Extinction: Evolution and the End of Man. Michael Boulter
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