Science Literature

After cellulose, lignin is a major constituent of plant material, comprising 25-33% of the dry mass and imparting mechanical strength and conducting water. There are three major types of lignin, depending on the monomers involved in the formation of this material. "Lignins are derived mainly from the phenylpropanoid monomers p-coumaryl, coniferyl, and sinapyl alcohol, which give rise to p-hydroxyphenyl, guaiacyl, and syringyl subunits when incorporated into the lignin polymer." These different types are significant for the research being considered here. "Generally, ferns and gymnosperms deposit lignins that are derived primarily from guaiacyl monomers together with a small proportion of p-hydroxyphenyl units, whereas angiosperm lignins are guaiacyl/syringyl copolymers that also can contain some p-hydroxyphenyl monomers." Since angiosperms appear in the Late Cretaceous, and the other types of vascular plant are found from the Silurian, some have thought that syringyl lignin is a late arrival in the evolutionary story of plants.

Selaginella moellendorffii (Source here)

The plant studied is a lycophyte - Selaginella moellendorffii - popularly known as spike moss. It can be considered a living fossil, and is described by the researchers as "a relict of an ancient vascular plant lineage". Its genome has been sequenced and it is the subject of numerous studies relating to lycophytes.

In their discussion, the researchers summarise their findings in this way:

"The distribution of syringyl lignin in the plant kingdom suggested two possible models for the evolution of F5H [an essential component in the formation of syringyl lignin]. First, the enzyme could have arisen early in plant evolution, was lost in ferns and gymnosperms, but was not lost in angiosperms or Selaginella. Alternatively, F5H could have evolved independently in lycophyte and angiosperm lineages after they had diverged. Our results suggest that the second model is correct and that F5H from Selaginella is functionally equivalent to, but phylogenetically independent from, angiosperm F5Hs. This conclusion is further supported by the observation that syringyl lignin derivatives are not detected in extant members of the Lycopodiaceae and have not been found in fossils of the extinct lycophyte Sigillaria ovata (order Lepidodendrales)."

The press release for the research paper summarises the achievement in these words:

"Biologists have discovered that a fundamental building block in the cells of flowering plants evolved independently, yet almost identically, on a separate branch of the evolutionary tree - in an ancient plant group called lycophytes that originated at least 420 million years ago. Researchers believe that flowering plants evolved from gymnosperms, the group that includes conifers, ginkgos and related plants. This group split from lycophytes hundreds of millions of years before flowering plants appeared. The building block, called syringyl lignin, is a critical part of the plants' scaffolding and water-transport systems. It apparently emerged separately in the two plant groups, much like flight arose separately in both bats and birds."

The conclusion, that this complex material "evolved independently" deserves careful scrutiny. We are dealing here with some very complex chemistry that does not exist in isolation from other aspects of the plant. The idea that almost identical innovations occurred independently may be the best of the explanations considered above, but there is at least one other option that needs to be considered: and that emerges from a design inference. Complex chemistry like this does not just happen: it suggests the presence of intelligence. This is confirmed by the potential this research offers for engineering plants for easier lignin breakdown in the manufacture of biofuels:

"Findings from studies such as this really have implications regarding the potential for designing plants to better make use of cellulose in cell walls," said Gerald Berkowitz [. . .] "Different forms of lignin are present in crop plant cell walls; engineering plants to express specifically syringyl lignin could allow for easier break down of cellulose. Overcoming this obstacle is an important next step for advancing second generation biofuel production."

These researchers are proposing to use their expertise in biotechnology and design engineering to bring about changes to plants that bear comparison with changes evolutionists attribute to mindless tinkering processes. This particular plant provides a remarkable example of convergence that argues against contingency in evolution and for intelligent agency in the way plant materials are constructed.

Independent origins of syringyl lignin in vascular plants
Jing-Ke Weng, Xu Li, Jake Stout, and Clint Chapple
Proceedings of the National Academy of Sciences USA, June 3, 2008, vol. 105, no. 22, 7887-7892.

Lycophytes arose in the early Silurian (~400 Mya) and represent a major lineage of vascular plants that has evolved in parallel with the ferns, gymnosperms, and angiosperms. A hallmark of vascular plants is the presence of the phenolic lignin heteropolymer in xylem and other sclerified cell types. Although syringyl lignin is often considered to be restricted in angiosperms, it has been detected in lycophytes as well. Here we report the characterization of a cytochrome P450-dependent monooxygenase from the lycophyte Selaginella moellendorffii. Gene expression data, cross-species complementation experiments, and in vitro enzyme assays indicate that this P450 is a ferulic acid/coniferaldehyde/coniferyl alcohol 5-hydroxylase (F5H), and is capable of diverting guaiacyl-substituted intermediates into syringyl lignin biosynthesis. Phylogenetic analysis indicates that the Selaginella F5H represents a new family of plant P450s and suggests that it has evolved independently of angiosperm F5Hs.

See also:

Fundamental Building Block In Flowering Plants Evolved Independently, Yet Almost Identically In Ancient Plants, ScienceDaily (May 28, 2008)