The Earth is estimated to have formed, by the accretion through large and violent impacts of numerous bodies, approximately 4.5 billion years ago (Ga). Traditionally, habitable worlds are considered to be those on which liquid water is stable at the surface. On Earth, both the atmosphere and the oceans may well have started to form as the planet itself did so. Certainly, life is thought to have originated on Earth quite early in its history, probably after about 3.8–4.0 Ga, when impacts from large bodies from space are likely to have declined or ceased. It may have originated in a shallow marine pool, experiencing intense radiation, or possibly in the environment of a deeper water hydrothermal vent. Because of the subsequent recrystallisation and deformation of the oldest sediments on Earth, evidence for early life must be found in its metabolic interaction with the environment.
The earliest, and highly controversial, evidence of life, from such indirect geochemical data, is from more than 3.83 billion years ago (Dauphas et al. 2004). Relatively unambiguous fossil evidence of life dates to 2.7 Ga (López-García et al. 2006). Either way, life has thus been present throughout much of the Earth’s existence. Although inevitably attention tends to fall on more immediate concerns, it is perhaps worth occasionally recalling this deep heritage in the face of the conservation challenges of today. For much of this time, however, life comprised Precambrian chemosynthetic and photosynthetic prokaryotes, with oxygen-producing cyanobacteria being particularly important (Labandeira 2005). Indeed, the evolution of oxygenic photosynthesis, followed by oxygen becoming a major component of the atmosphere, brought about a dramatic transformation of the environment on Earth. Geochemical data has been argued to suggest that oxygenic photosynthesis evolved before 3.7 Ga (Rosing and Frei 2004), although others have proposed that it could not have arisen before c.2.9 Ga (Kopp et al. 2005).
These cyanobacteria were initially responsible for the accumulation of atmospheric oxygen. This in turn enabled the emergence of aerobically metabolizing eukaryotes. At an early stage, eukaryotes incorporated within their structure aerobically metabolizing bacteria, giving rise to eukaryotic cells with mitochondria; all anaerobically metabolizing eukaryotes that have been studied in detail have thus far been found to have had aerobic ancestors, making it highly likely that the ancestral eukaryote was aerobic (Cavalier-Smith 2004). This was a fundamentally important event, leading to heterotrophic microorganisms and sexual means of reproduction. Such endosymbiosis occurred serially, by simpler and more complex routes, enabling eukaryotes to diversify in a variety of ways. Thus, the inclusion of photosynthesizing cyanobacteria into a eukaryote cell that already contained a mitochondrion gave rise to eukaryotic cells with plastids and capable of photosynthesis. This event alone would lead to dramatic alterations in the Earth’s ecosystems. Precisely when eukaryotes originated, when they diversified, and how congruent was the diversification of different groups remains unclear, with analyses giving a very wide range of dates (Simpson and Roger 2004). The uncertainty, which is particularly acute when attempting to understand evolutionary events in deep time, results principally from the inadequacy of the fossil record (which, because of the low probabilities of fossilization and fossil recovery, will always tend to underestimate the ages of taxa) and the difficulties of correctly calibrating molecular clocks so as to use the information embodied in genetic sequences to date these events. Nonetheless, there is increasing convergence on the idea that most known eukaryotes can be placed in one of five or six major clades—Unikonts (Opisthokonts and Amoebozoa), Plantae, Chromalveolates, Rhizaria and Excavata (Keeling et al. 2005; Roger and Hug 2006). Focusing on the last 600 million years, attention shifts somewhat from the timing of key diversification events (which becomes less controversial) to how diversity per se has changed through time (which becomes more measurable).
Arguably the critical issue is how well the known fossil record reflects the actual patterns of change that took place and how this record can best be analyzed to address its associated biases to determine those actual patterns. The best fossil data are for marine invertebrates and it was long thought that these principally demonstrated a dramatic rise in diversity, albeit punctuated by significant periods of stasis and mass extinction events. However, analyses based on standardized sampling have markedly altered this picture. They identify the key features of change in the numbers of genera (widely assumed to correlate with species richness) as comprising:
(i) A rise in richness from the Cambrian through to the mid-Devonian (525–400 million years ago, Ma); (ii) a large
extinction in the mid-Devonian with no clear recovery
until the Permian (400–300 Ma);
extinction in the mid-Devonian with no clear recovery
until the Permian (400–300 Ma);
(iii) A large extinction in the late-Permian and again in
the late-Triassic (250–200 Ma)
the late-Triassic (250–200 Ma)
(iv) A rise in richness through the late-Triassic to the present
(200–0 Ma; Alroy et al. 2008).
(200–0 Ma; Alroy et al. 2008).
Whatever the detailed pattern of change in diversity through time, most of the species that have ever existed are extinct. Across a variety of groups (both terrestrial and marine), the best present estimate based on fossil evidence is that the average species has had a lifespan (from its appearance in the fossil record until the time it disappeared) of perhaps around 1–10 Myr (McKinney 1997; May 2000). However, the variability both within and between groups is very marked, making estimation of what is the overall average difficult. The longest-lived species that is well documented is a bryozoan that persisted from the early Cretaceous to the present, a period of approximately 85 million years (May 2000). If the fossil record spans 600 million years, total species numbers were to have been roughly constant over this period, and the average life span of individual species were 1–10 million years, then at any specific instant the extant species would have represented 0.2–2% of those that have ever lived (May 2000). If this were true of the present time then, if the number of extant eukaryote species numbers 8 million, 400 million might once have existed.
The frequency distribution of the numbers of time periods with different levels of extinction is markedly right skewed, with most periods having relatively low levels of extinction and a minority having very high levels (Raup 1994). The latter are the periods of mass extinction when 75–95% of species that were extant are estimated to have become extinct. Their significance lies not, however, in the overall numbers of extinctions for which they account (over the last 500 Myr this has been rather small), but in the hugely disruptive effect they have had on the development of biodiversity. Clearly neither terrestrial nor marine biotas are infinitely resilient to environmental stresses. Rather, when pushed beyond their limits they can experience dramatic collapses in genetic, organismal and ecological diversity (Erwin 2008). This is highly significant given the intensity and range of pressures that have been exerted on biodiversity by humankind, and which have drastically reshaped the natural world over a sufficiently long period in respect to available data that we have rather little concept of what a truly natural system should look like (Jackson 2008). Recovery from past mass extinction events has invariably taken place. But, whilst this may have been rapid in geological terms, it has nonetheless taken of the order of a few million years (Erwin 1998), and the resultant assemblages have invariably had a markedly different composition from those that preceded a mass extinction, with groups which were previously highly successful in terms of species richness being lost entirely or persisting at reduced numbers.
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