Who were the first plants? At the risk of a scolding from some of my fellow botanists, I’m going to define plants broadly as organisms that use sunlight to make carbohydrates, and release oxygen gas as a byproduct - in other words, organisms that photosynthesize. There is in fact a diverse array of organisms that photosynthesize, including some bacteria and various kinds of algae that are only remotely related to one another; not to mention photosynthetic sea slugs! So used in that way, "plant" is more of an ecological term for organisms that provide the base of the global food chain, not a precise taxonomic category. (see my post of October 5, 2011, "Plants and animals and kleptoplasts - oh my!")
My goal today is in fact to explore where photosynthesis came from, and we won't be distracted by taxonomic issues. The first organisms to practice photosynthesis were bacteria, and the cyanobacteria were the first to employ modern photosynthesis, in which oxygen is released as a byproduct. Bacteria are prokaryotic organisms that have a simple cell structure without nucleus or internal organelles. They don’t have chloroplasts like true plants or algae, but their entire cell is adapted to conduct photosynthesis. In my October 5, 2011 post, I described how cyanobacteria in fact were captured and domesticated to become the first chloroplasts. I also argued as devil’s advocate that cyanobacteria are the only true plants – the multicellular organisms that we call plants are merely the luxury condominiums in which those captured cyanobacteria live!
Photosynthesis was invented only once, and passed on to various eukaryotic organisms as chloroplasts, which have been captured, stolen, and recaptured many times. But when did it all start? We travel to Australia, where we find some important clues.
In a few shallow, highly saline lagoons along the west coast of Australia, peculiar knobby pillars of rock called stromatolites stand like the disarrayed remnants of a terracotta army, eroded and distorted beyond recognition. These monoliths were not carved by some ancient civilization, however, but are built up very slowly by microscopic living organisms. On the tops of the knobs, you can find mats of living microorganisms, held together by a mucilaginous glue. The glue is secreted primarily by cyanobacteria. Many bacteria form mats like this. It’s a good way to anchor yourself to a suitable location.
|Stromatolites at Shark Bay, Western Australia. Photo by Paul
Harrison via Wikipedia.
Stromatolites turn out to be one of our most important clues as to the origin of plant life. They have been around for about 3.5 million years. Ancient fossilized stromatolites, which can also be found in parts of Australia, are in fact among the earliest signs of any kind of life on this planet. They are abundant throughout much of the geological record, but became rather scarce around 500 million years ago. This was the time of the “Cambrian Explosion”, when many new kinds of animals appeared. Stromatolites came under attack by voracious grazing animals equipped with hard, scraping mouth parts. After that, they survived only in restricted sites too salty for such animals.
The early photosynthesizers must have gradually built up enormous populations, for the oxygen they produced eventually transformed the vast oceans of our planet, and then the very atmosphere itself. The scarcity of oxidized (“rusted”) minerals in the Earth’s oldest rocks (older than 3 billion years), indicates that there was very little free oxygen in the atmosphere at that time, so the oxidizing of crustal rocks is also evidence of plant life. Iron is particularly abundant on Earth, and quite prone to rusting. In the ancient seas there was a steady supply of iron bubbling up from underwater volcanic fissures, and from eroding surface rocks. In its unoxidized state, iron is soluble in water, but when it oxidizes it forms insoluble molecules of hematite or magnetite, which sink to the bottom of the sea. When oxygen became available in comparably huge quantities there were spectacular depositions of iron. This resulted in distinctive and extensive rock layers known as the Banded Iron Formations. The “rusting of the earth,” as it was called by Schopf (2006), is the source of most of the iron ore that is being ravenously consumed by modern civilization.
Website for images and more information about stromatolites in Australia: www.sharkbay.org (under "Nature of Shark Bay")