BIO-Complexity, Vol 2012

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Lignin--Designed Randomness

Matti Leisola, Ossi Pastinen, Douglas D. Axe


Humans have long used wood as a structural material for some of the same reasons that trees use it—it combines great strength, flexibility and durability with a relatively low density. These desirable properties depend partly on lignin, a major chemical constituent of many plants, including trees. Lignin is the most abundant aromatic polymer on earth and the second most abundant organic polymer of any kind, exceeded only by cellulose. It is estimated that 30% of the earth’s non-fossil organic carbon is in the form of lignin. Considering its massive abundance and its high energy content (40% higher than cellulose, gram for gram), it is striking that no organism seems to have tapped it as an energy source. After posing this as an evolutionary enigma, we prepare to address it by reviewing what is known about the structure, biosynthesis and biodegradation of wood in general and of lignin in particular. Then, returning to the enigma, we ask whether it is more readily explained within a Darwinian framework or a design framework. The Darwinian account must somehow reconcile 400 million years of failure to evolve a relatively modest innovation—growth on lignin—with a long list of spectacular innovations thought to have evolved in a fraction of that time. How can one mechanism have been at the same time so effective and so ineffective? That tension vanishes completely when the design perspective is adopted. Terrestrial animal life is crucially dependent on terrestrial plant life, which is crucially dependent on soil, which is crucially dependent on the gradual photo- and biodegradation of lignin. Fungi accomplish the biodegradation, and the surprising fact that it costs them energy to do so keeps the process gradual. The peculiar properties of lignin therefore make perfect sense when seen as part of a coherent design for the entire ecosystem of our planet.

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