I was thinking something similar, in reading a couple of books on the amazing parallels between ancient Greek and Indian philosophy. The two cultures had little interaction except briefly through the Persians. One possible commonality would be the use of psychedelics and the impact on the human brain.

You wouldn't have to go as far as realism about the effects of psychedelics. The commonality might just be the effect of the drug on the brain structures which are of course the same in all societies. No historical contact would be needed if the drugs were similar. McEvilley doesn't seem to consider this possibility in "The Shape of Ancient Thought."

The question is which societies used which drugs at which times.