Intelligent Life Billions of Years Older than Humanity

Fascinating!

"...Earth-like planets that orbit in a continuously habitable zone are essentially found only around [metal-rich G-type stars which first formed about 10 billion years ago]...Assuming that the development time for human-type intelligence is about 4.6 billion years (as on Earth), then the first intelligent societies could have appeared about 5 billion years ago. We thus have to face the possibility that there are intelligent societies billions of years older than ours."

-- p. 207 of Ulmschneider, P. 2003. 'Intelligent Life in the Universe'. Springer Physics and Astronomy, Berlin.