For one thing, they learned that the Spiders had no knowledge of any previous civilization:

Also have him scan as best he can from his position for any previous civilizations on this planet.

They sought for, and found, artifacts of previous spacegoing civilizations.

All previous civilisations have died through the insufficiency of their underlying principles.

The Asarions don't really know much about the previous civilization, save for a few sites such as this one.

The three children find a cave containing relics of a previous civilisation.

In folklore from all over Europe, giants were believed to have built the remains of previous civilizations.

Above them is another ring, representing the gifts of previous civilizations.

I think that if we looked hard at all the sites we'd find evidence of previous civilizations.

Therefore it is likely that previous civilizations faced fatal problems with first contact.