A Lego brick on its own is nothing special. But link some of them together, and you could end up with anything from a medieval castle, to a pirate hideout or a space ship. Since 1947, the Lego company has released thousands of sets under the banner of a variety of different themes. Every one of these bricks in Lego’s history has remained compatible with other bricks. A sixty year old brick will still lock together with modern one.
This is only possible because Lego bricks are made according to a specific set of design rules. Every stud on every brick is round and has a diameter of 5 mm, for example. In theory, the studs could have any size or shape. They could be shaped as squares, triangles or even little hexagons and work just as well. Yet these alternative, but plausible, Lego bricks don’t exist. The potential space of Lego bricks is much larger than the actual brick space. A Lego brick with square studs will never be made because it would be incompatible with every other Lego brick in existence. The square brick would be an outcast among bricks, unable to connect with its rounder brothers.
Life on earth is like Lego in this regard, with the exception that life’s building blocks are molecules, not bricks. Of all the molecules that exist in our universe, life only uses a select subset. Every cell and every virus consists of five nucleotides, some sugars, a few lipids and twenty different amino acids. And that’s about it. Every living thing on this planet is made from different combinations of these building blocks.
Life’s building blocks are everywhere on our planet, in every scoop of dirt or every bucket of ocean water. But their presence alone is not enough to distinguish our living planet from a sterile one. Amino acids such as glycine have also been found in some abundance in non-living environments such as meteorites and comets.
What really sets the earth apart, chemically speaking, is the skewed distribution of molecules. In sterile environments, there exists a continuous range of molecules, with a bias for molecules that are stable and easy to form. But life doesn’t produce a range of molecules. Life thrives because it selects and amplifies only those molecules which it needs. Natural selection and historical contingency* shaped the final set of molecules which came to to define life on earth.
Life’s bias in favour of certain molecules might be useful for detecting life on other worlds. While it’s hard to say anything meaningful about alien biochemistry, general principles that apply to life on earth should also apply to life on different planets. Even if aliens are nothing like us earthlings, perhaps the alien set of biomolecules still sets them apart from a non-lving background distribution.
The idea that life can potentially be recognized by a skewed distribution of molecules is almost fifty years old. It was first called the ‘Lego Principle’ by the astrobiologist Christopher McKay, but it harkens back to earlier ideas about (alien) life, in particular those formulated by James Lovelock in the 1960s.
The Lego principle sounds logical and sound – but does it hold up outside the realm of theory? Without a second sample of life, opportunities to test the Lego principle in the real world are slim. Still there exists a place where this hypothesis can be tested. I’m not referring to the laboratory or a test tube, but inside a computer core.
There, scientists have created living and evolving organisms. These lifeforms are unrelated to life on earth, and thus provide an opportunity to test the most universal aspects of evolution. One such digital world is Avida. The elementary building blocks (the basic chemistry) of an Avidian organism are simple computer instructions, such as ‘add’, ‘stop’ and ‘substract’. Together, these instructions form a simple computer programme (the organism) that competes with other programmes for computing time in the central processor. By performing certain tasks, doing certain calculation for example, they get more access to computing time.
If the Lego principle is true, Evan Dorn and his colleagues reasoned that the distribution of computer instructions in living Avidians should also differ from that of an ‘abiotic’ background environment **. They started out with a virtual world that where mutation rates were high. You can imagine this world as if it is being bombarded with virtual radiation. Living programmes could not survive in the hostile environment: all the useful instructions mutated into gibberish within a few generations. By lowering the mutation rate (or intensity of the radiation) step by step, the Avidians eventually took hold and evolved. After a 1000 generations, Dorn and his colleagues went back and saw how the total distribution of instructions had changed over time.
What they saw was that within a couple of hundred generations, the uniform distribution of instructions at the start of the experiment had changed into a specific signature. Certain commands were favoured, such as the logical operator NAND, while others were purged from the Avidian genomes, like JUMP-F. It is easy to explain why these instructions changed in frequency. The NAND is a vital ingredient of mathematical operations, while JUMP-F instructs the program to jump forward and skip large parts of the original programme. NAND mutations are more useful than JUMP-F mutations, which are often lethal.
But it doesn’t really matter which instructions changed in abundance. The fact that the distribution of building blocks changed when life took over, suggests that such skewed distributions of building blocks are indeed a universal feature of life. So if we want to look for life elsewhere in our galaxy, it seems wise to first try and understand what non-life looks like. By understanding sterile environments, it becomes possible to detect environments that deviate from this sterility. Hopefully, this will make it possible to find life even without knowing on what kind of biochemistry this life is based. Science is cool like that.
* There is no reason to assume that the set of life’s building blocks is the most optimal or has an essential composition. There are instances where life could have picked a different molecules to do the same job. Valine and isovaline are chemically similar, yet the former is everywhere on earth, while the latter is nowhere to be found
** In their paper, Dorn and colleagues test the slightly different hypothesis that traces of life can be found in a different distribution of molecules, whereas the Lego principle states that life only uses a select subset of natural molecules.
Lego bricks and their dimensions by Cmglee
Diagram of abundance of molecules in abiotic and biotic environments from reference 1.
Graph of change in abundance of instructions in Avida from reference 2.
McKay CP (2004). What is life–and how do we search for it in other worlds? PLoS biology, 2 (9) PMID: 15367939
Evan D. Dorn, Kenneth H. Nealson, & Christoph Adami (2011). Monomer abundance distribution patterns as a universal biosignature: Examples from terrestrial and digital life J. Mol. Evol. 72 (2011) 283-295 arXiv: 1101.1013v1
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