As I have shown above, there are several ways to synthesise prebiotic organic compounds, all of which seem to be realistic, though the extent of their contribution to the early Earth will in most cases depend on the exact environmental circumstances in the place and at the time that they arise. Nevertheless, any one or (most likely) all of the following processes will allow for the synthesis of at least some organic molecules that would contribute to making the Earth habitable (Bernstein, 2006; Cady, 2001; Ehrenfreund et al., 2002, 2005; Ehrenfreund & Cami, 2010; Zahnle et al., 2007, 2010):

Spark discharge synthesis: in practice this means lightning but also includes the effects of high energy solar radiation, especially UV, in the atmosphere. The outcome depends on the oxidation state of the atmosphere and on how much hydrogen is/was present; but although yields of organic products might be limited in the general atmosphere, considerable quantities of biogenic organic molecules could be made locally during volcanic eruptions (Parker et al., 2011).

Hydrothermal vent synthesis: high temperature chemistry in the water outflow of deep-ocean (black smoker) vents can generate interesting chemical pathways but the stability of amino acids and other compounds in these environments remains problematic, and such vents may actually purge the seawater of biogenic molecules. Cooler, alkaline (white smoker) vents support a wide range of organic synthesis and readily support production of bubbles bounded by an inorganic membrane across which a proton gradient is naturally established (Martin & Russell, 2007; Simoncini, Russell & Kleidon, 2011), which might be the primeval ancestor of chemiosmotic coupling; ‘an energy source that must have been available to emergent life’ (Russell, 2010).

Although fungal hyphae have few unique morphological features and most fungal structures are poor candidates for preservation over long periods of time as fossils, a respectable fossil record for fungi has been assembled in recent years. By far the most impressive fungi of the Ordovician/Devonian Period are specimens of the fossil genus Prototaxites, which were terrestrial organisms found from the mid Ordovician (460 million years ago) to the early Devonian, suggesting that they lasted a period of at least 40 million years (Boyce et al., 2007; Hueber, 2001). These fossils are among the ‘nematophyte phytodebris’ that constitutes the earliest evidence for terrestrial organisms. This ‘nematophyte phytodebris’ nomenclature was assigned in the middle of the nineteenth century and has no relevance to present-day understanding of taxonomy (that is, it does not necessarily indicate that the stuff was of plant origin); though it does indicate that confusion over the identification of the material is over 150 years old (see discussion in Hueber, 2001 and Taylor et al., 2010). Prototaxites specimens are generally large: over a metre wide (Wellman & Gray, 2000) and up to 8 m tall (Hueber, 2001) (Figs. 12.1 and 12.2; illustrated in colour in Moore et al., 2011, pp. 33–34). Prototaxites was also so common that it was a major component of these early terrestrial ecosystems, both in terms of abundance and diversity. Some of the earliest examples found were tree-like trunks constructed of interwoven tubes < 50 µm in diameter (concentrically arranged in transverse sections), and the fossils were interpreted to be small coniferous trees, though we now know that environments at the time Prototaxites was fossilised did not (yet) include large vascular plants.

In the narrative of life on Earth we have now reached the Archaean Eon (3.8 to 2.5 billion years ago); the age when chemistry came alive. During this period the Earth day increased from about 15 hours long to about 18 hours and the Sun brightened to 80% of its current level. At present, few data are available that are able to specify the atmospheric, oceanic or geological conditions on the early (prebiological) Earth. It can be reasonably assumed that conditions were very hostile due to volcanism, radiation, and continued bombardment by objects large and small from space; but it is likely that the average climate was temperate rather than extremely cold or hot (Kasting & Howard, 2006). As I have discussed in Chapter 6, there is no agreement on the gaseous composition of the primeval atmosphere apart from the general acceptance that oxygen was absent. According to Lazcano & Miller (1996): ‘atmospheric chemists mostly favor high CO2 + N2, whereas prebiotic chemists mostly favor more reducing conditions’. In fact high levels of carbon dioxide in the early atmosphere are indicated by the high level of carbonate minerals in rocks of that age.

One of the reasons for postulating reduced gases in the atmosphere of the early Earth is that such an atmosphere would have had a greenhouse effect, which would compensate for the dimmer Sun of the day and keep the Earth from freezing over (Sagan & Chyba, 1997). It is thought that there was a global glaciation in the mid Archaean, approximately 2.9 billion years ago, which might have been caused by a hydrocarbon haze shielding solar radiation, the haze resulting from methane photolysis after the newly evolved methanogens increased the atmospheric methane/carbon dioxide ratio. This glaciation ended as continued production of methane produced greenhouse warming. Subsequent to the success of photosynthetic organisms, increase in atmospheric oxygen decreased greenhouse warming by methane and probably caused global glaciation just after the end of the Archaean (approximately 2.4 billion years ago) in the early Palaeoproterozoic (Kasting & Howard, 2006). These glaciations show the fine balance required to avoid global freezing (‘snowball Earth’; see Fig. 11.2) primarily because of the faint early Sun. Sagan & Chyba (1997) calculated that an atmospheric mixing ratio of ammonia (the mixing ratio CX of a gas X is defined as the number of moles of X per mole of air) of only about 10–6 to 10–4 would have been sufficient on the early Earth to cause enough greenhouse warming to counteract the effects of reduced solar radiation.

In Chapter 4, I stated that ‘interstellar matter provides the raw material for the formation of stars and planets’ and it should now be evident that interstellar matter may well have also been important starting material for the origin of life on Earth (Bernstein, 2006; Chyba et al., 1990; Ehrenfreund & Cami, 2010; Ehrenfreund et al., 2002). In fact, in Hazen’s phrase:

This being the case, and as there are so many plausible mechanisms for the synthesis of biogenic compounds in situ on planet Earth, it is surprising that so much attention has been given to the possibility that life came to Earth ready formed rather than originating here. The panspermia hypothesis claims that life exists throughout the Universe, with microbes drifting through interstellar and interplanetary space, transmitting life to the next habitable body they encounter. According to this view the Earth of long ago was colonised by microbes that had somehow escaped from their home planets to drift across the vast distances between the stars until they arrived on that primeval and sterile Earth. Panspermia should not be confused with pangenesis; the latter was Charles Darwin’s conjectural mechanism to explain heredity (he argued that inheritance depended on particles produced by each organ being transmitted from parent to offspring).