Symbioses and the Origin of Life on Land

At least two of the biggest events in the history of life involve symbioses, which are intimate interactions of two organisms literally “living together”. One would be the symbiosis between the eukaryote host cell and the two organisms that became mitochondria and chloroplasts, and the other would be the symbiosis between fungi and liverworts, the most ancient lineage of land plants. The invasion of land by green aquatic organisms was certainly a major event without which our familiar environs would never have appeared. Liverworts are simple bodied land plants, although the one shown here (Conocephalum) is more sophisticated internally than most people would suppose, but they lack roots and leaves although the plant body itself might be considered "leafy" in the sense that it is way broader than deep. The so-called higher fungi appear at about the time life invaded land, and their filamentous bodies invade intercellular spaces and the body cells of modern liverworts functioning much like the mycorrhizal fungi so familiar in association with flowering plants. A recent study has demonstrated that such fungi associated with liverworts can enhance the uptake of critical mineral nutrients, thus providing one of the functions of a root system, especially under conditions where such nutrients are hard to come by. This enhances the photosynthetic output of the liverwort even after “paying off” its debt to the fungus. Such experimental work demonstrates the value of this symbiosis, which would is even more important under harsh terrestrial conditions, and 500 million years ago terrestrial conditions were very harsh because without its familiar mantle of plants and soil, the environmental extremes and weather would have been quite severe.

What was Prototaxites?

Prototaxites has been a classic enigmatic fossil, which means no one knew what it was other than it appeared to be really big when nothing else really was. Prototaxites appeared to be some type of cylindrical axis , which when stood upright would have looked like telephone poles on a landscape of plants standing little taller than your living room carpet. That’s because Prototaxites existed during the Late Silurian and Devonian, a period of time when vascular plants were just getting their rhizomes under themselves. Prototaxites fossils display a strange spiral inner organization and a filamentous composition.

Not too long ago, the filamentous organization of Prototaxites led to its identification as a fungus, the “humongous fungus” and this is the actual cover of the journal this research was published in.

But a team of my ever creative colleagues lead by Dr. Graham (no, not the one of cracker fame) have just published their reinterpretation of these fossils by actually recreating plant remains that have the same appearance as Prototaxites. Dr. Graham is well known for torturing liverworts to see if any of the degraded remains bear similarities to enigmatic fossils of early land plants. The evidence they present suggests Prototaxites formed a helical cylinder when a large mat of liverworts was rolled up probably by the winds that would have howled across a treeless landscape. This further reinforces the hypothesis that plants with a bryophyte level of organization (mosses and liverworts) were pioneers on land long before vascular plants appeared. The Phactor hopes that PATL (People Against the Torture of Liverworts) doesn’t begin picketing her lab.