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If an ancient civilization existed of the same size as humans, we probably would have found evidence in fossils or from tools. Styracosaurus Rider (Speak to the Rider, your Lord and Master) {Look upon my servants} 19:19, November 13, 2011 (UTC)

1. You said yourself there could be thousands of undiscovered fossils.

"2. If their tools were primarily made of a substance such as wood, we might never discover their tools.

Pinguinus impennis 19:44, November 13, 2011 (UTC)
If the civilization was developed to such an extent as ours, there would have been evidence. Most fossils are known from limited ranges and limited timespans. A large civilization would be located over all sorts of locations that could be good for forming fossils, not by choice but by sheer need of space. Styracosaurus Rider (Speak to the Rider, your Lord and Master) {Look upon my servants} 20:30, November 13, 2011 (UTC)
If humanity disappeared tomorrow, all cars would become piles of rust within 75 years, bridges and skyscrapers within 100, and reinforced concrete would lose integrity after around 300 years as its supports corrode. Things cast in bronze or other durable metals can be expected to last, as well as certain types of plastic, but when we looks back into the past we see how much is lost from a civilisation's record in a short time. The signs we can expect to find are evidence of rises in the emission of industrial gases, which we could find in ice cores or oil wells. HolbenilordTalk 20:35, November 13, 2011 (UTC)