Indian Philosophy: Pre-Upanishidic Era

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It is well known that the Upanishads are the fountainhead of Indian philosophy: every orthodox and heterodox system is rooted in the Upanishads. Karma, absolutism, theory of momentariness of worldly things, cause of birth and death cycles all come from the Upanishads so do concepts like Atman, Brahman, and Maya. But what about the pre-Upanishadic era? Can we find any evidence of philosophical thought in the Brahmanas or Mantras? Or are they, as popular history books write, a collection of “ritual hymns and liturgical directives?”
Most books concentrate on the sacrificial angle, less on philosophy, and proceed with the statement that the Upanishads were a reaction to this Athirathram type rituals. Even some books on philosophy declare that there is not much of it in this period. But in fact there is a gradual evolution of thought from the Mantras to the Aranyakas.
While analyzing this evolution, it is common to use Western terminologies like monotheism, polytheism, monism, and anthropomorphic polytheism. But when you read Indian philosophy, you have to discard those terms. When scholars say the philosophy was monotheistic, there is enough evidence to indicate otherwise. If you argue that the Vedic texts were polytheistic, then you are discarding the portions which talk about the Supreme Soul and Guardian of the cosmos. A simpler way is to understand Indian philosophy as the work of spiritual people who wrote about their mystic experience in prose and poetry.

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Most ancient religions originated with people wondering about the power of nature and creating rituals to appease those powers. Eventually those powers were anthropomorphized and worshipped. Like in Egyptian, Roman and Greek civilizations, you see similar evolution in Vedic religion as well. So philosophically did the pre-Upanishidic era have only such naive cause-and-effect model of the world and nothing more complicated?
As you move beyond this cause-and-effect model, you will see that Vedic gods were not just gods of natural forces, but they also maintained moral order. Even though the sacrifice was important — the purusha sukta attributes creation to sacrifice — moral values were important as well. Thus Varuna is not just the god of sky and water, but also the one who fixed the inviolable laws of the physical universe. Vedic gods upheld moral law and were hostile to those who violated them, thus forcing people to choose righteousness.
Another change in the period is a move towards abstract concepts or symbolism. Thus we see the clay pot and the sacrificial post being worshipped as well as Agni, the offspring of water, being represented by a lotus leaf. Gods are collectively referred to as Visva devas. Visva karman was originally meant to represent Sun and Indra, but in turn becomes a god by himself: a logical abstraction becomes a god. Along with this, there is a change in the spirit of sacrifice – it becomes a way to compel the gods to provide what the person offering the sacrifice wants.
In some verses, Varuna is praised as the supreme one and in others Indra. But soon, these efforts of hoisting one god over the other is abandoned and there is an attempt at merging/unifying disconnected gods. As various divine beings — Sun, Fire, Dawn — overlap and merge and it becomes difficult to figure out which deity is being praised in certain verses. The divinity that is common among gods is given prominence: a declaration is made that Agni, Yama, and Mātariśvan are all one, which comes from the realization that there is an ultimate cause behind all diversity.
The climax of this search results in the ‘Song of Creation’. There are no anthropomorphic gods or mythological stories. The creation gets as abstract as it can with an impersonal creation and the declaration of ‘Tad Ekam’ (That One).
Next: The Upanishads (But not immediately)
References:

1. M. Hiriyanna, Outlines of Indian Philosophy (Motilal Banarsidass Pub, 2000)
2. Chandradhar Sharma,A Critical Survey of Indian Philosophy (Motilal, 2000)