If we are going to talk about biology, and organisms and cells and such, we have to define what we mean by life. This raises a problem peculiar to biology as a science. We cannot define life generically because we know of only one type of life. We do not know whether this type of life is the only type of life possible or whether radically different forms of life exist elsewhere in the Universe or even on Earth, in as yet to be recognized forms.
While you might think that we know of many different types of life, from mushrooms to whales, from humans to the bacterial communities growing on the surfaces of our teeth (that is what dental plaque is, after all), the closer we look the more these different “types of life” are in fact all versions of a common underlying motif: they represent versions of a single type of life. Based on their common chemistry, molecular composition, cellular structure, and the way that they encode hereditary information in the form of molecules of deoxyribonucleic acid (DNA), all topics we will consider later on, there is no reasonable doubt that all organisms are related; they are descended from a common ancestor. We will have to deal with viruses, too. Are they life and do they share an common ancestor with the rest of life?
We cannot currently answer the question of whether the origin of life is a simple, likely, and predictable event given the conditions that existed on the Earth when life first arose, or whether it is an extremely rare and unlikely event. In the absence of empirical data for life elsewhere, it is challenging for astrobiologists to rigorously use scientific methods to search for something - life - that might not exist. That said, asking questions that are seemingly impossible to answer, provided that empirically-based interpretations can be generated, has often been the critical driver of scientific progress. Consider, for example, current searches for life on Earth, almost all of which are based on what we already know about life. Specifically, many methods rely on the fact that all known organisms use DNA to encode their genetic information; these methods would not be expected to recognize dramatically different types of life; they certainly would not detect organisms that used a non-DNA method to encode genetic information. Alternatively, we also look for evidence of metabolism - chemical reaction catalyzed by life. If metabolism occurs on a different time scale than our observations, we will not recognize it as a product of life. If we could generate living systems de novo in the laboratory or if we better characterize organics on other planets or moons, we would have a better understanding of what functions are necessary for life and how to look for possible “non-standard” organisms using better methods. It might even lead to the discovery of alternative forms of life on Earth, if they exist.28 We also lack realistic models for alternative forms of life; even though scientists have studied alternative chemical and organic systems, no one has yet been able to construct a model that predicts life-like behavior for any system significantly different from life as we know it on Earth. That said, until someone manages to create or identify such non-standard forms of life, even in a theoretical model, it seems quite reasonable to concentrate on the characteristics of life as we know them from Earth.
Life as we know it from Earth
So, let us start again in trying to produce a good definition for life, or given the fact that we know only of one version of life, a useful description of what we mean by life. First, the core units of life are organisms, which are individual living objects. From a structural and thermodynamic perspective, each organism is a bounded, non-equilibrium system that persists over time and, from a practical point of view, can produce one or more copies of itself. Even though organisms are composed of one or more cells, it is the organism that is the basic unit of life. It is the organism that produces new organisms.
Why the requirement for and emphasis on reproduction? This is basically a pragmatic criterion. Assume that a non-reproducing form of life was possible. A system that could not reproduce runs the risk of death (or perhaps better put, extinction) by accident. Over time, the probability of death for a single individual will approach one – that is, certainty. In contrast, a system that can reproduce makes multiple copies of itself and so minimizes, although by no means eliminates, the chance of accidental extinction, the death of all of its descendants. We see the value of this strategy when we consider the history of life. Even though there have been a number of mass extinction events over the course of life’s history,31 organisms descended from a single common ancestor that appeared billions of years ago continue to survive and flourish.
Organisms have a structure. which confines certain chemical reactions, but they also exchange resources with their environment. So what does the open nature of biological systems mean? Basically, organisms are able to import, in a controlled manner, energy and matter from outside of themselves and to export waste products into their environment.32 This implies that there is a distinct boundary between the organism and the rest of the world. All organisms have such a barrier (boundary) layer. The basic barrier for all organisms (except viruses - if they are organisms) is a cell membrane, and it appears to be a homologous structure – that is, cell membranes were present in and inherited from their common ancestor. The cell membrane modulated the transport of chemicals and thus energy. The importation of energy, specifically energy that can be used to drive various cellular processes, is what enables the organism to maintain its non-equilibrium state and its dynamic structure. The boundary must be able to retain the valuable molecules generated, while at the same time allow waste products to leave. This ability to import matter and export waste enables the organism to grow and to reproduce. We will address the ideas of metabolism - energy producing reactions - later in this course.
We see evidence of the non-equilibrium nature of organisms in their activities, including their ability to move, to change their environmental chemistry, and to grow. It is important for all aspects of the living state. In particular, organisms use energy, captured from their environment, to drive a wide range of thermodynamically unfavorable chemical reactions. These reactions are coupled to thermodynamically favorable reactions. An organism that reaches thermodynamic equilibrium is a dead organism because it no longer has energy to live.
There are examples of non-living, non-equilibrium systems that can “self-organize” or appear de novo. Hurricanes and tornados form spontaneously and then disperse. They use energy from their environment, which is then dispersed back into the environment, a process associated with increased entropy. These non-living systems differ from organisms in that they cannot produce offspring - they are the result of specific conditions. They are individual entities, unrelated to one another, which do not and cannot evolve through inheritance. Tornados and hurricanes that formed billions or millions of years ago would (if we could observe them) be similar to those that form today because they emerge from the same processes. Since we understand (more or less) the conditions that produce tornados and hurricanes, we can predict, fairly reliably, the conditions that will lead to their appearance and how they will behave once they form. In contrast, organisms present in the past were different from those that are alive today. The further into the past we go, the more different they appear. Some ancient organisms became extinct, some gave rise to the ancestors of current organisms. In contrast, modern tornados and hurricanes originate anew, they are not derived from parental storms.
Question to answer and ponder:
- Speculate on how you might look for evidence of life on another planet or moon. What are characteristics might be universal to all life and what might be specific to life on Earth?
- Are viruses alive? You might not know much about them now, so feel free to speculate. How would you look for evidence of something like a virus on another planet or moon?
- Make a model of what properties a biological boundary needs to possess. Using your current knowledge, how would you build such a boundary layer?
- Ponder the non-equilibrium nature of a specific form of life, maybe your pet or something from your favorite natural area. What about it tells you it is alive? What happens to it when it dies? How does that relate to thermodynamics and equilibrium?
- Ponder a non-living structure that has a pattern. How can you tell that it is not living? For example, how might you be able to demonstrate that the great red spot on Jupiter is not a life form?
- What do you think about considering Earth and all the life on it as a single organism, similar to the Gaia concept? What aspects of the Earth-life system fit a traditional definition of life and which do not?
28 The possibility of alternative microbial life on Earth: http://www.ncbi.nlm.nih.gov/pubmed/18053938 Signatures of a shadow biosphere: http://www.ncbi.nlm.nih.gov/pubmed/19292603; Life on Earth but not as we know it (with the caveat that this is a relatively old article; we now know that most of life in desert varnish is related to the rest of know life; however, this article is still an interesting example of the arguments that scientists have on what is actually present on Earth): https://www.theguardian.com/science/2013/apr/14/shadow-biosphere-alien-life-on-earth