260 – 245 million years ago
Alfred, Lord Tennyson, wrote his poem “In Memoriam AHH,” in response to the death of his friend Arthur Henry Hallam. Several cantos consider the bleak lessons of paleontology – not just the myriads of deaths, but the specter of species extinction. Tennyson finished the poem in 1849, a decade before “The Origin of Species,” when the possibility of non-divinely-directed evolution and the reality of mass extinctions like the end-Permian were becoming part of general awareness.
Are God and Nature then at strife,
That Nature lends such evil dreams?
So careful of the type she seems,
So careless of the single life;
That I, considering everywhere
Her secret meaning in her deeds,
And finding that of fifty seeds
She often brings but one to bear,
I falter where I firmly trod,
And falling with my weight of cares
Upon the great world’s altar-stairs
That slope thro’ darkness up to God,
I stretch lame hands of faith, and grope,
And gather dust and chaff, and call
To what I feel is Lord of all,
And faintly trust the larger hope.
‘So careful of the type?’ but no.
From scarped cliff and quarried stone
She cries, ‘A thousand types are gone:
I care for nothing, all shall go.
‘Thou makest thine appeal to me:
I bring to life, I bring to death:
The spirit does but mean the breath:
I know no more.’ And he, shall he,
Man, her last work, who seem’d so fair,
Such splendid purpose in his eyes,
Who roll’d the psalm to wintry skies,
Who built him fanes of fruitless prayer,
Who trusted God was love indeed
And love Creation’s final law—
Tho’ Nature, red in tooth and claw
With ravine, shriek’d against his creed—
Who loved, who suffer’d countless ills,
Who battled for the True, the Just,
Be blown about the desert dust,
Or seal’d within the iron hills?
For one answer to Tennyson’s anguished question about human extinction, there’s an argument that says we can estimate how much longer humanity has got from just basic probability theory. It comes from astrophysicist Richard Gott, and goes like this: Homo sapiens has been around about 200,000 years. It’s not very likely that we’re living at the very beginning or very end of our species’ history, just like it’s not very likely that a name chosen at random from the phone book will come at the very beginning or the very end. Specifically, there’s only a 2.5% chance that we’re living in the first 2.5% of our species’ life span, and only a 2.5% chance we’re living in the last 2.5% of our species’ life span. So do the math, and there’s a 95% probability that our species will last somewhere between .2 million and 8 million years.
This might also explain the Fermi paradox – we, and other intelligent species aren’t likely to colonize the galaxy. But it’s only fair to add that a lot of other people (the physicist Freeman Dyson, for example) think this gloomily Tennysonian conclusion is an abuse of probability theory.
For more on Bayes’ Rule, and the future of humanity, here’s a recent book, The Doomsday Calculation.