The origin of life

By August 23, 2019Science

Working out when life on Earth started has always been difficult because the Earth is constantly recycling, albeit slowly, the rocks in its crust. As a consequence, the older the rocks are, the less likely they are to have survived intact, and these are the rocks in which fossils of the earliest life would be likely to be preserved. Currently, the oldest known fossils are from Greenland and are dated at approximately 3.7 billion years old. These are about 200 million years older than the previous record holder from the Pilbara in Western Australia, which were dated at a round 3.48 billion years ago1. Both groups of fossils are bacterial colonies called stromatolites, modern versions of which can still be found in Shark Bay in Western Australia, and other places around the world. However, it is clear that these stromatolites are not the oldest organisms to have existed on the planet. What was the oldest, is very difficult to even estimate; and that does not take into account the difficulty in determining what constitutes life. Indeed, some researchers consider it to be a mistake to try to define life. This is mostly because we cannot define it such that no ‘borderline’ cases exist2.

Despite these difficulties, various attempts have been made to work out when life on Earth began. One of the more recent attempts used a DNA analysis in what is termed a ‘molecular clock’* analysis, which is calibrated using fossil occurrences3. This analysis has found that the Last Universal Common Ancestor (LUCA; the last shared ancestor of all life on earth) was likely to have predated the Late Heavy Bombardment (LHB)**, probably over 3.9 billion years ago3. This is a little problematic, because it is suggested that the LHB came after the origin of life, and although microbes can survive just about anywhere, the time of the LHB might have been a bit difficult for them. However, a recent study of the LHB indicate that it may have happened much earlier, at about 4.48 billion years ago4.

The two primary divisions of life, Bacteria and Archaea probably emerged much later, perhaps soon after 3.4 billion years ago. This indicates that the stromatolites from Greenland and the Pilbara were populated not by these major groups but by precursors to them. In addition, this molecular clock study indicated that the Eukaryota (the groups made of complex cells and almost all the life you can see with the naked eye, including us) did not arise until much later, at about 1.8 billion years ago3.

One of the major problems with understanding the origin of life is trying to work out how the first cells arose. Cells make up all living things and all of the simplest versions have three fundamental elements. They have molecules that encode information and can be copied. These comprise DNA and its simpler relative, RNA. They have assorted proteins which perform various tasks within the cell. Lastly, they have a membrane isolating these elements from the outside world, and this is constructed from fatty acids. This is essentially what the first cell, the ‘protocell’ would have been: RNA and proteins in a fatty acid membrane. Without the membrane all these molecules would just dissipate into the world and nothing would happen. When placed in water, fatty acids self-assemble into hollow spheres. If these spheres contained RNA and protein, then the basics are present. It does seem too good to be true, and it was, because it is likely that life first arose in salty oceans and salt prevents the fatty acid spheres from forming, However, some ions, like those you get in a salty ocean, are necessary for RNA to function. Now this paradox seems to have been solved. It has been shown that these fatty acids can self-assemble in salt water in the presence of amino acids, the building blocks for proteins. The amino acids stick to the fatty acids and give them stability. In turn, the fatty acids concentrate the amino acids, perhaps allowing them to coalesce into proteins5.  Given that amino acids have been found in several meteorites, including the Murchison meteorite, that fell in Victoria in 19696, this ability of fatty acids to self-assemble in the presence of amino acids in salty water, just adds more support to the theory that life may be inevitable7  here and in the rest of the cosmos. Whether there is intelligent life in the cosmos is difficult to determine. I just hope there is, because, to paraphrase Monty Python, there seems to be bugger all down here on earth8.

*A molecular clock analysis uses the mutation rate of DNA or protein sequences to determine the time of separation of evolutionary lineages from each other. This rate of mutation is relatively constant over time and measuring the difference in DNA or protein sequences between different organisms can give an estimate of when they last shared a common ancestor19.

**The Late Heavy Bombardment was an interval of time in Earth history initially thought to be from about 3.9-3.8 billion years ago, when there was an intense ‘bombardment’ by meteorites and comets of the Earth’s surface10,11.



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