What factors impede the development of successful scientific theories? How can the development of such theories be facilitated? These questions arise independently of which conception of scientific theory one endorses. They are especially important for biology since we currently lack a scientifically fruitful, universal theory of life; as many biologists are fond of admonishing, “give me a general principle of biology and I’ll find an exception.” At best (assuming that life has a universal nature, which is not certain) we are still in the earliest stages of formulating such a theory. For as the next chapter (Chapter 5) explains, recent advances in biochemistry and molecular biology have established that familiar Earth life provides just a single example of life. Biologists have also discovered that complex multicellular eukaryotes are highly specialized, biologically fragile, latecomers to our planet. Yet (as discussed in Chapter 1) the latter, especially animals and plants, have served as prototypes for biology since the time of Aristotle. In a nutshell, in addition to generalizing on the basis of a single example of life, we have been seeking universal principles for life from an unrepresentative subsample of it.

Some artificial life (ALife) researchers contend that we are on the verge of creating novel life forms either in the form of information structures in a computer (soft ALife), robots made of plastic and metal (hard ALife), or synthetic organisms composed of unnatural biomolecules (wet ALife or synthetic biology). In the words of computer scientist Chris Langton, a founder of ALife, “Artificial life can contribute to theoretical biology by locating life-as-we-know-it within the larger picture of life-as-it-could-be” (Langton 1989, 1). Similarly, in a discussion of synthetic life, Mark Bedau and colleagues note that “[o]ne of artificial life’s key goals is constructing a life form in the laboratory from scratch” (Bedau et al. 2000, 365). This chapter evaluates whether ALife research can live up to its hype and deliver truly novel forms of life. As will become apparent, the inventions of ALife are very closely based on characteristics of familiar Earth life. In addition, they tend to fall outside of the Goldilocks level of abstraction (Section 4.3) in being either too abstract (soft and hard ALife) or too concrete (wet ALife).

Not everyone who advances a so-called definition of life has in mind the traditional notion of definition. This is especially true of scientists. Biochemist Steve Benner (2010), for instance, contends that definitions encapsulate theories, and speaks of the need for formulating a “definition-theory of life” (p. 1022). But it is also true of some philosophers who are well aware of the limitations of traditional definitions. As an illustration, Mark Bedau (1998) presents a “definition” (his term) for life and characterizes it as the “general form of my theory of life” (p. 128). Definitions of this sort are nonstandard in the sense that their authority does not derive from analysis of human concepts or alternatively mere stipulations of meaning. Their acceptability depends upon successful empirical investigations. Nonstandard definitions nonetheless resemble traditional definitions structurally insofar as they supply necessary and sufficient conditions (identifying descriptions) for membership in a presumed natural kind. Some recent proposals for “defining” life, briefly discussed in Section 3.5, are even more radical, rejecting the received view that a central function of definition is classification; on such a proposal, a definition of life need not even provide necessary and sufficient conditions for life.

When most people think about extraterrestrial life they envision intelligent, often technologically advanced, humanoid creatures, such as the Prawns (District Nine) and the Na’vi (Avatar), robots, such as Gort (The Day the Earth Stood Still), and the hive-like Borg (Star Trek). Most astrobiologists, however, are not looking for intelligent life. They are looking for bacteria-like organisms. For as discussed in Section 5.2, microbial life is almost certainly far more common in the universe than complex multicellular organisms, let alone intelligent, technologically sophisticated creatures. Because they are so tiny, detecting and identifying extraterrestrial microbes is especially challenging.

This book presents a long, multifaceted argument for pursuing universal biology in the face of (in William James’s colorful words) the “blooming buzzing confusion” offered by familiar Earth life to researchers. As Chapter 5 discusses, the central challenge for the program of universal biology is that familiar Earth life – the only form of life of which we can be certain – represents a single example and there are positive reasons for worrying that this example is unrepresentative of life. Biologists have discovered that life as we know it on Earth descends from a last universal common ancestor, and hence represents a single example. Moreover, biochemists have established that life elsewhere could differ from familiar life in certain ways at the molecular and biochemical levels, and they do not know how different it could be from familiar Earth life. Finally, as Chapter 6 explains, contemporary biological theorizing about life is founded upon what we now know is an unrepresentative form of familiar Earth life, namely, highly specialized, latecomers to our planet (complex multicellular eukaryotes). Indeed, a central theme of this book (Chapter 1) is that much of contemporary biological thought is still implicitly wedded to a defective, neo-Aristotelean, theoretical framework for life based on animals and plants.

As discussed in Chapter 1, Aristotle divided all life into two taxonomic categories, plant and animal, a view that, as Section 5.3.2 recounts, dominated biology until less than two hundred years ago. When one considers that Aristotle’s observations were limited to what could be seen by means of unaided human vision, namely, plants, animals, and certain fungi, for example, mushrooms (which he classified as plants), this is hardly surprising. In the seventeenth century, Antonie van Leeuwenhoek, who first observed and described them under a microscope of his own devising, classified microorganisms as tiny animals (“animalcules”). It was not until the mid-nineteenth century that unicellular organisms were placed in their own (a third) taxonomic category, Protista, by Ernst Haeckel. What is surprising is how long Aristotle’s classification system survived in the face of mounting empirical evidence that unicellular organisms defy classification as plant or animal.

Mars is the fourth planet from the Sun and the outermost of the rocky, terrestrial planets that make up the inner solar system. Mars is the second smallest planet; only Mercury is smaller. Surface gravity on Mars is 3.71 m s–2, which is 37.6% that of the Earth. The present atmospheric pressure is low (~0.6 kPa) relative to Earth’s (101 kPa), and the atmosphere is mostly carbon dioxide (95%). The obliquity of Mars (tilt of the axis of rotation relative to the plane of orbit) is presently 25 degrees and may have varied by tens of degrees over the past tens of millions of years and longer (Laskar et al., 2004).

The rugged highland terrains of Noachis Terra and Terra Sabaea dominate the region. The higher-standing, tectonically deformed, and densely cratered Terra Sabaea contains Scylla and Charybdis Scopuli. Also present are Denning, Bouguer, Lambert, Dawes, Pollack, Schiaparelli, Tuscaloosa, and Bakhuysen craters. To the west, the relatively subdued, but still rugged highland region of Noachis Terra has Newcomb, Wislicenus, and Mädler craters; and Marikh and a portion of Evros Valles. Numerous other moderately to highly degraded craters are scattered throughout the area. Valley networks ranging from tens of kilometers to thousands of kilometers in length dissect much of the topography. Wide grabens scar parts of Terra Sabaea. The region slopes from close to 3,000 m above datum in Terra Sabaea to as low at –1,500 m in the northwest. The northeast region is a portion of the zone that occurs between highland terrains to the south and transition terrain of Arabia Terra to the north (MC-12).