I came across a good deal of information about the Charvakas in Naturalistic Tradition in Indian Thought by Dale Maurice Reipe. (Overheard about it at Vidya's blog).
Interesting point is the comparison between Indian and Greek traditions. All I can say is, the connection can be found in the spread of Y-Haplogroup J2a.
I suppose the Atharva Veda, the fourth Veda predates Charvakas (around 500 BC?). However, only three Vedas are mentioned in Charvaka rebuttals. I suppose those are Rig, Yajur and Sama.