Читать книгу Extinction: Evolution and the End of Man - Michael Boulter - Страница 9

Children have always enjoyed fairy stories. Their early imagination conjures fantasies that make the spine tingle yet leave them safe within the protection of the parental storyteller. The contrasts between good and bad couldn’t be clearer and the eventual triumphs of right over wrong are an inevitability. The importance of these perpetual fairy stories as metaphors of human thought is well recognised by psychologists, and there appear to be complex reasons to account for their continuing popularity in the face of so much modern competition.

As in Jurassic Park and Walking with Dinosaurs, the images of the land of the dinosaurs are frightening, yet the protection of knowing they are virtual images is comforting – like Little Red Riding Hood’s wolf, they aren’t around any more. They are no longer a direct physical threat but remain as frightening images in our minds.

Imagine the scenery while striding along the quayside at Lyme Regis in Dorset, the location of The French Lieutenant’s Woman and countless television films about eighteenth-century swashbuckling. Centuries ago, the curve of the quay was built of large blocks of local rock which still protect the small harbour from the weather of the English Channel. The stones show remnants of earlier life embedded in the rock, fossil mussels and oysters from the Early Jurassic sea that flowed along this coast 200 million years ago. We walk down the quaint old steps, slipping on familiar slimy green seaweeds, ancient algae which also grew in the Jurassic sea. Down at the bottom of the steps the water laps as it has almost always done. We climb into our small rowing boat and the oarsman unties the ropes to cast off. Out of the small harbour and into the bay we float to a world apart: miles of ocean, a warm wind. The boat rocks with the waves and transports us to the edge of fantasy. We are off to see the dinosaurs.

Two hundred million years ago the sea there was much warmer than today, with very little wind and only small waves. Visitors like us would find breathing difficult with less oxygen in the atmosphere, and with the high humidity we would feel distinctly uncomfortable. We cool off many times by swimming from the boat in water much less salty than what we are used to. One of our group swims into a large submerged object shaped like a circular buoy, which then squirts sea water all over us in the boat as we row off quickly, scared by the turbulence. It is an ammonite.

These predecessors of the modern nautilus have a flat spiral body protected by a tough round shell. They move sluggishly through the waves, just a little faster than us. Some of them are about the same size and just as seaworthy as our boat. Their large buoyancy chambers under the carapace suck and blow air like a jet engine, and their mouths flush out fish and plankton with clumsy movements. From evidence in their fossil graves we think that many of the smaller species did not dare leave the shore, fearful of being chased by larger predators. They basked in the strong sunshine waiting until the high tide for their next meal. Many of the fish in the same sea have long sharp snouts specially designed for fighting, while in the air there are the flying monsters, which we can see from our exposed position on the rowing boat.

A flock of more than twenty skin-winged pterosaurs swoop down to attack, their spearlike teeth ripping into the surface-feeding fish that we have attracted around our boat. The sea turns dark as the energy of these beasts churns up the water to rock our boat. But they are as scared of us as we are of them, and they fly off with their prey as fast as they arrived, leaving behind a trail of debris. That draws new attention to our terrifying position, this time from feathered foe: scavenging birds and Archaeopteryx. Their visit is as timid as the ammonites’. These early ancestors of the birds have left evidence in the rocks that they swallowed the remnants whole and then regurgitated the indigestible remains, as owls do today.

There is also evidence of the food chain to show that the ammonites ate the plankton, that fish ate the ammonites, and that dinosaurs ate the ammonites and the fish as well. Through the earlier Triassic Period, sharp-toothed dinosaurs called nothosaurs, 4m long with small heads, long necks and tails, swam with small paddle-like limbs and ate the fish. We know that fish ate ammonites because their teeth marks have been found on ammonite shell tests. Sometimes the food chain was extensive and new or different species got involved, arthropods for example (invertebrates with segmented bodies, like lobsters). Broken bits of their bodies have been found in the fossilised dung and stomachs of marine reptiles as well as the regurgitated pellets from birds of prey. In their turn, the arthropods may have fed on small ammonite species, which then lived on the plankton ooze.

