Foremost among the great mysteries is whether or not there are other Gaias out there.

The Italian physicist Enrico Fermi, in pondering the question, left us a paradox. It involves the simple question of why, despite the antiquity of the heavens and the vast number of stars and planets we know exist, have we not yet detected intelligent life?

There are 250 billion stars in the Milky Way alone, so surely some of those should have spawned Earth-like planets, and some of those should have developed life. Ferni assumed that it's a characteristic of life to colonize suitable habitats, its spread thereby making it more likely to be visible to us.

If a civilization used even the slow kind of interstellar travel almost within our grasp today, it would have taken only five million to fifty million years to colonize our galaxy. And that is just the blink of an eye in the fourteen-billion-year history of our universe. Fermi’s paradox would be resolved if infantile Gaias rarely, if ever, survive. If this is the explanation, then perhaps the Medea hypothesis is correct after all: intelligent global super-organisms may carry within themselves the seeds of their own destruction, and so begin to extinguish themselves at the moment of birth.

But there is another possibility. Perhaps Fermi’s paradox tells us that we really are alone in the universe, simply because we are the first global super-organism to ever exist. After all, it’s taken all of time – from the Big Bang to the present – to make the stardust that forms all life, and to forge that stardust, through evolution by natural selection, into us and our living planet. If we really are the first intelligent super organism, then perhaps we are destined to populate all of existence, and in so doing fulfill Alfred Russel Wallace’s vision of perfecting the human spirit in the vastness of the universe. If we ever achieve that, then Gaia will have reached puberty, for she will have then become reproductive, nurturing the spark of life on one dead sphere after another. From our present vantage point we cannot know such things. But I am certain of one thing: if we do not strive to love one another, and to love our planet as much as we love ourselves, then no further human progress is possible here on Earth.

Tim Flannery is the Chair of Environmental Sustainability at Macquarie University. He was named Australian of the Year in 2007. This excerpt is from Here on Earth: A Natural History of the Planet (HarperCollins, 2011).