Before there were gods, there were...more gods?

I'm a little confused by this article in the New York Times today about a discovery of a deity that predates Zeus worshiped by the Greeks before they were Greeks. (I guess Greek-speaking people came later from the Balkans. At least that's what the article says.) I mean first I'm confused about how they look at these shards of pottery without any markings on them and determine that they were vessels in which offerings to gods were left:



Maybe it was just a good place to store meat. But more confusing is that I thought there was already established gods before Zeus. Much of Delphi was built over a temple to Gaia (otherwise known as Mother Earth). There's a cave there, which is off limits to visitors due to danger of collapsing, that was believed to be the birthplace of the deity; a crack that goes to the center of the earth and from which she crawled onto the surface and created life.

Maybe these new gods are supposed to pre-date her too, but Zeus is more widely known, so they used him? Either way, they should really explain it better.