How to recognise alien life

Life on other planets may not be carbon- and water-based but the familiar principles of natural selection will almost certainly apply

There are good scientific reasons to think that life exists elsewhere in the universe beyond earth, but how would we recognise alien life if we saw it? This interesting question is asked, and answered, by Samuel Levin in a paper published by Aeon in December 2018.

The only life we currently know is biological life on earth. This life is composed of solid carbon-based components and a liquid water phase. The simplest way to think about extra-terrestrial life is to assume that such life is also carbon-based and with a watery liquid phase. This allows us to predict the planetary conditions that could allow such life to exist beyond earth, for example, the planet would have to orbit its sun at a distance that would allow liquid water to exist on the planet, and we can concentrate on such planets in our search for extra-terrestrial life.

But, what if extra-terrestrial life is not carbon-based and its liquid phase is not water? It is much more difficult both to predict where such life might arise in the universe and also how we might recognise such alien life if we ever saw it. Levin proposes an elegant approach towards solving this conundrum.

Living biological organisms as we know them are each composed of many parts working together to allow the organism to fulfil its “purpose”, which is to live and to reproduce – they are clearly “designed” towards this end. And the designer is – wait for it – natural selection. I suppose some of you were having palpitations expecting I was about to wheel a deity onstage! On the assumption that alien life will also display design that allows it to live and reproduce, Levin proposes that such life will be designed by natural selection since natural selection is the only unconscious way to produce such design.


Natural selection, the mechanism that drives biological evolution, has been intensively studied by biologists and is well understood. It works automatically on things that have three properties – variation, heredity and differential success. Think of any population of biological organisms, for instance a herd of antelopes. The individual antelopes will not all be identical – some will be bigger than their companions, some will be faster, and so on (variation), and when individual antelopes breed they pass their characteristics on to their offspring (heredity). Faster-running antelopes, for example, will be better able to outrun predators, such as lions, than slower antelopes and will therefore be more likely to survive and to procreate than their slower companions.

Naturally selected

Therefore, faster-running antelopes will be naturally selected by the natural environment and the next generation of antelopes will be enriched in faster antelopes (differential success). The environment naturally selects those organisms best suited to survive and reproduce and the organisms appear designed to suit the environment.

Discovered by Charles Darwin (1809-1882) and Alfred Russell Wallace (1823-1913), natural selection is probably the most fertile scientific principle in biology. In biological organisms, natural selection works on carbon-based life and DNA-based heredity but it is not dependent on the specific chemistry of these details – it would work equally well with life based on another element, say silicon-based life, and heredity based on a chemical other than DNA.

Life is special because its apparent design is produced by natural selection. Alien life will also have an apparent design produced by natural selection. Natural selection “selfishly” selects genes that confer a competitive advantage for reproduction in the organism’s environment, thereby ensuring these genes are passed on to the next generation – hence the name of Richard Dawkins’ brilliant and best-known book, The Selfish Gene.

Evolution, operating through natural selection, follows rules and produces organisms constrained as to the forms, goals and evolutionary pathways they can take. For example, as Levin explains, natural selection produces life “with a hierarchy of entities, with the interests of each lower level aligned with the levels above”, and astrobiology, the study life on other planets, “can use the theory of natural selection and the mathematics of evolution to make predictions about aliens”.

The alien life we eventually discover may look very strange, but it will be based on familiar principles.

William Reville is an emeritus professor of biochemistry at UCC