Stephen Jay Gould was fond of emphasising the contingent nature of life - and in particular the emergence of humans.
The earth is 4.5 billion years old, and it had been estimated that we are at about the halfway point in the planet's existence.
As we saw previously, life emerged about 3.5 billion years ago, about as soon as it theoretically could. However, it was not until around 540mya that complex multicellular animal life (as we know it) appeared.
A new paper by a University of Sussex astronomer, Dr Robert Smith, actually calculates that the Earth has another 7.6 billion years to go. But now for the bad news. According to Smith's team, the slow expansion of the sun will cause temperatures to rise well before that: "the oceans will boil dry and the water vapour will escape into space. In a billion years from now the Earth will be a very hot, dry and uninhabitable ball."
So, we actually have less than a billion years left on this planet. We're here near the end of its cycle, not the middle.
Of course, a billion years is a long time for us to come up with other means of survival. If we don't kill the planet earlier.
But the point is, in the great contingent nature of our planet's history, there's effectively only a small window of time in which such complex life as ours could have developed.
Lucky our war against prokaryotes (see previous entry) has been balanced as much in our favour as this. But are we already starting to tip the balance back the wrong way?