The definition of life is an old philosophical problem, which has been made even more complex by the advent of the current understanding of nucleic acids and the impact of this on genetics, and the ascendancy of molecular biology. (For careful, detailed and entertaining discussions of the topic ‘what is life’ I would recommend the following references: Benner, 2010; Davies, 2006; Dyson, 1999; Fenchel, 2002; Gánti, 2003; Hazen, 2005; Koshland, 2002; Luisi, 2006; Lurquin, 2003.) But the most significant quotation, and the most important to place right at the start of this chapter is this:

I put high significance on this particular quotation for the phrases ‘everyone knows what life is’ and ‘there is no simple definition of life’; these are warnings of the nature of the road ahead if you choose to venture down this path. It is also evident that most authors are concerned to present (and bring to prominence) their own uncompromising opinion: Hazen (2005) writes:

In the story of life on Earth we have now reached the start of the great spreading out of the eukaryotes, from about 2 billion years ago. This is called the Orosirian Period (2.05 to 1.8 billion years ago). The climate of the Earth at this time is reasonably temperate (the Huronian ice age ended 2.1 billion years ago); the Sun’s luminosity is at about 85% of the present-day level and the day length is about 19 hours (see Table 13.1, located at the end of this chapter). By the end of this period, bacteria are abundant and there has been sufficient prokaryotic photosynthesis for oxygen to start accumulating in the atmosphere. Geologically this is a period of intensive mountain development, which means that there were extensive expanses of terrain above sea level as well as high mountain areas and all the potential habitats that such regions represent. The period was bracketed by two major impact events: the impact 2.02 billion years ago that produced the 300 km meteor crater at Vredefort, South Africa, and the impact of 1.85 billion years ago that formed the 250 km crater at Sudbury, Ontario, Canada (you can find details in the Earth Impact Database at this URL: www.passc.net/EarthImpactDatabase/index.html).

This is the global habitat into which ELCA, the eukaryote last common ancestor, first emerged, still in its biofilm and still surrounded by prokaryotes. Those prokaryotes gave birth to eukaryotes, in the sense that ELCA diverged from a prokaryotic ancestor; but they also compete with ELCA, so to become a success ELCA must immediately compete in the life game and win. By the time ELCA emerged the prokaryotes had had at least 1.5 billion years to diversify and spread themselves throughout the world. The descendants of ELCA will compete with all of these in due time, but at the beginning ELCA was in competition only with local prokaryotes within the biofilm in which it arose. Remember that one of the rules of the life game is that revolutionary advances in evolution take place in small volumes.

As I have shown in Chapter 1, with the quotation from Whittaker (1969), by about the middle of the twentieth century the three major kingdoms of eukaryotes were finally recognised, and a crucial character difference was their respective modes of nutrition:

1. (a) animals engulf

2. (b) plants photosynthesise

3. (c) fungi absorb externally digested nutrients.

As you might expect, many other differences can be added to these – some general differences, some highly specific. Some of these kingdom-specific differences are absolute, but most have to be qualified in some way. For example, you might, with some reason, say that a characteristic of animals is that they move, and contrast that with the characteristic immobility of plants. But coral reefs are made up of animals and yet are pretty immobile; and the next time you stroll through a meadow in late summer and the breeze stirs up an atmosphere filled with flying seeds, look around and remind yourself: ‘the plants are migrating’.

Consequently, although it is possible to assemble panels of biological characteristics that are specifically expressed by each kingdom, you have to recognise that those characteristics may be subject to the context in which they are expressed and that in some circumstances there may be serious exceptions. When you try to establish evolutionary relationships there are more difficulties, the prime one being how to decide whether a character is ancestral or adapted. Intuitively, you might expect the ancestral character to be the simpler, and the adapted character to be the more complex.

Our present understanding is that the Universe is between 12 and 15 billion years old and recent experiments and observations suggest that for almost all of that time most of the elements that we now know in the Periodic Table have been present and there has also been an abundance of spontaneously synthesised molecules, most of these being organic molecules. These exist in the interstellar medium of our own Milky Way Galaxy and other galaxies, and in our Solar System.

Max Bernstein (2006) starts the abstract of his article on prebiotic materials with these sentences:

Before showing how Max Bernstein answered his own questions, I want to ask (and answer) the question where did these ‘universal presumptions’ come from? As with many aspects of modern biology, we can look back with expectation of enlightenment to the writings of Charles Darwin.

Prokaryotes have dominated the Earth for the bulk of its history (I have put that statement in a tense that suggests they do not dominate the Earth now, but the truth might be other than this). LUCA must have emerged close to the start of the Archaean Eon, about 3.8 billion years ago, because, as noted above, some of the oldest microbial fossils are fully differentiated, photosynthetic bacteria (cyanobacteria) found in Western Australian sediments that are 3.5 × 109 years old (Boal & Ng, 2010; Derenne et al., 2008; Schopf, 1993). By contrast, eukaryotes are generally thought to have appeared no earlier than about 1.5 billion years ago (and some people put their emergence somewhat later than that). So, for at least 2 billion years the only living organisms on the planet were prokaryotes together, presumably, with their associated viruses.

