"Nature abhors a vacuum" (Aristotle)
The origins of life remain cloaked in mystery. Whether it is a universal occurrence or an isolated phenomenon, life’s enigmatic beginnings continue to elude us. Yet over the last two centuries great strides have been taken towards a fuller understanding of our own place within the history of life on this planet. It has become clear that we are the product of ceaseless evolutionary processes.
It can be startling to consider that each one of us represents an unbroken lineage, a chain of survival and reproduction, which spools back some 4 billion years to a single common ancestor - to LUCA the Last Universal Common Ancestor. Not one of our personal antecedents died before successful reproduction throughout all those eons. So 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 currentlythought to have emerged somewhere between 4.0 Ga and 3.8 Ga during the early Archean period.
Effectively, almost as soon as life was capable of existing on this planet it did so. How did that come to be? What are the evolutionary forces underpinning our own existence?
There are several hypotheses about how life might have first arisen on Earth.
PANSPERMIA. One hypothesis is that elements for life rained in upon the early planet - 'seeding' life 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 in on Earth generating life. A number of asteroids and carbonaceous meteorites have now been examined for signs of complex organic compounds and some have indeed been discovered. Recent spectrographic examinations of interstellar dust have further revealed that organic compounds abound throughout the cosmos. Bacterial DNA has also been collected from the exterior surface of the International Space Station, suggesting that some forms of life may be able to survive in space. Proponents of panspermia argue that it would help explain how life appeared so early on 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 the early planet. It is thought that these conditions possibly coalesced molecules into a 'Primordial Soup' of elements that were then catalyzed into complex organic compounds (amino acids) by electromagnetic lightning strikes. This hypothesis has proven hard to model successfully although recent experiments that incorporate photon irradiation from a volatile young Sun with modified atmospheric composition have been successful in generating amino acids - the first step towards life. The primordial soup hypothesis - or variation upon - is probably the one most supported by the scientific community. One of the problems however with this model is the lack of a continual supply of energy and 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.
The Earth is known to have undergone numerous changes during its long history - ranging from super-heated greenhouse conditions to frozen ‘snowball’ events. These climactic changes have been punctuated by many extinction events, both large and small, and collectively these forces 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.
