Supernovae and Life

Part 7 - Life

There is no clear evidence of life for 600 million years after water became abundant although fossilized microorganisms discovered in thermal vent precipitates in Quebec, Canada, may have lived as early as 4.28 billion years ago, not long after the oceans formed.

Thermal vents are underwater volcanoes where water can remain liquid at a temperate of up to 300 degrees Celsius because of the pressure (at sea level pressure, water boils at 100 degrees Celsius). Volcanoes provide a variety of chemical compounds and the energy that may have created early forms of life.

The earliest traces of life are found in the few rocks that have avoided erosion or being pushed down into the magma and melted. Greenland is one of the few places remaining where the surface rocks are about 3.8 billion years old. There are no fossils, but there is a thin vein of graphite, a form of carbon.

Carbon has two stable isotopes C-12 and C-13 (which has one extra neutron in the nucleus) and the graphite is unusually enriched with C-12. Such enrichment suggests that the source was the carbon remains of life because all living organisms prefer to use the lighter C-12. Similar traces, that might have been caused by living organisms, can be found in Western Australia where the rocks are 3.6 billion years old.

Evidence of life is also suggested by fossilized stromatolites that may be 3.7 billion years old. Stromatolites were formed by single celled microbes which glued themselves to rocks or rock particles forming mats. Instead of drifting with the water currents, they were anchored in place and the current brought nutrients to them.

The first strong evidence of life is provided by microbe fossils, that died about 2.6 billion years ago, which show variations in the isotopes of carbon, sulphur and nitrogen typical of living organisms.

One plausible theory, as to what happened during the 600 million years before the earliest evidence of microbial life in the seas, suggests that in early seas, rivers and lakes, various elements combined in crystal like formations and that these broke up to form other crystals. Over time these acquired other elements forming more complex molecules until these began to look more like proto-life forms.

A planet simulator at McMaster University in Canada recently provided evidence that life may have begun in warm pools of water containing inorganic salts, clays, lipids molecules and nucleotides.

Such complex molecules could grow only by chemically linking together elements such as hydrogen and carbon. But most of these would be already be combined with other elements and would not be available until these molecules were broken up by reactive chemicals, lightening strikes, volcanic heat or radiation. Very little of the sun's radiation would have penetrated the deep oceans but radiation from the decay of radio-active isotopes, principally uranium-235 and its daughter products, would have been more intense than it is now. Such fragments would have been 'food' for the growing proto-life forms.

This would have been a very hit or miss process if the molecules were stationary but in water the molecules are free to drift around and when the water is heated they move around faster and with more energy. In the process they bump into each other and into any molecules dissolved in the water. This process would have been one way for molecules to meet each other.

Proto-life forms may have resembled modern viruses which are relatively large, complex molecules not contained in a cell. Most modern viruses are inert and show no signs of life until they force their way into a cell which they can used to replicate themselves. Typically the cell eventually bursts thus releasing the new viruses into the environment.

There is evidence that early viruses contained more complex molecules, were much larger than modern viruses and had the ability to reproduce without the aid of a host cell. No modern descendants have yet been discovered.

Viroids, the smallest biologically active molecules yet discovered, are short circular, single-strand RNA without protein coats from 246 to 467 nucleo-bases in size compared to the smallest known viruses with about 2,000 nucleobases. They infect only plants as far as we know. Viroids may be relics of an RNA based, pre-cellular life form, before the evolution of DNA or proteins. They represent the most plausible RNAs capable of performing crucial steps in the evolution of life from inanimate matter.

Another agent that may have influenced evolution is an ancestor of the modern Prion, an infectious agent composed of a protein that can fold in multiple ways. One of these is responsible for spongiform encephalopathies commonly known as scrapie (in sheep) and mad cow disease which can also infect people who eat the meat from these animals. There is no effective medical treatment and the illnesses are invariably fatal.

As an infectious agent, this protein is unlike all other known agents such as viruses, bacteria, fungi, and parasites, all of which contain DNA, RNA or both. Prions appear to propagate by acting as a template to guide the mis-folding of more proteins.

Modern viruses are so tiny (smaller than the wave length of light) that we can observe them only by forming them into crystals and then using electron microscopes.

Visible light has wavelengths in the range of 400–700 nanometres (nm). There are 1,000,000,000 nm in 1 metre or 25,400,000 nanometers in one inch.

A sheet of paper is about 100,000 nanometres thick. A human hair is 80,000 to 100,000 nanometres wide. A strand of human DNA is 2.5 nanometres in diameter.

There are very large numbers of viruses on the planet. There are hundreds of millions of them in every millilitre of sea water in the upper levels of the oceans, about ten times more than all the microbes and bacteria.

Viruses are one of the causes of gene transfer between microbes. When they invade a cell, they reproduce very quickly, typically rupturing the cell wall to release the new viruses while also spilling the cells genetic information into the environment where it may be picked up by other microbes searching for leftovers.