In Valmiki Ramayana 1-3 where the plan of the Epic is described, we come across a verse “Vaidehyaah ca visarjanam” (1-3-38) stating that Valmiki described in the Epic the exile of Sita and also how Rama ruled the country. This means Uttara kanda was composed by Valmiki only. In the next verse it is stated  he had written ‘anAgatam ca’ - what is going to come and ‘Uttare Kaavye’ that covers what is yet to happen at the time.  In the next sarga 1-4-2 we are once again told that Uttara Kanda was also written by Valmiki himself - ?a? ka??ani tatha uttaram, where Uttaram refers to the end piece, Uttara Kanda. Earlier, when Brahma met him, He granted that whatever had happened and whatever was yet to happen would be known to him (1-2-34). So there can be no doubt who wrote Uttara kanda.

But Valmiki had not released all at once is what we gather from the next verse onwards. While he was wondering who would render his composition, (1-4-3) Lava-Kusha touched his feet (1-4-4) and Valmiki decided to teach them the kavya. Now that he has chosen Lava- Kusha, the question comes which parts of his composition he taught them. Definitely not the futuristic one when Valmiki himself enters the scene before Sita exits the world. Definitely not from the beginning that we have today, but from the 5th sarga of Bala kanda “SarvA pUrvam” when the story starts… once upon a time... and ended with Pattabhishekam, what we have now in the Epic.

Ramayana as taught to Lava-Kusha ended with Pattabhishekam. Always we follow a tradition of ending the renditions on a happy note and a phala sruti. Pattabhiskheka sarga has the Phala sruti of what one gets by reciting Ramayana. That is why even after finishing Sundara kanda parayanam at home we recite Pattabhisheka sarga and offer sweets / payasam as neivedhyam. The Phala sruti we find in Pattabhisheka sarga contains futuristic elements of Rama’s rule.

Based on all these we can conclude that Uttara Ramayana was written by Valmiki only. He had framed Ramayana in groups – one for Lava-Kusha starting from the 5th sarga and ending with the 6th Kanda; another for us starting from the background of composition that we have now. And another i.e., Uttara Kanda which must have been made known to the world after Rama exited.

We find a similar pattern in Mahabharata too. Bharatam was made known to the world not until Dhritarashtra, Pandavas etc. (the characters of the Epic) exited the world (Mbh: 1-1-55 & 56) which means the 1st version of 24k verses was already made by Vyasa after the war was over but not released until Krishna and Pandavas left. He made the same story with differing number of verses meant for different people, but this 24K Bharatam was meant for us. Then he added 150 verses. Again he made 100,000 verses and made an entry in the Epic in Janamejaya’s sarpa yaaga – much like how Valmiki, the composer of Ramayana made an entry in Uttara Kanda. These two Epics follow a certain pattern – of delivering only that part of incidents that were over, and releasing the rest later. Uttara kanda must have been released after Rama’s exit. All incidents of Rama’s life were conceived by Valmiki by the blessings of Brahma (1-2).

When did Valmiki compose Ramayana?

Though no explicit mention of the time of writing the Epic is found in Ramayana, we can make a fair guess. From the boon of Brahma that Valmiki must write the Epic from whatever that was revealed and unrevealed or known to him or not yet known (1-2-34), it is known that he had already known something. Since the kids (Lava–Kusha) were around when he finished the Epic we can deduce that Sita was in the ashram during most time of his writing of the Epic. This means Valmiki was aware of Sita’s side of the story, her plight.

Did the first hand information of her story that he got from her leave him undecided about Rama’s stature? This question arises from the fact the Ramayana begins with Valmiki asking Narada who he thinks is a virtuous person. Doesn’t he know about Rama before? When Narada picks out Rama’s name and compares him with Vishnu, Valmiki must have been convinced about the divinity of the couple.

However, the plight of Sita was upper most in his mind is revealed from the ‘Ma Nishada” verse that flowed from his mouth on seeing the plight of the Krauncha bird that lost her lover to the arrow of the hunter. The grief of the female bird continued to haunt him till Brahma consoled him and ordained him to compose Ramayana. The name Ramayana was repeated quite a few times by Brahma in his conversation with Valmiki, but Valmiki considered Ramayana as “Seetaayah Caritam Mahat” (1-4-7).

His devotion to Sita is again demonstrated in his Tamil verse found in Purananuru (Sangam verse) wherein he gives primacy to Sri, and not to her counterpart in not letting down anyone who gave up desires. She didn’t let down Kaakaasura and Trijata – both who agonized her. All these make me think that Sita’s story was haunting him right from the time she reached his ashram.

Three turning points occurred after he was fully aware of Sita’s story – meeting with Narada, seeing the plight of Krauncha bird and getting blessed by Brahma. Writing the Epic started after that right in the presence of Sita, the glorious Sri or Lakshmi, the motivating factor who points out the lakshya, the aim of the Epic and of anyone who reads the Epic.

Valmiki and Agastya were contemporaries. Since Ramayana happened at the junction of the 1st and 2nd Tamil Sangam around 7000 years ago when sage Agastya migrated to the South to restore the literary tradition of the Tamil lands affected by sea floods, Valmiki’s contribution to the revived Sangam Assembly at Kavaatam sounds plausible. In fact Valmiki had composed Tamil grammar too that was studied till thousand years ago as per Naccinarkkiniyar’s commentary to Tolkappiyam.

[On Valmiki’s contribution to Tamil:

http://jayasreesaranathan.blogspot.com/2013/03/valmiki-of-ramayana-knew-tamil-spoken.html]

Bogar the Siddha also remembers Valmiki as “Vedanta Valmiki’ says that he didn’t attain Samadhi but had physical existence for more than 700 years. (“Bogar 7000” verses 5723 to 5725) Bogar also refers to the birth date of Valmiki in the star Anuradha in Kanya month (Purattasi).

Courtesy 

https://jayasreesaranathan.blogspot.com/2020/05/did-valmiki-compose-uttara-ramayana.html