Before we consider the possibility of life in the universe, we have to inspect terrestrial biology. Although most people think they know what life is, it's actually quite hard to distinguish life from non-life. Occasionally, there can be complex, interesting and apparently sentient behavior in something that's not living at all. There are a standard set of definitions by which biologists and chemists use to distinguish living from non-living matter. Doctor, what is the definition of life? That is a big question. Why do you ask? I'm searching for a definition that will allow me to test an hypothesis. Well, the broadest scientific definition might be that life is what enables plants and animals to consume food, derive energy from it, grow, adapt themselves to their surroundings and reproduce. All right. You suggest that anything that exhibits these characteristics is considered alive. In general, yes. What about fire? Fire? Yes. It consumes fuel to produce energy, it grows, it creates offspring. By your definition is it alive. Fire is a chemical reaction. You could use the same argument for growing crystals but, obviously, we don't consider them alive. What about me? I do not grow, I do not reproduce, yet I am considered to be alive. One of the prime ingredients of life is order as opposed to chaos. We can see patterns in life ranging from microorganisms to the largest scale creatures on the Earth. This regularity is an essential ingredient of biology. Another essential ingredient is the use of energy. The fundamental source of energy for all life on Earth is the sun, but life can involve primary use of that energy through photosynthesis or secondary or even tertiary use of that energy. In all cases, however, when it's traced back, the life source is radiant energy from our star. More generally, we can make a list of properties that biological organisms seemed to share. They all have order, they adapt to their environment, they regulate their physical conditions, they process energy, they grow and develop, they reproduce and they respond to their environment in an interactive way. Are all of these ingredients essential? What are the necessary and sufficient conditions for biology? We don't know the answer to that question. Another extraordinary attribute of biology is it's multi-scale nature. There's a hierarchy of biological organisms. At the high-end, this ranges to an entire ecosystem with interrelated parts each biologically regulated. We can take that down in scale to a rain forest which also has interacting aspects in terms of different organisms and then down to the single organisms themselves perhaps a tree, then down in scale to the leaves or individual components of the tree and finally to the tissue material and the different types of cells, each one, a chemical factory. At the base of this, are small units within a cell carrying out discrete biological functions and the individual molecules themselves. The reductionist approach to biology fails. The interrelated parts produce emergent behavior at higher scales that are not predictable from the sub-components. Astrobiologists think there are two essential ingredients for life both on earth and elsewhere. One is water. All life on earth contains water ranging up to 99 percent for jellyfish down to 30 percent for bugs and beetles. We as humans are in the middle of this range with about two-thirds of our mass water. Water is a solvent and facilitates chemical reactions. We think water is essential for life and life formed in water on the Earth, and this will be the case elsewhere as well. The other essential ingredient is carbon. Carbon facilitates long complex molecules. In fact, the toolkit of carbon chemistry or organic chemistry is infinite. These are not special requirements for life beyond Earth, because water is one of the most abundant molecules and carbon is one of the most abundant elements. The context for astrobiology is the nature of biology on this planet. Even the definition of life is difficult. For example, we could use a very specific definition that homes in on the particular attributes of life on Earth and that might exclude variance that we might find beyond Earth. On the other hand, if we use too general definition, like processing energy and organizing information, we might not be able to recognize life. Terrestrial biology is a multi-scale phenomena that operates on the size of the planet down to the level of individual molecules. The interrelationship between these component parts is complex and not completely predictable.
