Front-loading Trees

If metazoan life was front-loaded, we would expect to find the components of metazoan machinery embedded in single-celled life forms. And this is indeed what science is finding. For example, on my old blog, I posted a series of essays showing that the components for epithelial tissue, the nervous system, and even the endocrine system, are found in various protozoa (I’ll repost these essays shortly).

But I have also noted that we would expect such front-loading to be coupled to terraforming, as a metazoan life form would be unlikely to exist on an otherwise sterile planet. Thus, it is encouraging to see that recent research provides some more evidence for front-loading that echoes both themes of deep homology and terraforming – a single-celled algae has been discovered to contain an important extracellular component previously seen only in land plants – lignin.

As this article from ScienceDaily explains:

Land plants’ ability to sprout upward through the air, unsupported except by their own woody tissues, has long been considered one of the characteristics separating them from aquatic plants, which rely on water to support them.

Now lignin, one of the chemical underpinnings vital to the self-supporting nature of land plants – and thought unique to them – has been found in marine algae by a team of researchers including scientists at UBC and Stanford University.

Lignin, a principal component of wood, is a glue-like substance that helps fortify cell walls and is instrumental in the transport of water in many plants.

In a study published in today’s issue of the journal Current Biology, lead author Patrick Martone and colleagues describe using powerful chemical and microscopic anatomy techniques to identify and localize lignin within cell walls of a red alga that thrives along the wave-swept California coast.

So not only do single-celled life forms possess vertebrate hormones and the information need to form synapses, but it turns out they also possess the “glue” needed to form trees.

“All land plants evolved from aquatic green algae and scientists have long believed that lignin evolved after plants took to land as a mechanical adaptation for stabilizing upright growth and transporting water from the root,” says Martone, an assistant professor in the UBC Dept. of Botany, where he is continuing his work on lignin.

This old view would be the purest expression of a non-teleological perspective – lignin would appear to fulfill a function to satisfy the adaptive needs of proto-land plants. The appearance of land plants would be just a lucky accident, as lignin just happened to be part of the pool of variability at the time the environmental conditions pressed for something like it.

But now we can see that lignin was always there ‘in waiting,’ ready to fulfill its full functional potential when the time came:

“Because red and green algae likely diverged more than a billion years ago, the discovery of lignin in red algae suggests that the basic machinery for producing lignin may have existed long before algae moved to land.”

Of course, it might even be more interesting:

Alternatively, algae and land plants may have evolved the identical compound independently, after they diverged.

“The pathways, enzymes and genes that go into making this stuff are pretty complicated, so to come up with all those separately would be really, really amazing,” says Denny. “Anything is possible, but that would be one hell of a coincidence.”

Such convergence would tie in nicely to Conway Morris’s views about evolution entailing the emergence of humanoid intelligence (see his book, Life’s Solution: Inevitable Humans in a Lonely Universe).

Anyway, here is the abstract for the research paper:

Lignified cell walls are widely considered to be key innovations in the evolution of terrestrial plants from aquatic ancestors some 475 million years ago [1,2,3]. Lignins, complex aromatic heteropolymers, stiffen and fortify secondary cell walls within xylem tissues, creating a dense matrix that binds cellulose microfibrils [4] and crosslinks other wall components [5], thereby preventing the collapse of conductive vessels, lending biomechanical support to stems, and allowing plants to adopt an erect-growth habit in air. Although lignin-like compounds have been identified in primitive green algae [6,7], the presence of true lignins in nonvascular organisms, such as aquatic algae, has not been confirmed [2,3,8,9]. Here, we report the discovery of secondary walls and lignin within cells of the intertidal red alga Calliarthron cheilosporioides. Until now, such developmentally specialized cell walls have been described only in vascular plants. The finding of secondary walls and lignin in red algae raises many questions about the convergent or deeply conserved evolutionary history of these traits, given that red algae and vascular plants probably diverged more than 1 billion years ago.

I would say that one of the key innovations in the evolution of terrestrial plants from aquatic ancestors actually originated hundreds of millions of years prior to the appearance of land plants clearly supports the hypothesis of front-loading evolution.

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