The food chain is part of the system, all changing slowly in tune with the environment. Together they form part of a gentle rhythm. If one part changes, the rest is affected. Nowadays, very similar rhythms are generated when crowds of people create an atmosphere of bustle of the kind you sense in airports and hotel lobbies. Out of the season there is a constant stream of people walking in different directions, positive with a sense of purpose, living and working with quiet efficiency. However, around Christmas and the high summer holidays they are different places, overcrowded beyond what the system was designed to carry, with people jammed, waiting, arguing in frustration.

The pace of life on the planet also changes through time. Ice ages and periods of high volcanicity are the equivalents of the busy, rushed days before Christmas at Heathrow. Conversely, the Jurassic and Cretaceous were off-peak times, with only one or two spurts of intense activity. Throughout this time biological evolution continued at a slow and steady pace in response to the equally modest environmental changes. Just as in airports and hotel lobbies outside rush hours, there were a few new journeys started, a few new species registering, but no major upsets. The relatively stable environment, the ecological balance and the steady growth in diversity saw evolution work mainly at the species and genus level. It was a bustling rhythm but without major divergences or catastrophes.

Back in our Jurassic boat we approach the shore, where we can see species of the two dinosaur groups. The Saurischia, with hips like lizards, stand on two legs to fight other animals in the famous Tyranno-saurus pose. Those with the heavier bodies and smaller heads, the Ornithischia, browse on the tough leaves of cycads and conifers, and walk around passively on four legs. This other group, with hips like birds, were vegetarians. They were also heavily armoured against attack from early Saurischia like Tyrannosaurus.

We know so little about their inter-relationships that the view from the rowing boat remains a fairy story. A popular view is that all this lighting, all this competition between individuals and species, is the motor of evolution. That is a myth from Victoriana, placed under the Darwinian banner of ‘survival of the fittest’. It’s an old-fashioned concept that should be banished to the annals of what is wrong about biology. Now, we know that the complex relationship between the organisms and the environment is also important. Evolution is less to do with winning battles between species and individuals, more to do with being able to live well together in the same environment. It is not necessarily the strongest that succeeds, but the most adaptable to new environments that might develop suddenly and unexpectedly.

In the tranquil times of the Jurassic and Cretaceous there were very few and undramatic environmental changes. Temperature and CO₂ concentrations steadily increased well above today’s values. The vicious battles between individuals and groups of Mesozoic monsters did not encourage major evolutionary changes. New species took over from earlier ones, a few new Families originated when there was a major altercation in battle with other animals or with any of the rare environmental changes. A few species and even genera became extinct. There was peace and relative quietness on Earth: evolution happened on a small scale, origins mainly at the species level, a few genera and fewer Families. Without big environmental changes there are few, if any, big evolutionary advances. Especially during the middle of the Jurassic there were only small and subtle changes in the marine and terrestrial environments. Without catastrophe there were only small evolutionary changes during the time, usually at the level of the species and genus.

Of the many important things to be learnt from these most tranquil of ages, there is one that most people do not expect. A popular view is that all the fighting, all the business of one thing eating up another, is the primary drive of evolution. They say it leads to the evolution of man and our seeing ourselves as the most powerful beings, sitting at the top of the evolutionary tree. This is not how nature works. The ammonites that ate most fish or resisted attacks from a soaring Pteranodon’s beak didn’t necessarily do any better than the more compromising species. So the bravest ammonites, charging off to battle in the front lines, perished in larger numbers than the more modest cowards who had found a safe niche.

What did survive were those most able to succeed when the environment changed. So the creatures that come to dominate at any given moment do so, not by power of fighting but by chance. They have just happened to fit into new surroundings at that particular time better than the others. As the environment, or internal biology, or social behaviour, changed, so they just happened to be in the right place at the right time with the right kind of biology. Now, humans think we are at the peak, just as the dinosaurs were through these Mesozoic times before the Cretaceous-Tertiary mass extinction. But once again the environment is changing dramatically.

