The great filter

My Times column on the paradox that planets seem
to be abundant, but signs of life are rare:

The search for another world that can sustain life
is getting warmer. We now know of 1,879 planets outside the solar
system. A few weeks ago, we (the planetary we, that is: no thanks
to me) found Earth’s twin, a planet of similar size and a habitable
distance from its sun, but 1,400 light years from here. Last week
we found a rocky planet close to a star just
21 light years away, which means if anybody lives there and tunes
in to us, they could be watching the first episode
of Friends.

Also last week the Philae lander’s results showed that the comet it is riding on has
organic (carbon-based) molecules in its dust, the ingredients of
life. Even in our own solar system we know of a moon, Titan, where
it rains methane, and another, Europa, with an ice-covered ocean.
In short, it is getting ever more likely that there are lots of
bodies like Earth in our own galaxy alone: with liquid water and
the right sort of temperatures for the carbon chemistry of the kind
that life runs on here.

Which only underlines Enrico Fermi’s famous question, first
delivered over lunch at Los Alamos in 1950 during a conversation
about UFOs: “Where is everybody?” His point was that if there are
billions of habitable planets, and many have had billions of years
to produce intelligent life forms, then the chances are that some
of them must have had time to broadcast to, or even visit other
solar systems. So why is there not a whisper in the ether of their
version of Top Gear, let alone a glimpse of a
tentacled Clarkson careering through the air in a flying saucer, or
even a bit of ancient, rusty wreckage to show where he once
crashed?

The Fermi paradox gets ever more baffling, as the evidence grows
of other habitable planets. The silence is beginning to seem
ominous. Robin Hanson, the chief scientist of the prediction market
research firm Consensus Point, advises: “Take a minute to look up at the dark
night sky, see the vast, ancient and unbroken deadlands, and be
very afraid.” He thinks there may be an obstacle ahead of us that
has caused every previous planetary civilisation to collapse before
colonising the galaxy: nuclear war, or something equally
horrible.

He calls this argument the great filter, and defines it thus, “The sum total of all of the obstacles
that stand in the way of a simple dead planet (or similar sized
material) proceeding to give rise to a cosmologically visible
civilisation.” Have we got past the great obstacles, or are there
still some insuperable ones ahead?

We’ve certainly got through at least five big filters. First,
life evolved. Nick Lane in his magnificent new book The
Vital Question thinks that a peculiar feature of all earthly
life — that it traps energy in the form of protons pumped across
membranes — indicates that it began at warm alkaline vents on the
floor of the early ocean. Gradually that energy came to be used to
make information, in the form of genes, and the machinery to
replicate it.

Genetically and biochemically, there is only one form of life on
this planet, which might imply that it’s a rare and lucky accident,
but then later forms would have struggled to compete with the first
one, so it’s not clear how difficult this step was and how many
planets with the right conditions failed to take it. That life did
not then die out thanks to a global freeze-up or fry-up may have
depended — David Waltham argues in his book Lucky
Planet— on the good fortune of having a relatively large
moon that stabilised our spin and moderated our climate: another
possible stroke of luck that other planets perhaps did not
share.

Next, after a couple of billion years, creatures bigger than
microbes emerged, once (Nick Lane argues) an energy-per-gene limit
was breached by the invention of the mitochondrion, a specialised
energy-generating microbe living inside another cell. This gave us
large and complex cells of the kind found in plants, animals and
fungi, and eventually multicellular creatures making and using
oxygen. It took a very long time to achieve this step, so many
planets may have been filtered out at that stage, and be stuck with
microbes only.

Earth, then, had complex life forms for more than half a billion
years before anything remotely intelligent enough to develop
technology appeared. For 140 million years, the dinosaurs achieved
large size and nimble agility without ever threatening to do much
mental or physical (as opposed to genetic) innovation. Their
evolutionary experiment was cut short by a meteorite, and the
mammals took another 60 million years even to start on technology.
Even then, it was only one species, a primate, that became
technological, rather than the almost equally large-brained
dolphins. So that was a strong filter: intelligence without
technology is clearly possible.

Even once we had big brains, technology, language and culture,
human ancestors spent a few million years stuck in
hunter-gathering, before something triggered an explosion of
cumulative culture in one sub-species on one part of one continent,
Africa, about 200,000 years ago. I have argued before that this
step was enabled by the invention of exchange and specialisation,
which made us capable of “cloud intelligence” so we could
collaboratively build devices too complex for individual minds to
comprehend. Only then did we experience rapid cultural evolution
and innovate to the point where we could start exploring space.

It is, in other words, very likely that most planets would have
failed to clear all these hurdles, which may explain the silence.
Many may teem with microbes; others could be rich in fossils of
life that died out; a few possibly host herds of agile, even
ingenious, creatures, some of which communicate in languages; one
or two might have got to the point of inventing weapons of war
before some catastrophe intervened. But almost none have reached
the point where they could send messages and spacecraft out of
their atmospheres.

Is it not incredibly lucky that we live on a rare planet that
did make it? Well, no, because whoever lived on such a planet would
say that about themselves. It is for this reason that I am not
persuaded yet that the most severe filter, the one that stops most
planets colonising the galaxy, comes after the stage we have
reached. I think that’s unnecessary pessimism. And with that I am
off on a summer holiday.