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.

Don't take air for granted

Whether you knew it or not, the oxygen in Earth’s atmosphere, which you probably think of as a life-giving gas, is actually the toxic metabolic byproduct of the particular type of photosynthesis found only in cyanobacteria (blue-green algae) and chloroplasts, which is actually one and the same place as any loyal reader of the Phactor knows.

All evidence tells us that life on Earth when Earth was young was a very different from what we think of today even if we try to avoid the bias inherent in having a big organisms perspective. Life on Earth is still mostly unicellular. All organisms were anaerobic meaning their metabolisms did not require free oxygen. Actually most “modern” metabolic machinery operates in the absence of oxygen too; what changed was the last step where oxygen became a handy place to stick a leftover hydrogen with the added benefit of changing toxic oxygen to harmless water.

New evidence has been reported that pushes the “origin of photosynthesis” back by some 750 million years to about 3.46 billion years ago. Hmm, ever the prescient biologist, the Phactor has been telling students for years that oxygen-generating photosynthesis began about 3.5 billion years ago, clearly on the basis of some pretty sketchy evidence, but clearly what those fossils implied was correct. And how long did it take to convert the Earth’s atmosphere? "Once cyanobacteria appeared in one area of the ocean, it probably took less than 10 million years to fully oxygenate the atmosphere and oceans." Translation: not very long at all.

Now I hate to niggle, but this is research does not provide evidence for the origin of photosynthesis, but only an earlier appearance of oxygen liberating photosynthesis. Both the green sulfur and green non-sulfur bacteria (chloroflexi bacteria) are also photosynthetic, but neither liberates oxygen. This requires a set of metabolic equipment only found in the cyanobacteria (and then, after some became intracellular slaves, in chloroplasts). Chloroflexi bacteria are arguably the most ancient lineage of living organisms, so there is good reason to think other forms of photosynthesis are older. Perhaps photosynthesis using chlorophyll was derived from bacteriochlorophyll, and some photosynthetic bacteria are capable of functioning in the “dark” by using infrared wavelengths.

Still we shouldn’t take oxygen for granted; you never can tell when some bacteria may go some bacteria may go bad. The picture at this link gives new meaning to “black smoker”.