Geologists are the supremos at understanding environmental change. In the formations of rocks they recognise signs of change in the atmosphere, the land and the sea, all connected and dependent on one another, linking events through time. Among the more dramatic changes through the Earth’s history has been the rise and fall in sea level – affecting drifting continents, changing climate and wcather systems – different atmospheres, and changes in sea composition. The planet is still clothed in very many environments which are changing, even now, though at different rates. It is an incredibly complex system, which we are only just beginning to follow. Our Jurassic boat trip, offshore from Lyme Regis, can help with some observations of the kinds of thing that happen during hundreds of millions of years.

As well as its quay, Lyme Regis is famous for its fossil ammonites. Theirs was a very different sea to the present English Channel which laps up to the Dorset coast; indeed, it was quite different to any sea in today’s world. At the beginning of the Jurassic period, all the world’s land was one huge C-shaped continent called Pangaea (see figure 2.1). What is now southern England was low-lying land right inside the concavity of the C, the site of river deltas from the north, west and south. In this very sheltered coastline the shallow sea carried lots of debris from the land. This meant that the sea was brackish, with much less salt than usual. It was ideal territory for ammonites and dinosaurs, as well as marshy plants, but not good for many others.

Slowly, the continental plates in that region began to move, causing Pangaea to break up from the central axis of the C-shaped land mass. The sea level fluctuated, the dry landmasscs began to form, the marshes grew less brackish, and eventually a new shallow sea stretched westwards to the other side of that C-shaped continent. The complex interaction of processes swung backwards and forwards, but within a few million years North America had separated from South America. In turn, this caused global sea currents to change completely and rougher weather came to Lyme Regis as the sea level rose. You can see this changing global geography at the website (biodiversity.org.uk/maps/palaeo).

These complex pictures contain objects from many different disciplines. To make a sensible interpretation the trick is to work out how to separate all these oscillating signals from different parts of the system. It needs full and broad knowledge and often demands evidence through long periods of geological time. In the first place we have to recognise the changes by breaking the system down into its component parts. Then we have to put these facts back together again to prove that they really are working together as part of Earth’s whole system. At last, that trick is beginning to make some sense through a multi-disciplinary scientific approach.

Without clear evidence, there is a suspicion from the foggy images of the whole Earth system that nature changes very often. These changes can take anything from a matter of seconds to millions of years to occur, at different oscillations and cycles. In Lyme Regis, 200 million years ago, the evidence points to many different cycles of change for many different environmental factors. Although the ecological changes were on a small scale throughout the Jurassic, there were enough to stimulate small evolutionary changes.

Biodiversity is a complex system and was growing even then, with off-peak rhythms of change. The continents were drifting, food cycles were changed by slight shifts in climate and ecology, CO₂ concentration and temperatures were rising. But it was so long ago that the evidence is distorted, fragmented or often destroyed by erosion. It’s difficult and often impossible to understand the timescales of the changes.

When you are suddenly dumped into the middle of changing systems like these it’s hard to get your bearings. For example, our understanding of long-term changes in the weather depends on whether we have data about the changes over a broad sweep of space and time. No wonder it’s difficult to say what life was like 200 million years ago on the basis of describing the Jurassic rocks at Lyme Bay, especially since all the changes appear to have been relatively modest. But some ideas about how to make sense of complex systems came 150 years ago from a surprising source: a retired railway engineer.

Herbert Spencer stopped working on the railway at Derby in the 1850s to be a successful writer and thinker. At one time he was the deputy editor of the Economist magazine. His social theory was that the best-adapted humans reproduce most effectively. ‘The survival of the fittest’ was his phrase, enjoyed by Darwin and ever since by politicians and misguided students. In the 1950s, the famous American geneticist Sewall Wright applied the same ideas to animals in the wild, seeing them as ‘fitness landscapes’. Wright was one of the first to try to monitor genetic and evolutionary changes, along with J. B. S. Haldane, George Gaylord Simpson and many other bastions of evolutionary biology in the mid-twentieth century. They didn’t have help from DNA or from much environmental data but they did have ideas about processes. In population genetics the conventional variable for fitness is ‘W’, encouraging one of his students to ask Wright if that ‘W’ was after ‘Wright’. ‘No,’ came the reply, ‘it’s for “Worth”.’

