What would convince you that aliens existed?
The question came up recently at a conference on astrobiology, held at Stanford University in California.
Several ideas were tossed around unusual gases in a planets atmosphere, strange heat gradients on its surface.
But none felt persuasive.
Finally, one scientist offered the solution: a photograph.
But why would a picture be so convincing?
What is it that wed see that would tell us we werent just looking at another pile of rocks?
An alien on a planet orbiting a distant star would be wildly exotic, perhaps unimaginably so.
What, then, would give it away aslife?
The answer is relevant to our search for extraterrestrials, and what we might expect to find.
But what exactly is it that astrobiologists are looking for?
How will we know when its time to pop the Champagne?
One thing that sets life apart from non-life is its apparent design.
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Or for a squirrel to want to jump from one branch to the next.
Or for a plant to attempt to reach towards the Sun and soak up nutrients from the soil.
This combination of complex design and apparent purpose, also known as adaptedness, defines life.
We would see, clearly, the difference between a disappointing pile of rocks and an exciting alien design.
This is good news because theres only one way to get such design: natural selection.
For example, some of the Glipgloops we posited have longer eyestalks than others (variation).
Long-eyestalked Glipgloops have long-eyestalked babies (inheritance of the variation).
The result is that, over time, Glipgloops evolve to have elongated eyestalks.
As a result, over time, populations consist of individuals whoappear designedfor the purpose of reproducing.
Its exactlybecausethe selection criterion is always the same that design can develop.
This is why we dont get organisms who sacrifice for the good of their species.
But this algorithmic rigidity of natural selection also comes in handy for the astrobiologist.
Athread should be revealing itself: life is special because of its apparent design.
The only way to get design without a designer is natural selection.
Therefore, aliens must be the product of natural selection.
And natural selection follows certain rules and can produce only certain kinds of organisms.
We cant get complex life, even something as simple as a bacterium, without natural selection.
Even a postorganic, computer-based alien would ultimately be the product of a product of natural selection.
But lets consider a borderline case.
Imagine a collection of replicating molecules, like tiny naked genes, on an alien planet.
Would this be life?
Maybe, but it wouldnt be very exciting.
These replicators wouldnt suggest anything.
The argument from natural selection is robust, even at the boundaries.
Natural selection, on the other hand, is universal.
Its incredibly simple it just requires a few ingredients and its the only way to generate life.
Thats not the kind of prediction good evolutionary theory can make.
But natural selection tells us that its forms, goals and evolutionary pathways are constrained.
How would we recognize an alien?
This work of incorporating evolutionary theory into our astrobiological toolkit is only just beginning.
What else can Darwin tell us about aliens?
Presumably quite a lot.
The photo, if and when it comes, will be something entirely exotic to the naked eye.
But to the student of evolutionary biology, it might look surprisingly familiar.
This article was written bySamuel Levinand originally published at Aeon.
It has been republished under Creative Commons.
