Though I'll be pointing out some errors in this comment, my intention is to be critical of the ideas you've presented, not of you personally.
Your literal interpretation of the sacred texts of Sanatana Dharma has missed the point. If we judge things as "improving" or "degrading" we make a judgement. When we judge something to be good or bad we reinforce maya.
When I read through these texts I see allegory employed in order to teach us, not a literal history. It is easier for us to learn from allegorical stories than by reading pure theory. Also, the ineffable is by definition impossible to talk about, this is why so many sacred texts employ metaphor and other symbolism.
Besides that, your description of evolution, though common, is not quite right. First, monkeys and human share a common ancestor; neither of us came from the other. You didn't come from your cousin. You and your cousin both came from your grandparents.
Also, evolutionary theory is the result of the work of thousands of scientists, not one man. After On the Origin of Species, researchers quickly began testing his hypothesis. From their work we know that, for example, a giraffe doesn't lengthen it's neck over its lifetime—evolution works on the scale of entire populations.
Punctuated equilibrium is the pattern we see in both the fossil record and in genetics. These realization refined Darwin's ideas. In science, we throw out what is incorrect and tentatively hold onto what seems most correct. This is how his hypothesis has become a theory even though several of his ideas were wrong.
Furthermore, "cavemen" weren't "dumb." They are biologically indistinguishable from us. That means, they had the exact same capacity for knowledge we do. The difference is that we stand on the shoulders of our ancestors. We are able to understand more because our predecessors figured out so much before us. With proper teaching a "caveman" could understand nuclear physics.
For neanderthals, the science isn't as settled, but we do know they were at least as mentally capable as a 5-year-old human. We know this because bonobos have such a mental capacity and we are much more distantly related to them. We also know that Neanderthals buried their dead with garlands of flowers which indicates some social cohesion and perhaps a conception of "afterlife."
We know the Indus Valley Civilization goes back to a maximum extent of about 5500 years ago, not millions. Of course, the culture which became the IVC goes back further, but there weren't settled cities yet. There also wasn't writing in South Asia until this period, so the Vedas cannot be more than 5,500 years old. Liberal estimates put them at 4,000 years old at most.
These facts do not indicate that the sacred texts are wrong in their messages. It's just that they needn't be taken as literal historical accounts.
🙏नमस्ते 🙇