Aditi. Upon seeing Urvaśī, the celestial society girl, both Mitra and Varuṇa discharged semen, which they preserved in an earthen pot. The two sons Agastya and Vasiṣṭha later appeared from that pot, and they are therefore the common sons of Mitra and Varuṇa.
My question is, was this the main birth of the sage Agastya? Given that Vasishta was originally a sage born from Brahma and then later reborn as the son of Mitra and Varuna, is it possible that something similar happened in Agastya's case?
Do any scriptures describe a previous birth of Agastya?