divulgatum está dedicado a la difusión de conocimiento científico en español e inglés, mediante artículos que tratan en detalle toda clase de temas fascinantes pero poco conocidos.
divulgatum is devoted to the dissemination of scientific knowledge in Spanish and English, through articles that go into the details of all sorts of fascinating, if not
widely known, subjects.

Sunday, May 7, 2017

On the universality of life

Cyanobacteria were among the earliest life forms on Earth, and are still found in almost every habitat. Over millions of years, they created the atmospheric oxygen that we breathe today.
(Image credit: Josef Reischig/Wikimedia.)

HOW FREQUENT IS life in our universe? The answer depends, primarily, on how easy it is for life to arise in the first place.

Even in the case that life had the smallest imaginable chance of occurring — while not being impossible, as we can tell for ourselves — we would certainly not be alone in the cosmos. The sheer size of our universe simply rules out the possibility that ours is the only planet to harbour life. If only one in a trillion stars were to have an orbiting planet in conditions such that the emergence of life were possible — these relate to the presence of liquid water, which is thought to be one of the few essential requirements for life as we know it — the number of stars in the universe is so inconceivably huge that there would exist billions of such habitable worlds throughout the cosmos.

How life on Earth began, how matter suddenly self-organised into autonomous systems with the ability to perpetuate themselves over time, is undoubtedly one of the most fascinating questions ever presented to human curiosity. It is also among the most hotly debated ones, and there are still a number of possibly valid answers to it. The most widely accepted one is that life, by what seems an unimaginable stroke of luck, at some point spontaneously assembled itself from the different essential molecules, such as lipids and amino acids, that where ubiquitous in the hot primordial seas of young Earth. Alternatively, if life on our planet did not arise on its own, then it must have been introduced from outer space, perhaps as frozen primitive cells which hitched a long ride on a comet or an asteroid. Nonetheless, in most scientists’ view, it is far more likely that some extremely primitive form of life arose by fortunate assembly of organic molecules — which is not that fortunate after all, given that it took hundreds of millions of years for this to happen — rather than the aggregate probability that life originated in some other part of the universe and was subsequently brought to Earth by whichever means.

By studying nature over centuries, we have gradually worked out the set of intangible rules that govern all forms of life on Earth. Such rules, I believe, are directly derived from the physical laws that permeate our cosmos, and are therefore universal. If the fundamental laws of physics, such as those which describe the gravitational and electromagnetic forces, seem to apply evenly throughout the known universe, then the principles of life, being an elaboration of the former, should be equally valid in any setting where life is viable, provided that there is sufficient time for life to arise. Although long time spans seem to be required for the emergence of life, once this occurs, life must abide to the very same laws that direct it on our own world. If we were to gather these laws under a single name, we should turn to what is undoubtedly one of the most transformative concepts in the history of science — so much so that some persist to reject it still today, in quite the same way as others in the past rejected the notion that the Earth is not at the centre of the universe. The concept’s name is, of course, evolution.

As an omnipresent life-powering process, evolution arises from the combination of various properties of life itself. The first of these is life’s necessity to replicate itself in order to avoid death, for even an ideal organism that is not affected by ageing, given sufficient time, will ineluctably die of some cause. The process of replication of living things is bound to be imperfect, due to the impossibility of perfection being an intrinsic and evident property of our universe. Perfection is a concept devised by humans; in reality, there exists no perfectly smooth surface, no perfectly uniform substance, no matter or process which is free from the smallest degree of physical imperfection, anywhere in the cosmos. In fact, there are no such things as ‘perfection’ and ‘imperfection’ in the universe; anything that abides to its physical laws is just fine. The impossibility of perfection implies that, instead of being identical copies of each other, organisms will accumulate minute differences over generations, thus introducing variation, which is a central ingredient of evolution, in their populations.

Secondly, because of the pressures imposed by the environment in which organisms live — which can often be reduced to the restless struggle to eat and not being eaten — such differences between creatures of a same kind will make them not just different, but better or worse than others at playing the game of life. Specifically which individuals are better, however, is something that can vary at any time, in response to the whims of an ever-changing environment. One of the fundamental principles of life, which is but an application of simple causality, prescribes that those individuals which are better adapted to the environment will survive longer and replicate more, gradually displacing the less-fit individuals in their population and slowly shaping their species over time. This phenomenon, which seems obvious today, yet took thousands of years for us to grasp, is called natural selection. And, together with variation, it is the driving force behind evolution.

