this textbook, the word “life” is pervasive. You will come across discussions on “the origin of life,” “life in extremes,” “evidence of life in the fossil record,” “life on other planets,” “biosignatures of life,” and so forth. However, implicit in all these discussions is the idea that we all agree on what life is. This chapter will explore the idea of “life.”
Attempts to understand what constitutes life are important for many areas of astrobiology. If we seek to understand how and when the origin of life occurred, we must agree when the transition from mere molecules to “living” things occurred, and that requires that we have some agreement on what constitutes a living thing. If we are to seek life on other worlds, we must agree what it is we are looking for. We must agree what types of entities would be of interest as exemplars of living things and what types of materials would not constitute life.
Similar concerns exist in a field such as synthetic biology. Scientists must agree when they have artificially created a living thing in the laboratory. Another application of the concept of life is in the computational field of artificial life, where computer programming is used to make artificial digital entities that reproduce and evolve. To discern whether a digital entity created in the computer could be described as “artificial life,” we must agree on what a living thing is.
There are numerous books and papers that explore the characteristics of the material we call “life.” This textbook is not intended as an exhaustive scientific and philosophical review. The objective of this chapter is to introduce some of the basic ideas so that you can explore this topic for yourself. However, given the widespread use of the word life, it is an important topic to explore.
As with astrobiology itself, it is always useful to explore some historical perspective since it helps to provide an understanding of what people thought in the past and why certain paradigms might still persist.
2.2 What Is Life? The Historical Perspective
Speaking unscientifically, we might all agree that life seems at first glance to be different from non-life. Without any in-depth analysis, many people intuitively think that life is unpredictable, complex, and, in a certain undefined way, full of energy. This is the perception that occurred to the ancients. Almost all their attempts to explain life revolved around ways to separate, if you will, life from non-life. It is also certainly true that the birth of monotheistic religion, such as Christianity, encouraged a general idea that life has a special place in the Universe and that it is categorically different from non-life. The underlying motive for this was almost certainly encouraged by a desire to separate animals, and particularly humans, from non-life. This perceived exalted position of humans would be consistent with the discomfort felt by many people when Charles Darwin's The Origin of Species convincingly suggested that we all share animal ancestors and that our closest relatives include apes.
However, long before the major monotheistic religions emerged around 2000 years ago, humans had already convinced themselves that “life” was categorically different from “non-life,” and the challenge was to find what that difference was. Greek philosopher Empedocles (c. 494–434 BCE) proposed a model in which everything in the Universe is made of four different materials: “elements” of water, air, earth, and fire. He did not call them elements, but rather “roots.” Different types of matter are formed by different mixtures of these roots. The mixtures account for all the characteristics of matter.
Democritus (c. 460–370 BCE) (Figure 2.1) advanced the idea that all matter is made of indivisible “atoms.” This idea was certainly radical, but it did not equate with the atoms we understand in modern science. First, his atoms were indivisible. Second, different atoms had different characteristics that aligned with the matter that they formed. For example, water atoms were slippery, and air atoms were diffuse and airy. Seen from a modern perspective, Democritus's atoms are more like our modern concept of molecules. In this view, Democritus explained life as being matter that had “fiery” atoms that imbued it with a “soul.” Here we can see an example of the assumption that life contained something material that separated it from all other types of matter. This idea would, and still does, prove difficult to shake off.
Figure 2.1 Democritus. Ancient Greek philosopher. He proposed an early atomic theory of matter.
Source: Reproduced with permission of The Heinrich Heine University of Dusseldorf, http://www.phil-fak.uni-duesseldorf.de.
Nevertheless, lurking behind his atomic theory was the important concept, now referred to as “materialism,” that life was mechanistic, an idea that would be picked up in the seventeenth century by philosopher René Descartes (1596–1650), who famously considered animals to be essentially machines.
Democritus's idea of the “soul” was advanced by Greek philosopher Aristotle in a form of philosophy called “hylomorphism.” The philosophy, yet again, is underpinned by a desire to find a way to explain the difference between living things and non-life. All material in the Universe was made of matter and form. In this theory, matter was the stuff of everything in the Universe and would align with our modern idea of matter. However, mixed in with this matter was a substance called “form.” The form was the “soul.” The soul could be used to explain the different cognitive capacities of living things. The vegetative soul is found in plants and allows them to grow and reproduce. The animal soul allowed animals to move and feel things. The rational soul, reserved for humans, allowed consciousness and reasoning. This substance, the soul, allowed for a categorical difference between life and non-life, but it also allowed for a gradation in cognitive capacities of organisms that provided a safe, and superior position, for humans.
Aristotle's ideas encouraged a wider idea that persisted throughout the Middle Ages all the way into the nineteenth century. One might even claim that remnants of this “hunch” persist to this day. This is the idea of “vitalism,” that life has some sort of force or energy that distinguishes it from non-life. Vitalism especially gained credence when the true atomic structure of matter was understood. The idea that elements were constructed from identical subatomic constituents, a concept that would later explain the structure of the Periodic Table, ended previous ideas that the difference between types of material could be explained by “fiery atoms” or other special atomic arrangements. Vitalism allowed for that special separation to persist between life and non-life. They could both be made of elements constructed of identical building blocks, but life had an extra force. This idea was picked up in the twentieth century by French philosopher Henri Bergson (1859–1941) in his 1907 book Creative Evolution in which he coined the term, “élan vital.” This “vital impetus” had no physical explanation, but various suggestions, such as special electrical energy, were in no short supply.
2.3 Spontaneous Generation
A concept that looms large in early ideas about life is spontaneous generation. Aristotle (Figure 2.2), alongside his ideas on hylomorphism, advanced the idea of spontaneous generation, whereby non-living matter could be transformed into living things by acquiring soul. This idea provided a satisfactory explanation of how living things came into existence.
Figure 2.2 Aristotle, ancient Greek philosopher and early natural historian.
Source: Roman copy in marble of a Greek bronze bust of Aristotle by Lysippos, c. 330 BCE with modern alabaster mantle; reproduced with permission of Jastrow, https://commons.wikimedia.org/wiki/File:Aristotle_Altemps_Inv8575.jpg.
