Supernovae and Life

Part 10 - Endosymbiosis

Cyanobacteria evolved in the oceans and were probably the first microbes to produce oxygen by photosynthesis. These microbes began splitting water into its components to obtain the hydrogen and discarding oxygen into the environment as a dangerous element. This did not cause free oxygen to accumulate in the atmosphere as most of the hydrogen released when the organisms died quickly combined with any free oxygen. Also oxygen was rapidly absorbed by other elements resulting in the precipitation of iron in the form of iron oxide (rust) and by burning with methane gas producing water and carbon dioxide.

But the evolution of denser eukaryotic phytoplankton, which sank more rapidly than the earlier prokaryotic microbes, permitted free oxygen to accumulate in the atmosphere as the dead organic material accumulated in the sediment of shallow seas. After billions of years of sedimentary deposits, the underlying silt was compressed into rock and the organic material was formed into the oil and gas deposits that we current use.

About 2.4 billion years ago, about 200 million years after the first cyanobacteria, atmospheric oxygen increased to about 10% of the present value in the Great Oxygenation Event (GOE).  As oxygen is toxic to anaerobic organisms, the increase of oxygen in the atmosphere and dissolved in the upper part of the oceans, may have been responsible for one of the most significant mass extinctions in Earth's history. The anaerobic bacteria simply could not survive in the presence of oxygen.

About 500 million years ago, the amount of oxygen in the atmosphere increased to the range 50% to 150% of the present value.

In the 1960's scientists analysed the relationships between all life forms, comparing ribosomal RNA sequences to confirm that there were three distinctly different groups of life forms, Bacteria, Archaea and Eucarya. The Eucarya include all animals, plants and fungi.

Further analyses of the DNA of various parts of algae cells, which, like land plants, use sun light to separate carbon from carbon dioxide thereby releasing oxygen, revealed that three of the cell components;- the nucleus, chloroplast and mitochondria each had different DNA, indicating that all three had evolved independently before being incorporated into one cell!  How this incredible event occurred is known as endosymbiosis. It is not an uncommon process but rarely does it produce a new organism which survives. The fact that was successful is the reason we exist.

More than 2 billion years ago, an Archean cell engulfed a purple, non-sulphur, alpha-proteobacterium and both survived in a symbiotic relationship with the bacterium living inside the Archean cell. It was the first Eukaryote. One theory suggests that the purple bacterium was more ancient than cyanobacteria and could use the energy of light to make organic matter only in the absence of oxygen. If any oxygen was around, it went into reverse and started using oxygen to break down organic matter and supply energy to the cell. Animals have retained this Jekyll and Hyde character - the mitochondrion - in every cell.

There were a few problems. The new Eukaryote cell had to quickly obtain control over the old purple non-sulphur bacterium and this meant modifying the chemical signalling system (quorum signalling) already in use between independent microbes. It also required the transfer of some genes and the loss of many more. Over time the bacterium lost the ability to replicate itself outside the host but it retained the ability to produce energy and some proteins. Neither the original Archean cell nor the purple bacteria cell could produce oxygen. The only benefit seems to have been that the new Eukaryote cell was able to retain nutrients such as ammonium and phosphate that otherwise would have been lost.

And then, some time later a second major endosymbiotic event occurred. The new Eukaryote cell engulfed an oxygen producing cyanobacterium. This event probably occurred many times as in most cases it cause the death of the old purple bacterium. Over time the purple bacterium adapted to the poisonous stream of oxygen by salvaging and cobbling together parts of several organisms to supply electrons and protons (from hydrogen atoms) to convert the oxygen into water while controlling the release of chemical energy. This complex molecular process became cytochrome c oxidase and this would affect all life on Earth.

Oxygen supercharged the cells making them far more efficient and permitting them to make complex lipids like cholesterol and evolve hair like structures, flagella, with molecular motors that could propel the cells around.

But there were now three sets of genetic information in one cell. The host had one set, (the nucleus) the protomitochondrion another and the recently acquired cyanobacteria (the protochloroplast) the third. A set of signals were needed to ensure that none of the symbionts outgrew the others and that the reproduction information be transferred to the host cell. The three organisms had to cooperate with each other merely to survive.

But the new Eukaryotic cells made the green pigment chlorophyll-A used by all oxygen producing organisms to split water and they were about to remake the world.

I highly recommend Paul G. Falkowski's book - Life's Engines: How Microbes Made Earth Habitable, to anyone interested in this subject.

THIS STORY CONTINUES IN BOOK 2  -  We Eukaryotes