Have you ever thought about what life actually is? Sure, we can instinctively point at a flower and call it alive, but how do we know? On the flip side of the coin, there are many things that exhibit similar characteristics to living organisms, but we somewhat intuitively know that some things, like fire, are not alive. When we talk about life, what do we mean? On the most fundamental level, we can describe life as a process — a series of chemical reactions involving carbon-based molecules. In this process, matter and energy are taken into a system, used to assist in growth and reproduction, and are then expelled as waste products. We will note, however, that this leaves a lot of ambiguity when actually categorizing something as living or non-living based on this simple definition. When considering what is "alive", it is also important to note that we are limited to what observations we can make on Earth. Nevertheless, many biologists would agree that the following statements comprise a set of requirements for something to be considered living:
- Living things grow, evolve, and reproduce.
- Living things are highly organized chemical factories.
- Living things require energy, and they respond to their environment.
Part of the thrust of astro biology is to examine the nature of life on Earth to better inform our search for life elsewhere in the Universe. Therefore, it is important to characterize life on Earth as completely as possible. For instance, the smallest unit in which life processes occur is the cell. All known living things are composed of one or more cells, which in turn contain an intricate array of molecules. But what elements, exactly, is life made of? Interestingly enough, most of your body, over 99%, is made from just four elements: hydrogen, oxygen, carbon, and nitrogen.
In living organisms, each of these chemical ingredients has some interesting aspects. First, the large amount of hydrogen and oxygen, and in particular the ratio of these two elements in comparison to one another (two hydrogen atoms for every one oxygen atom), serves as an indicator of the high percentage of water that all life on Earth requires. Second, when we examine Earth's atmosphere, we note that nitrogen is the single most abundant element. In addition, it is an important component of all living things as it is found in many molecules, such as DNA. Finally, carbon is the element that is considered the basis for life on Earth. In fact, organic chemistry is sometimes defined as the chemical processes of carbon and its compounds, regardless of whether or not a living organism is involved. Carbon is unique in its ability to build large and complex molecules. In comparison, hydrogen can combine with oxygen to form only two molecules: water (H2O) and hydrogen peroxide (H2O2). Similarly, hydrogen can combine with nitrogen to form only two molecules: ammonia (NH3) and hydrazine (N2H2). On the other hand, the number of ways that hydrogen can combine with carbon is so large that it is unknown! The largest molecule listed in the Handbook of Chemistry and Physics has a chemical formula of C90H154. Carbon is, therefore, a versatile element for creating life forms and the perfect building block for complex structures.
All the life elements discussed above (except hydrogen) are created inside stars and are common in the universe. It is remarkable that the chemical composition of life on Earth resembles that of a star more than it does that of the Earth. Carbon and nitrogen, both necessary for life as we know it, are more common in the Sun than they are in our Earth. Iron, silicon, and magnesium are the most common elements in Earth — apart from oxygen — but play only miniscule roles in organic chemistry.
Although we have difficulty providing a concrete, all-inclusive definition of life on Earth, it is apparent (as illustrated above) that we can characterize some of the commonalities that life on Earth shares. What about life beyond Earth? Is carbon-based chemistry the only possibility for life? Chemists (and science fiction writers) have speculated about a life chemistry that is based on silicon, or some other element. Even more speculative is the idea of life based on some other organizing principle, such as electric or magnetic fields. Nobody has ever observed such life forms, so we cannot say anything substantive about them. However, the chemistry of the elements in the universe is well understood. In terms of a basis for life, it is generally agreed upon that carbon is superior to any other element in its ability to form complex chains and thereby serve as a building block for life.
What about water? It has been suggested that water is the one requirement for all life on Earth. More generally, it has been noted that life, at a minimum, must have a liquid solvent to facilitate chemical reactions. Must life rely on water, or could it utilize another liquid to accomplish biogenic tasks? First, we must recognize that water is quite possibly the most abundant liquid in the universe. Oxygen is far more abundant than silicon, the main rock-forming element. Therefore a rocky planet that uses up all its silicon by combining it with oxygen to make rocks will still have plenty of oxygen to combine with the most abundant element, hydrogen, to make water, or ice. Water remains a liquid over a wide range of temperatures and acts as a solvent, dissolving a variety of other materials to form a solution. In this role, water is vital to many cell functions; it dissolves and transports nutrients and waste products within a cell, regulates an organism's temperature, and even plays a role in shielding life from harmful UV radiation. (Water's solvent properties are a double-edged sword; it breaks down useful chemicals too!) It is thus not surprising that astrobiologists surmise that complex life forms originated in Earth's copious oceans. Even on land, a large percentage of the weight of plants (40%) and animals (70%) is comprised of water. Many solvents have been proposed as a life liquid, such as ammonia and ethyl alcohol. However, water is the most abundant and has demonstrates unique advantages for facilitating life processes.
Although our observations about life are limited to those that we can make on Earth, it is possible to speculate about life elsewhere in the universe. One problem we may encounter, though, is determining how far we can stretch the definition of life. Is a virus considered a life form? Viruses are simpler than many single-celled organisms (they lack reproductive equipment), but they can reproduce using materials from host cells that they invade. They cannot, however, fully function independently of other cells. Similarly, technology has advanced to the point where machines and computers have taken on many of the attributes of life. Some computers have been programmed to evolve and adapt. Can certain machines or computers, therefore, be called living entities? Regardless of where we draw the line on cases such as these, it is almost certain that we will have to re-evaluate our definition when we begin our exploration for life beyond Earth.