Lignin is a rigid, waterproof polymer found only (as far as I know) in plant cell walls. It's pretty interesting stuff: it's what makes trees rigid enough that they don't fall over and gives the water-bearing vessels in plants the ability to withstand large tensions without caving in. It's also inedible, at least to animals, and is sometimes used as a defensive compound. Because it's of economic significance (high-lignin wood makes good building material and fuel, but bad paper) a fair bit is known about it biochemically.
Why do I, an evolutionist, care? Because of this cool paper recently published in Plant Cell.
The paper discusses S-lignin, a chemical flavour of the compound which is found only in the flowering plants (Angiosperms) and the completely unrelated Selaginella. Selaginella (the beastie pictured) is a member of the horribly-named spikemosses (they aren't mosses), which is a rather small group of rather small plants. Way back in the Carboniferous era however, its most common representatives were very large trees (up to 30 metres) which dominated the landscape. Fossilized spikemoss relatives make up a large proportion of the coal seams that we mine.
Jin-Ke Weng and his colleagues were studying the role of a protein called F5H in Selaginella. F5H performs a specific function in Angiosperm biosynthesis, and they found that it can perform this function in Selaginella as well, but it also performs another reaction in the pathway. This second reaction, which is necessary for S-lignin production, is done by a different protein in Angiosperms.
Their next experiment was very clever. The group had access to Arabidopsis thaliana plants (thale cress: the fruit fly of botany) which were mutated in various ways that interrupt lignin synthesis. Through genetic engineering, they inserted the Selaginella F5H gene into these mutant plants where it filled-in for the non-functional, mutated proteins and allowed lignin to be produced. With a series of these experiments the authors were able to show that the Selaginella F5H protein can bypass four different steps in the normal Angiosperm lignin production pathway.
Since Selaginella is not closely related to angiosperms, and none of its close relatives produce S-lignin, it's almost certain that the two groups of plants discovered S-lignin by convergent evolution. The differences between the pathways show this as well: Selaginella pressed a single, multi-functional protein into service to perform several different chemical reactions where Angiosperms use different proteins for each. S-lignin is also found in several other unrelated lineages, suggesting that it has arisen many times over the course of evolution.
This paper also shows why evolutionary biologists should be excited about molecular biology: without the genetic engineering experiments, we would have to stop at saying that Selaginella and the Angiosperms discovered S-lignin independently. With molecular techniques we can discover exactly how evolution acted to produce these discoveries.
Weng, Akiyama, Bonawitz, Li and Chapple (2010). Convergent evolution of syringyl lignin biosynthesis via distinct pathways in the Lycophyte Selaginella and flowering plants. Plant Cell. 22: 10 (1909-22).