Evolution is therefore the consequence of two unavoidable phenomena in life: first, variation between living things, caused by imperfections in the processes by which these replicate themselves; and second, natural selection of those individuals which are better equipped for survival. Because variation provides the ‘pool’ of possible forms from which natural selection can ‘pick’, those organisms which are constantly exposed to very hostile environments, such as viruses or cancer cells, tend to vary extremely quickly, by favouring imperfect replication systems that allow them to adapt and evolve faster.

It is therefore sensible to affirm that, wherever in the universe life may arise, even if we cannot predict many of its qualities, we can be certain that it will abide to the universal laws of replication, variation and selection; quite simply because these are the only possibility for the survival of life, just as gravity is the only possibility for the existence of stars and planets. Moreover, there are some physical characteristics of living things that could indeed be predicted under certain conditions. For example, even if the concrete shape and lifestyle of alien organisms is unknowable to us, we can safely say that they will need to gather some information about their surroundings — they will need some way to sense. If we accept that life will occur within a fluid, such as water or air, it is virtually sure that, as their complexity increases, living beings will soon evolve some means of sensing mechanical or electrical changes in such fluid. If life is exposed to the light of a star — which is needed for liquid water to exist — evolution will no doubt come up with sensors to detect changes in this light: eyes, as simple or complex as we may imagine them. Even very basic organisms, such as the mould or cyanobacteria in our world, will benefit from some chemical way to learn, at least to an elementary degree, what is going on in their immediate vicinity. This is so important an advantage, that some of these sensors have already arisen multiple times here on Earth. For instance, the eyes of an octopus and our own eyes are not evolutionarily related — that is, the eyes of vertebrates and cephalopods were independently evolved — and yet they perform the same exact function and are staggeringly similar. This phenomenon, termed convergent evolution, gives a hint of just how powerful a life-shaping force natural selection can be.

It is also enlightening that, despite evolution sometimes giving rise to complex life forms, we find a striking variety of very simple organisms living around us today — so simple indeed that we cannot even see them. The fact that a huge diversity of microorganisms, including bacteria and viruses, still thrive on Earth four billion years after the beginning of life is a clear sign that complexity is not a requisite for survival. This is especially true in the case of intelligence, which we tend to regard as the pinnacle of evolution. Complexity, particularly in the case of advanced intelligence, is a very expensive characteristic to develop and maintain for a species, and therefore evolution does not go down that path very often; otherwise, there would be many more complex and highly intelligent species around us. Moreover, with all of man’s supposed superiority over all other life forms, one must consider the actual likelihood that our species will still be here in just a million years from now. In my view, it is difficult to overstress how unlikely our long-term survival is, especially as we are already failing to manage the development of our own populations, as well as the unsustainable overuse of natural resources which we regard to be infinite. On the other hand, the simplest life forms on this planet have been around almost since the beginning of life, and are likely to stay until the day Earth becomes completely uninhabitable.

If an overdeveloped intelligence is probably not very suitable for long-term survival, it does spark a unique phenomenon — the temporary alteration of the rules of life. Thanks to the development of our societies, technology and medicine, those who, according to nature’s dictates, are less fit for survival, nevertheless have an opportunity to defy natural selection. In other words, we have used our intelligence to bypass the very laws which dictate who survives and who perishes; and by doing so, although making life much fairer from our standpoint, we have effectively arrested the progress of our evolution. We have in fact just begun treading our own evolutionary path, which will be largely independent of the natural environment around us; the threats to our survival are no longer to come from the natural world, but rather from the artificial world that we have built for ourselves.

I maintain that life is far from a one-off accident. Life is a property of our cosmos. Given the appropriate setting, it will naturally occur, in the same way as stars, nebulae or black holes are triggered by a specific series of conditions and events. Our knowledge of life on Earth tells us that the laws which define such life, whatever its shape, size or ecological role may be, are but a natural consequence of the imperfect character of the universe and the plain interactions between life forms and their environment (including other life forms). Together with the unfathomable scale of the cosmos, of which Earth itself is just an unremarkable, negligible blue atom, the simplicity and universality of these principles indicate that life is no uncommon thing in our universe. On the contrary, life must be everywhere around us; although it may be too far away, buried in the void reaches of space, for us ever to discover it.