Читать книгу Wonders of Life - Andrew Cohen - Страница 14

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 elsewhere in the Solar System is so desperately important.

There is something on which everybody agrees, however: as first proposed by the late Lynn Margulis, the eukaryote is a chimera, formed by endosymbiosis in much the same way as the ancestors of plants and algae acquired their chloroplasts. The evidence for this lies in structures known as mitochondria, found in the overwhelming majority of eukaryotic cells alive today and responsible for the generation of ATP through respiration. We will meet mitochondria in more detail in Chapter 2. Just as for the case of the chloroplasts, however, mitochondria have their own loops of bacterial DNA that mark them out as ancient bacterial symbionts. Furthermore, eukaryotes bear an interesting genetic relationship with the two prokaryotic branches of life – bacteria and archaea. They share genes with both, which strongly suggests that the first eukaryote was the result of a merger between a bacterium and an archaea. The details are still the subject of debate, but it seems that something very unlikely indeed – the successful merger of two prokaryotic cells – had to happen before complex life could develop on our planet. The eukaryote is probably a happy accident. And, therefore, so are we.

This may have profound implications for the existence of complex life on other planets. The emergence of prokaryotes may well be inevitable, given the right conditions. We will explore this in much more detail in the following chapter. But there is no way we know of that prokaryotes will clump together to form animals, plants and people – at least they haven’t managed it during their 4 billion years on Earth. To build Apollo 8, you first need a eukaryote, and it seems probable that on Earth this key step occurred due to blind luck, followed by a lot of natural selection. It took almost 2 billion years for it to happen on our planet – 2 billion years in which the oceans remained a stable, hospitable home under a dangerous Sun. Could it be that living things capable of taking pictures of their home from space are rare or even unique to Earth? Perhaps one day we will know, but, lonely or not, we have surely learnt enough about life’s long struggle to complexity to treasure the good Earth.