In many ways, the question of life's origin can be compared to a jigsaw puzzle. Both are confined by some sort of boundary and there are clues as to where the pieces go. Despite these similarities, why is the origin of life so difficult to solve?
Grab a cup of coffee (or your drink of choice) and let us explore what makes the origins of life such a challenging problem and how scientists from different disciplines have joined together to tackle this riveting mystery.
Saying that solving the origin of life is trickier than a jigsaw puzzle would be the definition of an understatement. While both have their boundaries, connecting the dots between the first principles of chemistry and physics and the complexity that is biology is no trivial task. For example, almost every reaction in the cell is driven by one or more enzymes. These reactions, many of which organic chemists would struggle to achieve reasonable yields and efficiency can be performed by enzymes in a matter of seconds. However, current enzymes are made by enzymes that came before. These polymers, comprised of specific sequences from 100s to 1000s of units long, are extremely complex. Most scientists would agree that the chemistry prior to the origin of life is insufficient. This chicken-or-egg paradox is only one among the many, many gaps of knowledge that needs to be addressed before we can unravel the mysteries surrounding the origins of life.
Unlike jigsaw puzzles, where an image is provided with all the pieces present, most points of reference that could have been served as hints to the origin of life have been buried by the sands of time, leaving but a few disparate fragments. Over time, destructive forces such as degradation and weathering cause most evidence to disappear without a trace. Since it is not possible to travel back to when and where life started, there is no certainty with regards to the environment from which life emerged or the processes and chemicals that were present. Moreover, geochemical processes could have taken hundreds to thousands of years (or more) to kickstart life. In other words, humans don't live long enough! Even if we do, it would be near impossible to request funding for such an intangible project.
Despite these hurdles and many more, the question of how life originated has captured the curiosity and interest of many scientists, including myself. Research in this field is less interested in finding the exact history of how life emerged on Earth (since such an endeavour is impossible) and more in the necessary qualities that drive the transition from non-life to life. As Ram Krishnamurthy, a highly-regarded prebiotic chemist, eloquently wrote: “What scientists are trying to comprehend is how chemicals ‘can’ (and not ‘did’ or ‘could’) transform themselves into a functioning collection of molecular systems that can exhibit biological behaviour”. Unravelling a question as far-reaching as the origins of life requires the intellect of scientists across different fields of research, some of which will be highlighted below.
Planetary sciences afford the understanding of how planetary systems form and the processes involved. These studies combine computational simulations, space exploration, and experiments to shed light on the formation of our Solar System (and others), including what chemical feedstocks were present and estimate their abundance. The work of planetary scientists has led to our current understanding of how the Earth and Moon formed 4.5 billion years ago and the composition of Earth’s early atmosphere. Studies of the molecular entities on asteroids, comets, and other planets and their chemistry also fall under this category.
Geology studies rock records and the processes by which they are formed, providing evidence for some of the earliest signs of life. For example, the discovery of banded iron formation, a type of rock that is formed due to the presence of oxygen, allowed scientists to estimate when photosynthesis first emerged on the early Earth. Geologists also investigate environments that could have provided the right conditions to support the emergence of life, such as hydrothermal vents and hot springs. Their findings, in turn, provide prebiotic chemists with clues about different physical conditions and processes possible on the early Earth.
Prebiotic chemistry is the study of the chemicals and reactions that could have potentially happen on the early Earth in order for life to emerge. Since its initiation after the Miller-Urey experiment in 1953, prebiotic chemists have, for the most part, focused on the synthesis of biomolecules. While ribonucleotides, amino acids, and fatty acids (the building blocks of RNA, protein, and cellular membrane, respectively) are often sought after, other metabolites or compounds not utilised by modern biology are also investigated. This is because it is possible that the chemistry that gave rise to life on the early Earth bears little resemblance to modern biology. The complex mechanisms that regulate life today were not present back then, and thus it’s not inconceivable that a different set of mechanisms gave rise to primitive life which, through evolution, developed its own mechanisms to interact with the environment, grow, and replicate.
Even though biology is the study of life, its role in demystifying life's origin has been somewhat constrained. This is because even the simplest forms of life, bacteria, consist of a highly complex network of compounds that cannot easily form on the early Earth, especially without enzymes, a separate rabbit hole unto itself. Although this large gap in scale and complexity between chemistry and biology prevents the latter from playing a more active role in origins of life research, its role cannot be understated. Biomolecules and metabolic intermediates serve as end-goals for many prebiotic chemists and the use of metal ions in the cell inspired geologists to investigate the catalytic effects of different metal-bearing minerals.
Efforts in these fields and many others have made steady progress in our understanding of how life may have emerged. As our understanding evolves, so does the research. Bridges have been built to connect the walls which initially separated individual disciplines, paving the way for interdisciplinary collaborations. While the path towards a complete understanding is still far away, the words of Leslie Orgel, one of the founding fathers of the ‘RNA world hypothesis’, serve as an encouragement. “Anybody who thinks they know the solution to this problem (of the origins of life) is deluded. But, anybody who thinks this is an insoluble problem is also deluded”.