On the basis of protein sequence comparisons, LUCA probably had a complexity comparable to that of a simple modern bacterium and lived 3.2–3.8 billion years ago (Orgel, 1998). The abundant biological activity in the deep ocean volcanic hydrothermal systems of the present day, most of it being dependent on chemosynthesis rather than photosynthesis, has stimulated the widespread appeal of the theory promoting a ‘deep hot’ origin of life, and particularly Günter Wächtershäuser’s argument linking the chemistry of submarine deep ocean vents with the origin of life (Alpermann et al., 2010; Wächtershäuser, 2006):

The only known habitat for life is the planet we call Earth. The only place we know the life experiment has been carried out is this planet Earth. We do not know how life originated here, however. Nevertheless, we understand a great deal about the physical and chemical conditions, the environments, and some of the spontaneous (‘self-organising’) mechanisms that the physics and chemistry of this Universe make possible, so there is no difficulty in formulating reasonable models for the emergence and onward evolution of living things. One of the best existing books about this topic starts like this:

Although I intend to discuss here the notion of panspermia (see Chapter 5, below), I will not discuss the other mechanism that Pier Luigi Luisi says is excluded by prebiotic molecular evolution in the quotation above, namely divine intervention. I choose not to include this because personally I see no need to invoke divinely magical or mythological processes in the scientific story I wish to tell: basic physics and chemistry are enough. If you want to round out your reading on the topic then I suggest you start with chapter 1 in Luisi (2006) and chapter 6 of Lurquin (2003). For the real hard-core discussion you don’t need to go much further than Dawkins (1986, 2006) and Scott (2009). If you put the words ‘dawkins’ and ‘god’ together in Amazon’s search window, the software will display publications on both sides of the argument, and from the number of items offered you may get an inkling of why I decided not to venture into this part of the arena!

The first serious experimental attempts to make biogenic or prebiotic monomers by adding energy to simple gases were the spark discharge experiments that Stanley L. Miller carried out in Harold C. Urey’s laboratory at the University of Chicago. The original experiment consisted of running steam containing a simple gas mixture of hydrogen, ammonia and methane past electrodes supporting a corona spark discharge from an induction coil, then through a steam-condensing loop before emptying back into the boiling flask. The scale of the apparatus was modest; the steam was produced by boiling just 200 ml of water in a 5 l flask and after evacuating air, the apparatus was charged with 10 cm pressure of hydrogen, 20 cm of methane, and 20 cm of ammonia. The boiling and electrical discharge continued for a week;

The conditions used by Stanley Miller were based on what were then assumed to be realistic conditions on the ancient Earth (Urey, 1952); specifically, that the early Earth would have had a moist and chemically reducing atmosphere of hydrogen, ammonia and methane with a warm liquid ocean (represented by the water in the boiler) and frequent lightning discharges (represented by the spark discharge). For the time the outcome was amazing, for when Miller analysed the solution by two-dimensional paper chromatography run first in butanol/acetic acid/water followed by water-saturated phenol, a ninhydrin spray (the standard way of detecting amino acids) revealed:

‘The most striking feature of life is its similarity!’ (Fenchel, 2002, chapter 12, p. 123). Cavalier-Smith (2010a) called it stasis and illustrated it this way:

The principle I have used in this book so far is the general consideration that features that are common to all organisms that exist today are the fundamental cell functions that today’s organisms have inherited from their Last Universal Common Ancestor (LUCA).

Around about 5 billion years ago something truly remarkable happened here. I mean here at this unremarkable position in this unremarkable spiral arm of this unremarkable galaxy. At the time this place was a region of interstellar gas and dust, a very tenuous cloud with no particular place to go and no particular thing to do. The remarkable happening was that something disturbed this aimless chaos. It might have been something as simple as a star sweeping past on its own way in its own orbit; it might have been something as dramatic as a star exploding in the vicinity. Either way the gravitational disturbance was enough to give this region of that gas cloud a slight swirl, just the merest touch of concerted angular momentum. And that was enough to start the entire story of life on Earth.

Of course another few hundred million years or so was needed to establish a habitable planet. That initial disturbance set the gas and dust swirling and the resulting interactions caused the dispersed particles to begin to come together into a rotating disc, which retains most of the disturbed cloud’s angular momentum. This is a solar nebula: the beginnings of a star and its planetary system. A nebula is an interstellar cloud of dust, gases (and as we will see later, in Chapter 4, organic molecules). Originally, the word nebula was applied to any astronomical object that looked diffuse and cloud-like and many distant galaxies were called nebulae for this reason; indeed, the Andromeda Nebula (= Andromeda Galaxy) was so named even before the nature of galaxies was established. This is old-fashioned usage of the term, which should now be reserved for the clouds of dust and gases that are often star-forming regions.