Death enables complexity in chemical evolution

The road to answering the question of how life originated is guarded by Spiegelman's monster, named after the American molecular biologist Sol Spiegelman, who some 55 years ago described the tendency of replicators to become smaller when they were allowed to evolve. 'Complexity is a disadvantage during replication, so how did the complexity of life evolve?' asked Sijbren Otto, Professor of Systems Chemistry at the University of Groningen. He previously developed a self-replicating system in which self-replication produces fibres from simple building blocks and, now, he has found a way to beat the monster.

Death

'To achieve this, we introduced death into our system,' Otto explains. His fibres are made up of stacked rings that are self-assembled from single building blocks. The number of building blocks in a ring can vary, but stacks always contain rings of the same size. Otto and his team tweaked the system in such a way that rings of two different sizes were created, containing either three or six building blocks.

Under normal circumstances, fibres that are made up of small rings will outgrow the fibres with larger rings. 'However, when we added a compound that breaks up rings inside the fibres, we found that the bigger rings were more resistant. This means that the more complex fibres will dominate, despite the smaller rings replicating faster. Fibres that are made from small rings are more easily "killed." '

Experiments

Otto acknowledges that the difference in complexity between the two types of fibres is small. 'We did find that the fibres from the larger rings were better catalysts for the benchmark retro-aldol reaction than the simpler fibres that are made from rings with three building blocks. But then again, this reaction doesn't benefit the fibres.' However, the added complexity protects the fibres from destruction, probably by shielding the sulphur-sulphur bonds that link the building blocks into rings.

'All in all, we have now shown that it is possible to beat Spiegelman's monster,' says Otto. 'We did this in a particular way, by introducing chemical destruction, but there may be other routes. For us, the next step is to find out how much complexity we can create in this manner.' His team is now working on a way to automate the reaction, which depends on a delicate balance between the processes of replication and destruction. 'At the moment, it needs constant supervision and this limits the time that we can run it.'

Variants

The new system is the first of its kind and opens a route to more complex chemical evolution. 'In order to achieve real Darwinian evolution that leads to new things, we will need more complex systems with more than one building block,' says Otto. The trick will be to design a system that allows for the right amount of variation. 'When you have unlimited variation, the system won't go anywhere, it will just produce small amounts of all kinds of variants.' In contrast, if there is very little variation, nothing really new will appear.

The results that were presented in the latest paper show that, starting from simple precursors, complexity can increase in the course of evolution. 'This means that we can now see a way forward. But the journey to producing artificial life through chemical evolution is still a long one,' says Otto. However, he has beaten the monster guarding the road to his destination.