as quickly as the bacteria could release it. This is one plausible scenario, although there is not widespread agreement on the reason for this ‘Great Oxidation Event’, and it is still a very active area of research. Whatever the reason, the oxygenation of the atmosphere made possible by the evolution of oxygenic photosynthesis was critically important for the emergence of complex animals.
VARIATION OF ATMOSPHERIC OXYGEN CONCENTRATION OVER THE LAST 3.5 BILLION YEARS
Aerobic respiration, in which energy is released from organic matter, makes the existence of food chains possible because it is so efficient. Releasing energy from food using oxygen is around 40 per cent efficient, while oxidising food using iron or sulphur is only around 10 per cent efficient. This means that animals can eat plants, and in turn get eaten by a tower of predators that can still extract enough energy to flourish. It is almost certainly no accident that the Cambrian explosion – the rapid diversification of life resulting in the emergence of virtually everything we would regard today as complex – followed (on geological timescales) a rapid increase in atmospheric oxygen levels.
BRINGING IT ALL BACK HOME
The story of the emergence of today’s Earth is complex. That we understand not only the broad sweep of the narrative, but also the fine detail of at least some of the chapters, is one of the great achievements of science, and the presence of some uncertainties in the story of the emergence and development of a 4-billion-year-old biosphere is surely unsurprising. We have seen that water is a prerequisite for life on Earth, and most likely for life anywhere in the Universe. Likewise, an oxygen atmosphere, while not necessary for microbes, is a vital component of a complex ecosystem able to support large predators and prey, and probably therefore intelligent civilisations. As oxygen atmospheres are inherently unstable, oxygenic photosynthesis on a global scale is necessary to maintain high levels of this life-giving gas. And we know that this evolved only once on Earth. But there is one final ingredient that is more elusive and certainly beyond life’s control: time. It is a certainty that the evolution of complex life requires an ecosystem that is stable over many millions of years. But how many millions? This question will occur again and again throughout this book. Why did life emerge so soon after the birth of our planet, only half a billion years after its formation? And how did the first life blossom into the magnificent complexity we see on Earth today? A good place to start is to look at the evolutionary history of a single animal, and see how precisely we can trace its origins back into the deep past.
The family tree of the horse is the best known of any complex animal, partly due to the wealth of available fossil evidence. The first animal recognisably ‘horse-like’ is the Hyracotherium (once known as ‘Eohippus’ or ‘dawn-horse’), which lived around 50 million years ago. It was a fox-sized omnivore, and because many thousands of intact skeletons have been found, a great deal is known about its form and lifestyle. Alterations in the availability of food probably played a role in the later emergence of two other species, the Orohippus and Epihippus, both better adapted to a browsing diet of tough plants. Around 30 million years ago, changes in climate saw the emergence of grasslands and steppe landscapes across the planet. In North America, the Mesohippus emerged; with longer legs and a slightly larger frame, it was better adapted for life on the new grassy plains because it could run faster to avoid predators. At around the same time, a species known as the Miohippus appears in the fossil record. It probably lived alongside the Mesohippus, but over time gradually replaced it. This raises an important point relating to the construction of a family tree. It should not be read as a gradual transition from the simple to the complex, culminating somehow in the grandeur that is a modern domestic horse. The different species were adapted to different conditions, occupying different ecological niches. None was ‘better’ than another in any absolute terms. Because all that remains of these animals are their fossils, and our knowledge of the local ecology is generally rudimentary, it is often difficult to know why one species survived while another died out, or why a particular species arose through a process of ‘speciation’. We will explore the phenomenon of speciation in much more detail in Chapter 5. The domestic horse should therefore be seen as one of many branches of a complex family tree that survives today – it is not the culmination of a series of ‘improvements’ over its more distant ancestors.
That said, it is still instructive to follow the tree, because it illustrates the surprising pace of change delivered by the power of evolution through natural selection. Over around 25 million years, the Miohippus has given rise to a grand array of species, forever passing on a shifting mixture of genes, filtered by the sieve of natural selection. As conditions changed, some branches of the tree turned out to be evolutionary cul-de-sacs, while others lived on, branching or gradually changing. This continuously shifting selection and isolation of pools of genes is reflected in the diversity of the horse family that we can see today – from zebra to the Yucon wild ass, from the kiangs of Tibet to Equus ferus caballus (the domestic horse).
So we can see that changes in form can be rapid and surprising. The Hyracotherium looks more like a fox, or even a large rodent, than a horse, and yet in the history of every modern horse, Przewalski’s horse, zebra or donkey there are Hyracotherium ancestors who lived only 50 million years ago – the blink of an eye in the 4.5 billion years’ life of our planet.
If we sweep back still further, we encounter the first mammals around 225 million years ago. There is an explosion of complexity in the fossil record associated with the Cambrian period, 530 million years ago, which may have been related to a rise in oxygen levels. The first evidence of complex multicellular life appears around 600 million years ago in the form of the Ediacaran biota, named after fossils first found in the Ediacaran hills in Australia. Some of these organisms had a quilted, fractal appearance and were so bizarre that it has been suggested they were neither animals, nor plants, nor fungi, but some failed evolutionary experiment. Other Ediacarans were clearly soft-bodied animals, up to 2 cm in length with a head, and may even have burrowed slightly into the microbial mats on the sea bottom, thereby subtly changing the planet’s ecology, and opening the way for further evolutionary developments. The delicate and ambiguous nature of the fossils left by these mysterious organisms has made the study of the Ediacara one of the most intriguing parts of recent palaeontology.
Before the earliest Ediacaran fossils, dated at 655 million years old, there is no direct evidence of multicellular life on Earth. The next major milestone occurred around 2 billion years ago with the emergence of the eukaryotic cell. As we have already discussed, eukaryotes are cells with a nucleus and internal structures similar to our own – we are grand colonies of eukaryotic cells. At around 3.5 billion years ago, we find the first prokaryotes, the first free-living cells that emerged, perhaps, from hydrothermal vent systems on the ocean floor of the primordial Earth.
The reasons for these vast periods of apparent stasis in the development of life – over a billion and a half years from prokaryote to eukaryote and a similar amount of time from the eukaryote to the first evidence of multicellular life – are not understood. It certainly seems that the complex cell – the eukaryote – emerged only once in the history of life. There is no evidence of different versions of the eukaryotic cell emerging from bacteria or archaea during their 4 billion-year tenure on Earth. If they did, they have left no trace. All animals, plants, algae and fungi are self-evidently related to each other, sharing multiple traits from the structure of their DNA to the use of ATP. The form and biochemistry of their cells are very similar; only the colonies they form are radically different. This strongly suggests a common ancestor, and raises the intriguing question: was the emergence of the eukaryote an incredibly unlikely event, or was the billion-year delay just bad luck? We don’t know, because we have only one Earth to observe. This is why the search for microbial life