In the early 1960s, the independent scientist James Lovelock worked as a consultant for NASA developing ways to analyse extraterrestrial atmospheres. This work led him to the dramatic conclusion that life would leave an indelible stamp on the chemical make up of any planet.
Over billions of years, he said, the processes of life would create a fog of chemicals unlike anything that could form in an ordinary chemical equilibrium.
He even went further to suggest that this atmosphere and the life it supported would form a kind of self-regulating system that could itself be thought of as a living organism–the Gaia hypothesis. Lovelock famously says that as soon as he saw the first analysis of the chemical make up of the Martian atmosphere, which is almost entirely carbon dioxide and nitrogen, he knew the planet could not support life.
Since then, the search for biosignatures has become an important problem for astrobiologists. We know, for example, that large amounts of oxygen and small amounts of methane are generated by life on Earth. And that carboxylic acids in terrestrial life forms tend to have even rather odd numbers of carbon atoms, a fact that is used to identify contamination in meteorite samples.
The problem, of course, is that we have only one example of life to study. So the biosignatures of life on Earth may be of little use for identifying ET lifeforms.
Today, Evan Dorn at the California Institute of Technology and a couple of buddies suggest a solution. Their idea is to look for measurable properties of evolution, rather than mere life. Such a property ought to be present in any system that has evolved, they say.
That’s important because scientists have developed several systems in which evolution takes place, the most famous being the various attempts to make artificial life using computer code and silicon chips..Such a signature ought to be present both on Earth and in silico.
To find out, Dorn and co looked in various samples at the distribution of biomolecules, such as amino and carboxylic acids. They compared terrestrial sludge, which is obviously teeming with life, with the outcome of experiments to synthesise amino acids, which have no life. And they even looked at the composition of meteorites.
Their results are interesting. They found that the distribution of biomolecules in the absence of life generally reflects the thermodynamic cost of making them. So there are far more simple amino acids than complex ones, for example.
However, samples containing life do not follow this pattern. Where complex biomolecules play a role in the processes of life, and therefore confer some kind of advantage, they are much more common than can be explained by thermodynamic arguments.
That’s more or less what most astrobiologists would expect.
Next, Dorn and co did a similar kind of analysis on a system of artificial life called Avida. In this world, the building blocks of life are elements of computer code that carry out simple instructions. Connect several instructions together and you have a complex “molecule”. If these molecules have a code that allows them to copy, they can reproduce.
Environmental factors such as the rate of mutation are controlled externally by computer scientists who also inject a constant stream of code that organisms can consume as they evolve. Dorn and co then compared the distribution of code in Avidian worlds before and after evolution had occurred.
It turns out, that Avidian creatures make the same kind of stamp on their environment as terrestrial organisms do on theirs. Avidians ensure that certain bits of code are preferentially selected so that they are far more common in an evolved system that in one that is starting from scratch.
Dorn and co call this the “monomer abundance distribution biosignature” and hypothesize that it is common to all forms of life.
That’s a potentially exciting result–that there is a universal biosignature of evolution that could be used to spot any kind of evolved life. Call it an evosignature.
Dorn and co even say their evosignature “may show promise for detecting alien biochemistries.”
Perhaps. First, these guys will need to think about some potential problems. The crucial property of a biosignature is that it must be the result of life but not of other ordinary processes. It must be unique.
Otherwise you run the risk of all kinds of false positives (as indeed occurred with the Viking Mars lander tests designed to spot life).
It’s not at all clear that this is the case with Dorn and co’s signature. While evolution undoubtedly plays a crucial role in the development of life, it also plays an important role in other processes too. For example, computer scientists regularly exploit the process of evolution to solve problems such as factory scheduling and aircraft design. Would these processes also demonstrate a measurable evosignature?
It’s too early to tell. But Dorn and co need to check this.
Of course, there’s another problem here too. What this discussion highlights is the difficulty in defining life in the first place. It may be that we’ll never find a biosignature or evosignature that is an entirely unambiguous sign of life, merely just a good indicator
Whatever the outcome, the new approach of using aLife to test evosignatures looks like an important new way to investigate this problem.
Ref: arxiv.org/abs/1101.1013: Monomer Abundance Distribution Patterns as a Universal Biosignature: Examples from Terrestrial and Digital Life
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