"Nature abhors a vacuum" (Aristotle)
We live within a great mystery.
Each one of us represents an unbroken chain of successful reproduction that stretches back to a single common ancestor – to LUCA the Last Universal Common Ancestor. The chances of any one of us being here, or of life arising in the first place, would appear vanishingly small and yet here we are.
The Earth is understood to have formed approximately 4.6 billion years ago (Ga) - from accreting interstellar dusts that were drawn together by the force of gravity. It took around 600 million years for the planet to cool down to a point where it had a lithosphere capable of supporting life. The earliest agreed upon evidence of life on Earth currently dates to 3.5 Ga but new findings are continually being presented that point to it appearing much earlier than that. LUCA is currently thought to have emerged somewhere between 4.0 Ga and 3.8 Ga.
Datelines may adjust in the future but effectively, almost as soon as life was capable of existing on this planet it did. We still don't know how that came about and it remains one of the great mysteries of science.
The Earth has undergone continuous change throughout its history and life has existed upon it under many different conditions. It is hard to know what early conditions were like during the Archean Eon - whether there were high levels of hydrogen, ammonia and methane or high levels of nitrogen or carbon dioxide - but it is generally understood that there was little or no oxygen in the atmosphere (a reducing atmosphere) and only small amounts of land mass. The Archean Earth underwent numerous changes throughout its own long history but for the most part it was a low oxygen water world and this would have affected the look of the planet. There may have been bright green seas (high in iron content) coated with blooms of purple bacteria (Halobacterium) overcast by orange clouds of methane and carbon dioxide. The planet would have looked and been very different to the one we inhabit today.
Life first appeared on Earth at the beginning of the Archean Eon. There are a number of hypotheses about how that might have occurred.
PANSPERMIA. One hypothesis is that life on Earth was brought about by the planet being 'seeded' with organic compounds from the surrounding solar system. This hypothesis, known as panspermia, postulates that amino acids and other building blocks of life - even microbial life itself - were created elsewhere in the Universe, to eventually rain down upon the Earth generating life. Many meteor fragments have been examined for signs of complex organic compounds but with little success to date and so this hypothesis has fallen out of favor. However, bacterial DNA has been collected from the exterior surface of the International Space Station, suggesting that some forms of life may be able to survive in space and recent spectrographic examinations of interstellar dust have revealed some interesting signatures suggestive of possible organic material throughout the cosmos. Proponents of panspermia argue that it would also help explain how life appeared so early in the history of this planet.
PRIMORDIAL SOUP. A second hypothesis based on the Miller-Urey experiment of 1952, looks to the atmospheric conditions that may have existed on t= early planet. These possibly coalesced molecules into a 'Primordial Soup' of elements that were then catalysed into organic compounds (amino acids) by electromagnetic lightning strikes and the Sun's irradiation. Although a few experiments have shown that under the right conditions amino acids can form naturally in this way, this hypothesis has proven hard to model successfully. One of the drawbacks of this model is the lack of a continual supply of nutrient elements that the creation of life would require. It has recently been suggested that radioactive geysers could provide an answer to this particular problem.
HYDROTHERMAL VENTS. A third hypothesis is that life was formed as a result of tectonic plate activity and the associated leaking of minerals and elements into deep ocean trenches via fissures in the Earth's crust. This hypothesis brings together a number of factors and conditions that includes super-heated mineral fluxes precipitating into pressurized cold water, resulting in a concentration of elements and the generation of electrical gradients. In this context it is speculated that a crystalline mineral may have acted as a substrate for the formation of simple chains of amino acid - the presumed first step towards cellular life. This hypothesis would suggest that biology is a direct product of geochemistry. Life arising from non-life in this manner is known as abiogenesis.
However life emerged it happened relatively early in the life of the planet - shortly after the initial geological stabilisation of the Earth's surface. Bacterial mats have now been identified that existed 3.8 Ga and global blooms of single-celled organisms were part of the early Earth's make-up, resulting in such things as the banded iron formations of the world, laid down around 3.5 Ga, and the Great Oxygenation Event of 2.3 Ga
The Earth’s atmosphere is known to have undergone numerous changes during its history - ranging from super-heated greenhouse conditions to frozen ‘snowball’ events. These extreme cycles are thought to have acted like a pressure cooker on the evolutionary development of single-celled organisms, eventually resulting in the emergence of complex multi-cellular lifeforms that are recognizable as creatures to us today.
One of the many puzzles for palaeontology is why after 3 billion years of single-cell evolution did complex multi-cellular life so suddenly appear along with all its associated diversity? The Cryogenian period was the time of the greatest ice age on Earth and it was followed by the Ediacaran period with its assemblage of complex multi-cellular lifeforms. Debate continues about how the two periods are related - did the Cryogenian period facilitate the emergence of theEdiacaran animals?
Another hypothesis is that complex life was made possible by the development of sexual reproduction. When lifeforms began to mix genetic information rather than simply clone information, the chance of variation in genetic coding increased exponentially. Variation underpins evolutionary theory and sexual reproduction would help partly explain the sudden leap into complexity.