The rugged peaks of a fitness landscape mimic the conflicts and constraints of biodiversity (see figure 2.2). Both have features that show a range of sophistication: good at this and bad at that, works well in one state and not in another – different certificates of fitness. The complex system of biodiversity, whether in the tranquil Jurassic or the fast-changing world of today, has the fittest individuals at the peaks where they are most likely to succeed.

What’s new and exciting about fitness landscapes is that some of the oscillations we find in the fossil record follow the same patterns that we find in many other natural systems. Just as natural landscapes can cope with the complexities of changing weather and ecology, so all features of life on the planet fit together systematically as they keep changing. Organisms at the peaks one moment slip down when some other part of the system changes, but equilibrium is maintained and the rhythms continue.

Right through geological time internal and external planetary factors have made enormous impacts on land shape and climate. Internal factors come largely from plate tectonics, external ones from things like impacts. It has been rare for the environment to stay the same for very long, because the system is so dynamic that a kind of chain reaction goes on in different directions and dimensions. At the time of the ammonites and dinosaurs, during the Triassic and Jurassic periods around Lyme Regis, the temperature and climate were increasing in very slow cycles. Within these complex ups and downs different sediments around the Dorset coast hold clues to what actually happened. Evidence of the changes in vegetation was left behind in the rocks as fossil pollen and spores from conifer and ferns. They show a steady rise in temperature then, which fits with increases in coral reefs, bivalves and many other groups of shallow-water marine organisms which enjoyed the warmth.

Not all scientists trying to reconstruct changes in the Jurassic climate agree with this account. For example, results from our own research group are in direct conflict with those from two other students of Bill Chaloner, Jenny McElwain and David Beerling. Our group’s work was done mainly by Richard Hubbard, who analysed counts of many thousands of pollen and spores from Triassic and Jurassic sediments, 210 – 190 million years old. One of his principal components was a group of plants we believe to be cold-loving, and they show up right on the boundary between the Triassic and Jurassic periods, 205 million years ago. Jenny McElwain and David Beerling found a warm phase at just that same interval. They had assumed that the density of pores on 2oc-million-year-old fossil leaves is a marker for CO, concentration. Fewer pores mean that less water is lost so temperatures are higher. We’ll have to wait for evidence from elsewhere to see who is right, whether it was a cold snap or a warm one.

Disagreements such as this are scattered through the scientific literature and occupy much time in tearooms and at conferences. The whole scientific way of thinking is based on the challenge of proving something wrong, on refusing to accept the conclusions of others and hopefully being able to prove the hunch right. Journalists and teachers do a bad job in conveying this conflict to members of the public, let alone to politicians. Both groups want straightforward answers to straightforward questions and don’t understand it when they can’t get them. Invariably they force out an unsatisfactory compromise from the bullied scientist and give ill-informed impressions about what’s going on. AIDS, BSE, GM food and foot and mouth disease are recent examples of confused public awareness of really complex controversies which, as yet, have no clear or complete explanations.

In the Jurassic landscape, animal and plant life was well adapted to the rising temperatures. It meant that many of the features familiar in our presently much cooler climate were missing. One is fur, which then would have made mammals’ bodies too hot. The thin leaves of deciduous trees and herbs would be scorched, so they were not evolved. The few small mammals that were beginning to evolve had no fur and the plants remained thick-skinned. As with other new groups the diversity and numbers of individual mammals were small, a measure of their limited power to compete for space and food with the other beasts. They were on the lower peaks of the fitness landscape. It was to take major changes in the environment before they were free to diversify and grow larger.

Today, the beaches and the cliffs of Dorset are full of the fossilised remains of these times, especially the dinosaurs and ammonites. You can also see Jurassic coral reefs preserved at the base of Church Cliffs, relics of the shallow Jurassic sea, and, as the sea deepened, so limestone and shales form the upper part of the cliff to bear witness to marine sedimentation. The majesty of famous Dorset coast cliff scenery comes from the series of small environmental changes like these. Other ancient events show up as bands of pebbles, upright yellow sandstone, slouching grey clay, folded strata, caves and bridges eroded out of softer rocks by the sea. These forms of geomorphology give further clues to the Jurassic landscape and how what’s preserved has been altered time and again since they were formed up to 2oo million years ago.