The Vedas were the way in which extreme consciousness developed in a time when very little was going on in terms of the human brain. Language had barely begun. Though it had indeed been around for a few hundred thousand years, it was an indicative rather then a communicative language.
As they sat in real silence, yogis began to understand the energy pathways that exist within the body parallel to those just outside the body. From this comprehension of how the body aligns the inner world and outer world, yogis began to sense how to stand, how to move, how to hold their hands, how to utter word-like sounds with no meaning attached. In those initial stages, mantra was merely a sound that caused an effect. Meaning was added later.
The yogis began to develop rapidly as they remembered this science, and they had to remember it all so they could tell each other, and then test each other, and keep working. Called the guru-chala relationship, it was bidirectional: a student couldn't learn without giving to the teacher, a teacher couldn't receive without giving to the student, and it thus became a continuum.