Photo credit: Pixabay
In the time since its conception, Darwin’s theory of evolution by natural selection, incorporated with the more recent significant advances in our understanding of genetics, has provided clear insight to the origins of many modern species. Using the ever growing pool of information available (especially genetic sequencing etc.) we have been able to build large parts of the tree of life on Earth; much of this work standing up to scientific scrutiny. Despite this, the further back we look in the history of Earth and life the less convincing evidence we find for evolutionary processes; especially the origin of life. In modern biology, with evolutionary theory as its backbone, there have been a number of hypotheses put forward to explain the origin of life; suggesting the conditions and requirements and attempting to determine the number of times this process occurred before the ancestors to all modern lineages appeared.
One of the most famous and revealing of all experiments on the origin of life is that of Stanley Miller and Harold Urey, carried out at the University of Chicago in 1953. This experiment speculated on the chemical composition of early Earth’s atmosphere, and was used to see if organic compounds key to life on earth, such as amino acids, could be formed naturally from these conditions. At the time, consensus said the atmosphere would have been reducing; full of hydrogen, methane, ammonia and water. The hypothesis was that conditions on early Earth led to chemical reactions that synthesized complex organic compounds from simple organic monomers and polymers (originally by A. Oparin and J. Haldane). Miller and Urey thought that with the aforementioned gasses acting as hydrogen, carbon, nitrogen and oxygen sources, organic compounds could be synthesized over time with an energy source (in this case an electrical source representing lightning strikes). At the end of the experiment 5 amino acids were detected originally including glycine and alpha and beta alanine. In 2011 the results of a later experiment by Miller were analysed for the first time. This experiment also included the gas hydrogen sulfide and revealed 22 amino acids, more than the 20 that naturally occur in life. This may have been a more accurate representation of early Earth than the original, because later evidence suggests major volcanic eruptions added carbon dioxide, nitrogen, hydrogen sulfide and sulfur dioxide to the atmosphere.
This is strong evidence for the version of the ‘primordial soup’ hypothesis that Oparin and Haldane originally put forward, showing that with the right basic materials (that were available on early Earth) and an energy source, organic compounds can be produced including amino acids; called the ‘building blocks’ of life because they combine to form proteins. Most stages of the process detailed by this hypothesis have been observed (in the Miller-Urey experiment and its successors). First, the chemically reducing atmosphere is exposed to energy; this produces simple monomers which become concentrated in the ‘soup’. The biggest problem with this hypothesis is the final stage, where monomers develop into polymers and eventually into cell-like structures and even later: the earliest life forms. In the Miller-Urey experiment, substances that react with amino acids were produced, effectively ending any peptide chains. It is also unclear how or why monomers would polymerise to form incredibly complex structures, when hydrolysis down to the simple forms may well have been favoured in the suggested environments. The ‘RNA world’ hypothesis is one that potentially solves the issue of this development stage, by proposing RNA (ribonucleic acid) as the precursor molecules to modern life instead of more simple organic monomers.
The biggest support to the RNA world hypothesis is our observation of RNA’s multiple capabilities. It can act the same way as DNA; storing, transferring and duplicating genetic code, and in the 1980s it was discovered that RNA can also act as a catalyst to biological reactions. In these cases the RNA molecules work like enzymes, and are called ribozymes (ribonucleic acid enzyme). This is important because it gives support to the self-replication hypothesis of an RNA world scenario, which requires an RNA polymerase ribozyme to undergo autocatalysis for its own synthesis. Performing all these tasks means that RNA could have once supported life forms without the use of DNA (as in some viruses today), and could also have feasibly survived long enough in early Earth conditions to allow development of cell precursors, and eventually the first life forms.
An issue with the RNA world hypothesis arises when considering how the original RNA molecules were formed (as opposed to how amino acids developed in the primordial soup hypothesis). RNA is a polymer of ribonucleotides, and while it has been shown that these polymers can form naturally, it is more difficult to understand how these ribonucleotides could have formed from ribose and nucleotide bases. In 2008 John D. Sutherland (et al.) at the University of Manchester, offered an explanation for how pyrimidine ribonucleotides could be formed not from ribose and nucleotides, but from small, relatively simple carbon compounds such as glycoaldehyde, cyanamide and cyanoacetylene (in the presence of inorganic phosphate). These could plausibly have been found in an early Earth (or ‘prebiotic’) setting and the conditions of the experiment were made to match potential conditions of the time. It has also been suggested that the building blocks of RNA can be formed extraterrestrially before the planet they will end up on has formed. A study from Copenhagen University confirmed the detection of the simple sugar glycoaldehyde near a distant star, suggesting that the materials needed to form RNA could have already been present when the planet formed. This gives more support to the idea of an RNA world, as we can back up with evidence both the suggestion that RNA formed naturally from available carbon compounds in prebiotic conditions as well as the suggestion that RNA, by acting as a ribozyme, can catalyse its own replication process.
A less common alternative hypothesis is that of ‘panspermia,’ which proposes that modern Earth life didn’t actually originate on Earth, but originated and began evolving long before being transferred to Earth by one of the many early solar system meteorite barrages. In a 2006 paper, A. Sharov used the size of an organism’s functional genome as a clock for the origin and evolution of life. He proposed that biological complexity has increased exponentially throughout evolution of life on Earth and that following this exponential hypothesis places the origin of life at 9.7 ± 2.5 billion years ago, billions of years before the formation of Earth; possibly from different, but functionally equivalent, heritable elements to nucleotides. In relatively cool parts of the outer solar system, organic molecules are quite common and it is suggested that material from such comets could have provided early Earth with enough complex organic material to start development and replication of even more complex materials. There is not a great deal of physical evidence for panspermia hypotheses, and they don’t explain the actual origin of life, just transfer it to another planet or comet etc. The main evidence for proposed Martian panspermia comes from meteorites found in Antarctica which contained carbonate material and possible bacteria fossils, though the validity of these findings are disputed. More recent studies suggested that most of the organic matter was terrestrial contamination.
In some ways, parts of the origin of life seems relatively simple: very basic and/or common organic materials combining into polymers in exactly the right conditions; maybe a primordial soup pool, deep underground in volcanic conditions or possibly around a deep ocean vent where even today there are thriving ecosystems. In other ways it is almost impossibly complex. In many hypotheses there is one stage where organic molecules are expected to become incredibly complex, despite that diverging from the favoured simple state in supposed conditions. In our current understanding, the origin of life is very complex because almost everything in modern biology is based on the principles of evolution; however it is impossible to apply these principles if there is nothing to apply them to. The complexity of such an event would suggest it had to occur multiple times before it was successful in the long run. Using models for diversification and evolution, a study by D. Raup and J. Valentine stated that the probability of survival without multiple origins is very low, suggesting that continued survival of life might only have occurred once in 10 independent origins. It was proposed that based on the early age of the first known fossils, life must have originated readily in the right conditions. This does not discount the highly supported hypothesis that all modern life has a single common ancestor, because high level of extinctions could effectively ‘hide’ separately originated lineages from our knowledge if they lasted less than around 50 million years.
Without a greater quantity and quality of evidence, it is impossible to make any conclusive remarks on the origin(s) of life. It would seem that the two key features are development of complexity and replication, which both the RNA world and primordial soup hypotheses attempt to address with limited success in some areas. Overall, the origin of life appears very complex in our current understanding, and it may well be that the best evidence we get for how life started on our own world may come as data taken from distant stars or alien planets undergoing very early conditions at a time we can observe them.