If civilizations exist around other stars they are likely to be just emerging across our Galaxy right now: like an apple orchard suddenly maturing and ripening in the autumn sun.

Carbon - the fundamental building block of life as we know it - may not have been widely available until the universe was about half its present age (made in stellar interiors).

Once a planet has formed with the right amount of carbon, it may take a long time (billions of years) for advanced life to develop (approximately 4 billion years on Earth). For example, it takes a long time (comparable to the age of the central star) for oxygen/ozone production through photodissociation of water vapor to reach levels sufficient to permit emergence of life on land.

Suggests that only now is the Galaxy ripe for the development of advanced life.