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Let's assume that life exists on other planets. Let's also assume that this life is built on the same biochemical foundation that characterises life on Earth: simple sugars, fatty acids, 20 varieties of amino acid that link together to form proteins, and an efficient genetic code, similar in shape and style to our own DNA. Given these common starting materials, can we say anything about how life on other planets might evolve? Can we predict what kind of creatures we might encounter?
Consider the possibilities. A theoretical protein consisting of a chain of 100 amino acids will have 20100 possible configurations. This is a number greater than the total number of atoms in the Universe. Add on the fact that a typical organism consists of many thousands of different proteins, and the chances of extra-terrestrial life bearing any kind of resemblance to life on Earth become vanishing small.
Of course, life isn't that simple. The vast majority of protein configurations will be unworkable. Physical and biochemical constraints will mean that only a tiny subset of the possibilities are likely. And the universal algorithm of Darwinian natural selection will ensure that possibilities will be channelled in the most competitive directions.
While all biologists accept these constraints, vocal, and frequently vicious, disagreements emerge over the amount of 'noise' in the evolutionary system. The debate centres on the issue of whether evolution is essentially probabilistic, or deterministic. Are we as humans, happy accidents, a product of chance and contingency, as much as the helping hand of natural selection? Or is there an element of predictability, perhaps even inevitability in an evolutionary outcome?
Convergence is a recurring pattern in evolution. Think of the iron-bearing protein, haemoglobin, for example, found in organisms as diverse as legumes, bacteria and animals; the camera eye common to octopuses, vertebrates and marine worms; the large brains of primates, whales and dolphins; the eusociality of insects, naked mole rats and the coral-reef shrimp; or the agriculture of leaf-cutting ants and humans. Wherever you look within the great diversity of life, you will find independent evolutionary lineages coming up with common solutions.
Patterns in evolution are impossible to deny. But the details of what kind of life we will find on other planets are still difficult to predict. There are sure to be some characteristics that we recognise, but perhaps they will occur in combinations that we don't.
Life, the Universe and Everything | A Difficult Recipe | Location, Location, Location | The Realm of Possibility | Alien Titchmarsh | Find Out More
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