Complexity of Life and Biological Regimes
Discussion
"The emergence of life implied the rise of a fundamentally new mechanism for achieving complexity. Unlike stars and galaxies, biological regimes do not thrive because they convert matter into energy within themselves from existing supplies. Life needs to continuously tap matter and energy flows from its surroundings in order to maintain itself and, if possible, reproduce (Lehninger 1975: 3–4). If living creatures were not to do so, they would very soon die and disintegrate. This is not a new insight. Already in 1895, the Austrian physicist Ludwig Boltzmann stated that all life is a struggle for free energy (quoted in White 1959: 34). Many academics have followed in Boltzmann's footsteps (for an overview, see White 1959: 34 ff.).
Unlike stars, living cells extract matter and energy from their environment and rework them at very moderate temperatures and pressures, while utilizing very complex molecular machinery. In addition, all the biochemical compounds produced by cells can be said to fulfil functions for either their own survival and/or for the survival of the entire organism. These are major differences between physical and biological complexity. All living organisms survive by using hereditary information, with the aid of which they program themselves. I therefore propose to define life as ‘a regime that contains a hereditary program for defining and directing molecular mechanisms that actively extract matter and energy from the environment, which matter and energy is converted into building blocks for its own maintenance and, if possible, reproduction’.
The Emergence of Life
We do not know how and when life first formed. Claims for the earliest evidence for life dating back to about 3.8 billion years have recently been challenged. Firm evidence for terrestrial life is about 3 billion years old. Given the fact that the Earth was formed some 4.8 billion years ago, there may, or may not, have been a long period of physical and chemical evolution leading to the rise of early life. Neither do we know whether life actually formed spontaneously on the Earth, or whether it was transported to us from elsewhere by whatever celestial object happened to dive into our atmosphere. If life did originate elsewhere in the Universe, we do not know where, when and how this happened.
If life originated on our home planet, more likely than not it was preceded by a long process of increasing physical complexity on the Earth's surface. This process is usually called chemical evolution. Under the influence of energy flows such as sunlight, volcanic activity, lightning and perhaps radioactive decay, increasingly complex molecules would have formed. At a certain point in time, a spontaneous process of self-organization leading to life would have kicked in. Next, Darwin's mechanism of natural selection would have started acting as a filter, allowing fitter organisms to produce more, and/or more efficient, offspring than others. This produced a selection for organisms that became both increasingly better at tapping matter and energy flows from their environment and at preventing themselves from becoming sources of matter and energy for others.
Early life may well have fed on the products resulting from chemical evolution. For a while, this would have provided enough matter and energy to survive and, if possible, reproduce. Yet after a certain period of time, life would have consumed more chemical soup than was formed anew. In the long run, therefore, chemical evolution could not possibly have sustained life. The earliest living blobs may also have extracted matter and energy from underwater volcanoes, the so-called black smokers. Such situations can be found today and may well have existed throughout the history of life on the Earth. And, as long as black smokers kept smoking and as long as no major mishaps took place, the continuity of life in such locations was assured.
Ever since the origin of life, the presence of sufficient water has been an absolute requirement for its continued existence. Without it, the matter and energy flows needed for life's sustenance could not have existed. Up until today, the distribution of water on our planet has set the boundary conditions for the areas where life and culture can develop. This suggests that life originated in the oceans, especially since the overall salt concentration within cells is very similar to that of the modern oceans (and, more likely than not, that of the ancient oceans also). In those early circumstances, the salt concentration of the pioneer cells could not have been very different, for that would have generated elevated energy differentials which would have destroyed those early cells almost immediately. Over the course of time, especially after life moved out of the seas onto land, such energy differentials did develop. As a result, mechanisms had to be evolved to protect cells against this new and hostile environment.
Early life forms were comparatively simple and could, therefore, handle only comparatively gentle energy flows. Yet these organisms must also have been pretty robust, because they were able to live under conditions of far higher external energy levels and flows than the ones which prevail in most places where life thrives today. Temperatures were higher; radioactivity and volcanism were far more prevalent than nowadays. Moreover, the Earth was bombarded by meteorites of many different sizes. Clearly, early life must have been adapted to these circumstances from the very beginning.
