I think I would perhaps be correct in claiming India has the largest body of oral knowledge created, preserved and transmitted than any thing else in the world. Jainism, buddhism,atheism owe their origins to vedic speculations being preserved and their debating /speculating culture. Philosophical speculation is just not possible without memory being preserved in some way, it just wasnt true for anyone else except for vedic people. So, i disagree on this issue. As for Indus valley, there seem to be aspects of goddess worship of some kind and associated human sacrifice. Many cults of human sacrifice continued with Goddess worship, kali for example.
Brihadaranyaka upanishad, for example is quoted by both carvakas and Buddha himself. It is probably the most ancient upanishad, it asks the interesting question as to what survives after death, speculation is from, atman, to karma, idea of all things going back to their source, earth element to earth, water element to water, it compares the answer to what survives after a plant is uprooted, so if one qualifies question with God, what survives after death then one ends up with most of the spectrum of beliefs that come from India.
atheism- no God,no cosmic karma,no atman
ajivika- No God, no karmic force, no atman, naturalistic force, fate
sankhya- (ancient sankhya is unknown now, latest sankhya has similar model to david chalmers(they really deserve credit, this theory has been brought back independently by david chalmers), ofcourse there are differences, but one can bracket sankhya with david chalmers somewhat. consciouesness, matter being origin for everything else including intelligence.
buddhism- no God, karma cosmic force, no atman
jainism- no God, karma cosmic force, atman yes
advaita- No God, karma cosmic force,atman yes
other hindu schools have God, karma,atman etc combination of things.
There are other reasons to believe this, upanishads record sense objects, sense organs and knowing through them, this is reflective of later buddhism, One of Buddha teachers was a person who taught him an ancient form of sankhya. Buddha himself is to have spoken of different groups of vedic teachers to be authentic than others. In absense of memory preserving culture I do not know how can any of these ideas could have been speculated , recorded and preserved for generations growing into a community. This isnt possible, it might be a case that the people were from non vedic social back grounds were taking parts of vedic,Upanishads principles bit by bit as they saw fit and then have own schools but the ideas do seem to come only from vedic system of knowledge. Again, in order to have any worthwhile philosophical speculation of any kind one must first possess an ability to remember and pass it along for generations. This ability was possessed by vedic system