Understand essential properties of life
Distinguish between living and non-living systems
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. However, asking questions that are seemingly impossible to answer, provided that empirically-based interpretations can be generated, has often been a significant driver of scientific progress.
The techniques scientists use to detect and characterize life depend on our current understanding of life. Essentially all of the methods are based on what we already know about life. When we search life in extreme environments on Earth, for example, we often rely on the fact that all known organisms use DNA to encode their genetic information; we use methods to detect and characterize that DNA. However, these methods would not detect organisms that used a non-DNA method to encode genetic information; DNA-based methods would not be expected to recognize dramatically different types of life. Alternatively, we can look for evidence of metabolism - chemical reactions catalyzed by life. However, if metabolism occurs on a different time scale than our observations, we will not recognize it as a product of life.
One challenge with developing alternative methods for detecting dramatically different types of life is that we do not know what to look for. We need models of water alternative life might look like to be able to recognize it. 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. The research might even lead to the discovery of alternative forms of life on Earth, if they exist.28 We can also work to develop 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. In the absence of a model for non-standard forms of life, even a theoretical one, the reasonable scientific approach is to concentrate on the characteristics of life as we know them from Earth, while keeping in mind that we may be excluding other types of life in the Universe.
Properties of life
So, let us try 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. From a practical point of view, an organism needs to be able to produce one or more copies of itself, and it consumes energy to do so. Even though organisms are composed of one or more cells, the organism that is the basic unit of life because it is the organism that reproduces.
Why do we include reproduction in our definition of life? This is basically a pragmatic criterion: organisms have to be able to reproduce to persist through time. 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 common ancestor that appeared billions of years ago, and they continue to survive and flourish - and reproduce.
Metabolism and structure
Metabolism consists of the chemical reactions that occur within organisms that allow them to persist. These reactions are essential to life because they provide energy and raw materials to make the cell structure, control the exchange of materials with the environment, and allow reproduction. Metabolism is the foundation of how life works. Much of an organism's structure is related to facilitating metabolism. Organisms confine their metabolic reactions to within cells, but they also exchange resources with their environment. Organisms are able to import, in a controlled manner, energy and matter from their environment and to export waste products into their environment. To control these fluxes, a distinct boundary between the organism and the rest of the world is required. The basic barrier for all organisms (except viruses - if they are organisms) is a cell membrane, and it appears to be a homologous structure across all organisms – that is, cell membranes were present in and inherited from the common ancestor to all life. The cell membrane modulates the transport of chemicals into and out of cells, and is thus critical to metabolism. The importation of chemicals to produce 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 with the environment 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.
Metabolism allows organisms to persist in disequilibrium with their environment. 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 thermodynamically favorable reaction to capture energy from their environment. They use this energy to drive a wide range of thermodynamically unfavorable chemical reactions, creating disequilibrium with their environment. An organism that reaches thermodynamic equilibrium is a dead organism because it no longer has energy to live.
It is often useful to think of systems that have some of the properties of life, but are not alive. There are examples of non-living, non-equilibrium systems that can “self-organize” or appear de novo. Hurricanes and tornados form spontaneously and then dissipate. 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 environmental 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 that lived 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 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 with its environment 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