Increasing Complexity
Living organisms are regimes which maintain a relatively stable steady state. This comparative stability over billions of years allowed sufficient time for many types of greater complexity to form both within and among cells. Not unlike the building blocks of most physical regimes, the basic construction kit of life consists mostly of spheres, the cells. This is not the result of gravity but of the fact that the molecules which make up the skins of cells attract one another and as a result cause surface tension. Since gravity does not play a major role in the formation and sustenance of cells, their interiors could become very complex.
At a certain point in evolution, some cells began to cooperate in harnessing matter and energy. Some of these cells may have adapted to others to the extent that they became mutually dependent yet remained biologically separate. This inter-species division of labour is perhaps the most common form. Other cells may have fused into larger complexes, which led to forms of intra-cellular division of labour. Such cells may have emerged about two billion years ago. Over the course of time, this led to the emergence of even more complex eukaryotic cells, which could handle far greater matter and energy flows than their more humble cousins, the prokaryotic organisms.
In eukaryotic cells, the nucleus serves as the hereditary storehouse. Organelles such as mitochondria specialize in energy metabolism, while chloroplasts devote themselves to capturing sunlight and converting it into energy. Because eukaryotic cells became more versatile as a result of this intra-cellular division of labour, they became the building blocks for all greater biological complexity. Yet many organisms remained small and comparatively simple. These are micro-organisms we know today. As a result, the tree of life differentiated into increasing numbers and shapes.
Another way of achieving greater complexity consisted in increasing the cooperation among cells with the same genetic make-up. At a certain point in time, such cells began to hang together. Both prokaryotic and eukaryotic cells were able to do this. But over the course of time, only eukaryotic cells learned how to cooperate and divide tasks. I call this latter process the inter-cellular division of labour. As a result of the inter-cellular division of labour, cells within one single organism began to differentiate. This allowed for greater levels of complexity. The selective force that drove such processes consisted of the new opportunities this division of labor offered to improve the extraction, and use, of matter and energy. As a consequence, ever more new life forms began to emerge with increasingly intricate shapes. Gravity, however, still sets the upper limits on the size and shape of life forms. It is no coincidence that the biggest living bodies developed in the oceans, where buoyancy and gravity balanced one another to a considerable extent.
Here we see a major difference between the differentiation of biological regimes and of physical regimes. All more complex life forms exhibit a clear differentiation of both forms and functions within their own regimes. Physical regimes, by contrast, do show a differentiation of forms but not of functions. Galaxies, for instance, consist of a great many different objects. But to say that all the stars and whatever objects galaxies consist of actively fulfil functions for one another in order for the galaxy to exist and thrive does not make any sense to me.
We do not know how stable micro-organisms are in an evolutionary sense. There are some hints of great stability. In the shallow waters off the Western Australian coast, for instance, the so-called stromatolites may have existed for about three billion years. Stromatolites are basically mounds of micro-organisms that cluster together. Single cells living in the oceans may well have been rather resistant to change also, because their environment would have not have altered a great deal during the past three billion years or so. In other words: comparatively stable matter and energy flows in the environment may well have caused comparatively little evolutionary change.
Yet evolution by chance, caused by random variations in the genetic program which proved to be advantageous in terms of survival – or at least not disadvantageous – has led to an ever-growing range of organisms, especially when the environment changed. In actual fact, the process of evolution itself has also changed the environment which, in its turn, would have stimulated the emergence of new species. This led to feedback loops that might well have speeded up evolution. As a rule, the more energy a species could extract from the environment, the more complex it became, and vice versa.
Tapping New Energy Flows
Over the course of time, life has succeeded in maintaining itself and spreading all over the world, including too many places that did not offer a free chemical lunch. This could happen because micro-organisms and later plants evolved that were able to exploit sunlight. This energy was used for combining the atoms of carbon dioxide and water into a great many organic substances, which became the building blocks of life. We do not know how life learned to exploit sunlight for its own purposes. But, surely, mastering this art laid the foundation for all further biological complexity.
In this process called photosynthesis, free oxygen is released. It may have taken two billion years, but eventually this led to an oxygen-rich atmosphere. Subsequently, through respiration the internal combustion of organic matter with the aid of atmospheric oxygen became the major energy source for animals. Over the course of time, photosynthesis would, therefore, provide most of the energy that drove biological evolution. The oxygen-rich atmosphere allowed for the formation of the stratospheric ozone layer, which started to protect life against ultraviolet radiation. Up until that time, the energy flow of sunlight had suppressed the rise of biological complexity on land. Now, for the first time, life could leave the cradle of its protective watery surroundings and begin to colonize the entire planet.
