How was sandhyavandanam performed before the age of Vishwamitra?
Since the two most important parts of sandhyavandanam - arghyam and Gayatri japam - involve the Gayatri (savitri) mantra, I was curious to know what our ancestors did before that.
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In every age the Gayatri mantra is heard by a sage named Vishwamitra.. In this section of Ramanujacharya's Sri Bhashya, his commentary on the Brahma Sutras, Ramanujacharya explains how sages with the same names hear the same mantras after undergoing Tapasya:
As words such as Indra and Vasishtha, which denote gods and Rishis, denote (not individuals only, but) classes, and as the creation of those beings is preceded by their being suggested to the creative mind through those words; for this reason the eternity of the Veda admits of being reconciled with what scripture says about the mantras and kândas (sections) of the sacred text having 'makers' and about Rishis seeing the hymns; cp. such passages as 'He chooses the makers of mantras'; 'Reverence to the Rishis who are the makers of mantras'; 'That is Agni; this is a hymn of Visvâmitra.' For by means of these very texts Pragâpati presents to his own mind the characteristics and powers of the different Rishis who make the different sections, hymns, and mantras, thereupon creates them endowed with those characteristics and powers, and appoints them to remember the very same sections, hymns, &c. The Rishis being thus gifted by Pragâpati with the requisite powers, undergo suitable preparatory austerities and finally see the mantras, and so on, proclaimed by the Vasishthas and other Rishis of former ages of the world, perfect in all their sounds and accents, without having learned them from the recitation of a teacher. There is thus no conflict between the eternity of the Veda and the fact that the Rishis are the makers of its sections, hymns, and so on.
The idea is the Brahma uses the Vedas as a blueprint for the creation of the world. In particular Brahma, knowing that certain mantras have certain sages' names associated with them, proceeds to create sages with those very names.
By the way, the fact that the Vedas describe things that are common to every age is the reason why the Vedas seem to refer to specific stories despite being eternal; see my answer here.
Sandhyavandanam is relatively recent as compared to Agnihotra. Earlier it was Agnihotra that was the daily morning and evening ritual. But when exchange of cultures started happening during interaction with other countries and races, sun worship came into play, most probably due to the Maga priest influence from Persia. Savitri mantra was just a mantra used during Upanayanam, and people used it in different chhandas, most prevalent among them being Gayatri and Anushtup. But during Upanishad period, Gayatri version became more popular. In Sandhyavandanam, the focus was on offering water and praising the sun, using upasthana mantras, Gayatri was just a part of it. But after the Tantra age, some sects incorporated some famous Vedic mantras and built Tantric rituals around them and even invented a goddess Gayatri. They made it so that everything except Gayatri was useless. So now we have elaborate Sandhya rituals which the majority can't follow.
In Sandhya vandhanam many slokas are taken from Lord Krishna's period .
Prathar narmadayai namo nisi
Namosthu narmadhe thubhyam trahi maam visha sarpatha
Apa sarpa sarpa bhadram the Dooram gacha maha yasa
Janamejayasya yagnanthe aasthika vachanam smaran
Jarath karer Jarath karvam samuthpanno maha yasa
Aasthika Sathya santho maam pannagebhyo abhi rakshathu.
The above shloka is cited during the Sandhyavandanam ritual as a part of Harihara Vandanam.
So, I think Sandhya vandhanam is just after Dvaapara yuga.