This section is from the book "Some Contributions Of South India To Indian Culture", by S. Krishnaswami Aiyangar. Also available from Amazon: Some Contributions of South India to Indian Culture.
The Acharyas of the Vaishnavas - Nathmtjni.
Nathamuni was the fifth ancestor of Rama-nuja and from the known age of Ramanuja calculating backwards, Nathamuni ought to have flourished in the first half of the tenth century A.D. According to the traditional account of Nathamuni's life, he lived in a village called Viranarayanapuram, and was following the life of an ordinary Vaishnava of those days. He heard a certain number of Vaishnavas in the course of their pilgrimage recite, in the temple'of his village, a particular verse from the works of Nam-Alvar.
The verse appealed to Nathamuni as embodying in the happiest phraseology, the sentiments of devotion, which must have infused the author. Making enquiries as to whose work it was, they were only able to recite to him the particular ten of which the verse under reference was one; but beyond that they were not able to recite any more of Nam-Alvar's works. As these fen related to God enshrined in the temple at Kumbha-konam, Nathamuni was referred to that locality. He proceeded there and found the people knew no more than the ten. Therefore he proceeded to the birthplace of Nam-Alvar, in the hope that perchance he might recover the whole there from somebody; but it had been so far forgotten that one among the Vaishnava inhabitants of the locality who bore the name Parankusadasa (Parankusa being a name of Madhurakavi) stated that they had heard of the name Tiruvay-moli, but knew nothing more of it. He could however recite the eleven stanzas ascribed to Madhurakavi in praise of Nam-Alvar. Nathamuni received these eleven stanzas ascribed to Madhurakavi, and is said to have recited these repeatedly till both Nam-Alvar and Madhurakavi appeared to him and gave him not merely the Tiruvaymoli, but also various others of the works, Sanskrit and Tamil, that went to constitute the basal authorities of Vaishnava teaching. The story goes on to say in so many words that Nathamuni had to remain practising single-minded contemplation upon Nam-Alvar (yogic practice) for full 340 years before the two Alvars appeared to him in the manner indicated above. This is only the hagiologist's way of stating something like a gap of 340 years, or roughly three and a half centuries between Nathamuni, it may be, and Nam-Alvar, or at the very best Tirumangai Alvar. The works were apparently in existence according to this tradition in the age of Tirumangai Alvar, and the traditional teaching of these had been broken since his time. Counting five generations backwards from Ramanuja and taking three centuries more for this interval, gives us the approximate age of Tirumangai Alvar or, at any rate, the latter end of the age of the Alvars. Other lines of investigation confirm this chronology more or less. What we are concerned with here is that the work of Nathamuni, the first Acharya consisted in the revival of the teachings of the Alvars and the organisation of that teaching by providing for its unfailing continuance. This last object was attained by the creation of a pontificate not of an official character like the Christian pontificate, but of a more or less popular character.
One other feature of Nathamuni's work that deserves attention is the setting of the tune and the prescribing of the form of recital of the works of Nam-Alvar. This arrangement would necessitate a class of people whose contribution practically it would be, to recite the work in the truly orthodox style, and even accompany the recitation with action of a suitable character. A class of people known by the name Araiyar at Srlrangam recite even to-day and perform in this style on the occasions of festivals in the temple. The adoption of the tunes and the singing was not confined to this class of men alone. A class of women whose profession had come to be music and dancing also adopted this as part of the temple order which continues, in certain places at any rate, up to the present time. All this seems to have formed part of the arrangement by which Nathamuni first made the collection of Nam-Alvar's works. And this was confirmed by the final arrange-ment made by Ramanuja who collected not merely the works of Nam-Alvar but even went forward to collect the works of the other Alvars and arrange them in the form in which the Prabandham 4000 is accessible to us at present. A similar arrangement, it was already pointed out, was made in regard to the works of the Saiva Adiyars by Nambi Andar Nambi under the patronage of a Chola ruler who is called Raja Raja Abhaya Kula-sekhara, in all probability Raja Raja the Great. It was on that occasion that the Saiva works also got set to tune and there is a reference under the later Cholas to a class of dancing women, who rendered these poems in the esoteric model (ahamargam). It may be pointed out again that it is not likely that either Nathamuni or Nambi Andar Nambi originated this mode. We have stated already that even from the classical age, there was a class of people who set devotional poems to tune, and that this arrangement is exemplified in the early classic Paripadal. All that this means is that the two classes of works got set to music for purposes of devotional recital by these two teachers res-pectively.
Though the Vaishnavas count the succession of Gurus in the line of descent from preceptor to disciple, the more prominently recognised apostolic succession of Vaishnavism passed from Nathamuni to his grandson Yamunacharya, whose sacerdotal name is Alvandar. He received the teachings of Nathamuni from two of his disciples, who may be taken to be in their turn preceptor and disciple.