The rise of an oxygen-rich atmosphere created another new type of energy differential. First of all, it provided energy for organisms that did not participate in the process of photosynthesis, both in the water and on land. But, perhaps even more importantly, it made possible the emergence of ever larger and more complex multi-cellular complexes. This was the case because oxygen could be transported to cells that were not in direct contact with the outside world. They could thus share in the exploitation of energy differentials. All the organisms that could not cope with the rise of the oxygen-rich atmosphere and the associated rise of energy differentials had two options. The first one was to limit themselves to places where the oxygen concentration remained low enough to handle. The second option was to become extinct.
The general trend seems clear: the more intricate biological regimes became, the greater the matter and energy flows were that they could tap. Apparently, over the course of time, biological evolution has created structures so intricate that they can handle increasingly larger matter and energy flows, at least for a time, without being destroyed by them (Christian, pers. com., 2003). The price to be paid for greater complexity was a growing vulnerability when the conditions changed. The huge matter and energy flows caused by volcanic eruptions and the impacts of extraterrestrial objects especially could spell the end of more complex organisms. In such circumstances, their less complex fellows appear to have had better survival chances. As a consequence, the life span of the more complex species as a whole decreased. In other words, the more complex species became, the quicker they became extinct. The overall result was the emergence of growing numbers of short-lived species exhibiting ever greater levels of complexity.
The Cambrian Explosion of Life
About 540 million years ago, the above developments led to the so-called Cambrian explosion of complex life forms. A great variety of multi-cellular complexes suddenly emerged, endowed with an ever greater variety of organs, all of which began to fulfil functions for one another to make it easier for the whole to survive and thrive. This led to the types of complex living organisms we are familiar with today.
The Cambrian explosion of life may have been caused by sudden changes of energy flows and levels on the Earth's surface. It seems that right before the Cambrian era, the Earth's surface had frozen over almost completely. This would have severely restricted the room for terrestrial life and may have wiped out many individuals and perhaps entire species. When for reasons yet unknown the big thaw began, suddenly a huge new niche opened up for the lucky survivors and their offspring (Walker 2003).
During the Cambrian explosion of life, two general types of complex organisms came into being that have continued to exist up until today. On the one hand, there are the ancestors of modern plants. They extract their energy from sunlight and their chemical elements from soil or water. With some exceptions, such organisms do not eat other organisms. Since they do not need to move and catch prey, they lack brains. Some parts of plants are actively involved in extracting energy. They tend to position themselves in ways that are the most favorable for capturing the right amount of sunlight. For the same goal, their photosynthetic mechanisms as well as their production of pigments are continuously fine tuned. According to Eric Chaisson, modern plants handle power densities of about 0.09 watt/kg (2001: 139).
On the other hand, there are animals. These are basically species feeding on other organisms. For the lucky ones, this implies the appropriation for their own purposes of supplies of energy and matter gathered by other creatures. The eaters use this energy constructively for themselves. Yet they became increasingly destructive for the unlucky ones that were eaten. During the process of evolution, therefore, living species became both increasingly constructive and destructive.
Since animals need to eat plants and/or other animals, they developed ways of purposefully moving around, including brains. They needed weapons to defeat their prey and suitable digestive tracts in order to eat them. As part of this process, animals became better at extracting both matter and energy. This meant that their power densities should be much greater. And, sure enough, according to Eric Chaisson, the power densities of modern animals would be in the order of 2 watt/kg (2001: 139). As a result, animals also became greater potential sources of matter and energy for others. In order to survive, they needed to develop ever better ways of defending themselves. Plants also began to defend themselves against predators, for instance, by producing toxins. The overall result was an increasingly complex biological regime consisting of ever more and more different species. Within this constantly changing regime, an increasing variety of matter and energy flows was exploited. This constant search for sufficient matter and energy in order to survive and thrive has been the major factor that has driven biological evolution up until today.
The development of a biological waste disposal regime must have been an absolute precondition for the continued existence of life on this planet. Without it, life would have choked in its own dirt a long time ago. One may wonder whether the rise of a biological waste disposal regime was an almost inevitable component of the successful evolution of life on our planet. It is not inconceivable that elsewhere in the Universe, life got kick started only to find itself being drowned by its own waste. Here we see another great difference with physical regimes. Although the Universe as a whole does function as a huge entropy trash can, galaxies, stars or planets have never evolved such garbage solutions of their own."