The theory of natural selection provides a way of understanding how life, starting from one or a few microorganisms, has evolved over time to give us the immense variety and complexity we see all around us now. But it does not, at least directly, tell us how the very first thing that we can call a living organism came about. Natalie Elliot provides a nice survey into what current research says about the origin of life and she says that this research has also resulted in significant changes in what we mean by ‘life’.
She starts out by reviewing what happens as we trace our ancestors back in time.
Somewhere at the end of the line lies life’s oldest ancestor. This ancestor has acquired a name: LUCA, the last universal common ancestor. It also has a hypothetical nature and place in the biological order of things: LUCA is a microorganism or group of microorganisms from which all life on Earth descends. Though scientists, such as the molecular biologist William Martin of Heinrich Heine University in Dusseldorf, and his team, have been able to infer some part of LUCA’s genetic profile, they don’t have a complete portrait. They also can’t see beyond LUCA: LUCA isn’t necessarily the first life, and scientists can’t see what other life forms could have cropped up before it. Ultimately, LUCA is the living system that scientists identify to say that, at least once, somewhere, spontaneously, life got its start on Earth.
Unfortunately, this road stops there and we cannot see beyond, that she likens to the event horizon of a black hole. We then need to think in a new way about how things that we don’t normally think of as living could organize themselves differently to become the self-replicating entities that we now see as life.
Darwinian natural selection has helped researchers develop hypotheses for thinking about the process by which chemicals organise themselves into living forms. Natural selection, the process that shapes evolution, tells us that, as populations reproduce and change, those species best adapted to their environment survive. Many researchers think that natural selection can also explain the process by which inanimate matter begins to organise itself into living forms. If new species emerge by natural selection, then there might be prebiotic chemical precursors that become capable of evolution – perhaps evolution marks the beginning of life.
The most well known model for how this replication might happen is of course the DNA spiral but she says that it only takes us so far.
Watson and Crick’s discovery was, of course, profoundly significant for evolutionary biology in general, and for molecular biology in particular. But what was its significance for origins-of-life research?
With a mechanism for replication, scientists began to explore the idea that early life, if not the first life, began with the onset of replication. There was one problem, however: DNA couldn’t be the first self-replicator – it couldn’t have emerged spontaneously through the chemicals of early Earth. Once it’s formed, DNA carries the information required for making proteins which do much of the functional work of life, from structuring cells to transmitting signals between organs. DNA also relies on specific kinds of proteins called enzymes in order to catalyse reactions that allow it to copy itself. But proteins weren’t present on early Earth, and they require DNA for their production. If neither DNA nor self-replicating proteins came first, what molecules began the process of replication?
More recently, attention has shifted to the RNA molecules which previously were formerly seen as having a secondary role in the replication process.That search explores whether the appearance of proteins might have signaled the beginning.
In the 1960s, scientists began to consider that one candidate for this process might be ribonucleic acid, or RNA. In living organisms, RNA helps DNA convert its information to the functional products made possible by proteins.
In recent years, RNA world has therefore met with some rival theories about Earth’s first replicators. In 2017, for example, the scientists Elizaveta Guseva, Ken A Dill and Ronald N Zuckermann proposed a theory that protein-like molecules might have been the first replicators.
The physicist England sees things another way. For him, life isn’t a surprising thing at all: it follows naturally from the laws of physics. In his hypothesis, called ‘dissipation-driven adaptation’, the laws of the Universe generate the ordered structure we call life. His theory addresses Schrodinger’s challenge to explain why life doesn’t follow the path of matter in closed systems toward greater entropy, and why it instead becomes more ordered and complex over time. As England explains in a lecture in 2014, and in his forthcoming book, in non-equilibrium systems with a powerful source of energy, such as the Sun, matter necessarily forms structures that help dissipate energy. For living things, one of the most efficient ways to organise in order to dissipate energy is to reproduce. In accordance with England’s theory, life forms increase in complexity not only because they are subject to Darwinian evolution, but also, more fundamentally, because they must improve at dissipating energy.
Hanging over all this research is the question of exactly what we mean by ‘life’ and has led to suggestions that that definition too has to be broadened.
As the evolutionary frame for origins-of-life research has expanded and frayed, so too has the definition of life. Once scientists start thinking about prebiotic chemicals spontaneously organising, the boundaries between living and nonliving begin to blur. For some researchers, such as the evolutionary biologist David Krakauer, challenges to the definition of life are welcome. According to Krakauer, the focus on the replication of forms that we call living things prevents us from thinking biologically about the fascinating array of emergent systems before us – things we wouldn’t say are alive.
These frames make us pay attention to life in different ways. When we recognise the universal ways that matter organises and replicates; when we entertain the possibility that the transmission of information across computational and cultural systems can mark the emergence of life; when we turn to the biosphere as a living system, we begin to look for life in places that often seem inanimate. We look for signs of life in the outer planets or in the interstices of rocks and ice; or we see life replicating in the iterative tapestries of culture. We look for ways that life surprises us. It almost seems as though life emerges precisely when our ideas about it begin to conform to the phenomenon that we are attempting to conceive.
This is a fascinating area of research. The origin of life was at one time considered one of the last great mysteries that might be forever inexplicable. It thus provided hope for religious people who fought the notion that science was making the idea of a deity unnecessary. Along with the origin of the universe, the origin of life was the ultimate ‘God of the gaps’, the explanation they gave for what they thought (or more likely hoped) science would never explain. Of course this ‘explanation’ just consisted of ‘making stuff up’.
But both those questions are no longer mysteries for which we throw up our hands in bafflement, not knowing where to even start. They have transitioned to becoming puzzles that scientists are working on and that is a major step towards finding a solution.