Читать книгу Extinction: Evolution and the End of Man - Michael Boulter - Страница 10

The variations in geomorphology come from changes in weather and climate, in the position of the continents, sea level and so much more. These factors, in turn, influence the pace of evolutionary change of the organisms in the same system. As with so much of nature, their world was knitted together into a humdrum rhythm of normality. In different ways all organisms reach this state of dependence on others, both other species and other members of their own. There is also a dependence on the particular environment for each individual and community. The eventual effect of this interdependence is for organisms living together to establish a state in which there is a more or less constant harmony. Together, the whole of biodiversity reaches a stable state. The mature systems appear to take care of themselves and don’t seem to change unless there’s some interference. Individual organisms become fully integrated into the community of their own species and into their wider ecosystem. A question we have to ask is whether or not such stable states can go on for ever, or whether ‘system Earth’ makes sure that, in time, some kind of change is inevitable.

I used to travel regularly from London to Devon, relaxing through the English scenery of Brunei’s Great Western Railway. It goes west from London to Bristol and then south to Exeter and Cornwall. The railway was one of the first of its kind, built during the 1840s just after Darwin had returned from his expedition to South America on the Beagle. Taking this train journey through the railway cuttings of western England is also a geological voyage. It follows the strata of rocks back through the sequence of geological time (see figure 1.2).

The clays of the Thames valley outside London were sedimented from shallow seas in the first half of Tertiary time. Heavy grey clays alternate with lighter sands, all getting older as we travel west. They are 55 million years old at Reading and 5 million years older at Newbury. An hour from London, the train passes out of this modest landscape into the grand sweep of the chalk downs through quaint market towns like Hungerford and Marlborough, past the white horse on the chalk hill near Westbury. Jurassic sandstone cottages show up around Bath. They pass over the 200-million-year-old rocks around the redbrick terraced houses at Taunton. It is the same sequence of rock formations that outcrops on the Dorset coast just to the south.

As the train passes the changing environments, the different local building stones still serve as reliable clues to what was sedimented 200 million years before. There are few parts of the world where it is possible to travel back so far into geological time so quickly and see the sequence of rocks as they were laid down in the changing environments of the past. The place on the planet now occupied by the British Isles was in a pivotal position through most of the Mesozoic. That spot was inside the C shape of Pangaea (see figure 2.1) before it split up. Sometimes it was land, sometimes sea, and often on the floodplain between the two. Placed between Euro-Asia and what is now North America, the region is a microcosm of the despositional history the two blocks experienced on a larger scale.

The train stops at Exeter, on extremely complex geological structures due to the prolonged influence of the Dartmoor granite nearby to the west. Potassium-argon dating shows that the granite comes from volcanic eruptions 280 million years ago (www.phdcsm.freeserve.co.uk/overview.htm). Dartmoor itself, as well as Bodmin Moor and Land’s End to the west, are the remains of hard granite from these volcanic structures of the early Permian (see figure 1.2). The region around Exeter and strips of land down to Torquay and due west to Crediton are what remains of the lava flow from the eruptions. The famous red soils of Devon are even more widespread signs of the iron-rich rocks formed more than 200 million years ago. In good scientific tradition, the age of these events has been in dispute for twenty or more years, as geophysicists refine their dating techniques.

As usual, one morning on that train I had some papers with me. On this occasion I was sifting through some graphs of results from a new search of our fossil occurrence database. Funnily enough the data were from fossil records in rocks showing the very same range in geological ages as the train journey: from the tropical swamps of the Tertiary from London to Reading, the deep oceans of the chalk near Hungerford, the crisp foliage and deserts of the Jurassic near Bath, to the swamp and ocean of the Carboniferous in North Devon. The graphs condensed data from all over the world, not just south-west England, and they were a horribly confused and chaotic mess. They plotted the number of records of each kind of fossil in every one-million-year interval of time. All my earlier attempts to interpret them had failed, so I had put them to one side, waiting for a quiet moment.

Most of the fossil records were from plants and microscopic marine plankton, long names, hard to pronounce, showing up the changes in vegetation of past forests and seas. They were from sediments from all ages back to 200 million years ago. On land, at first ferns and exotic evergreen conifers and cycads dominated the flora, and then flowering plants became common in the lower-lying land. In the sea, plankton blooms dominated most of the warm water, though through one interval they were conspicuous by their absence. The environmental changes of a catastrophic event seemed to be showing through.

Somewhere near Westbury – I can’t remember exactly where because I had become so excited – I noticed that just five of the hundreds of curves showed the same clear pattern of change. The changes also showed up on the five graphs at about the same time in the Cretaceous rocks, 90 million years ago, at the Cenomanian – Turonian (C–T) boundary. Each curve plotted the number of occurrences through geological time of the plant groups I had selected and all five suddenly rose at this same time. The groups touched on the most sensitive mystery of paleobotany: how did the flowering plants first evolve?

Surprisingly, birch and elm were among these first five to diversify suddenly then. There was also a rise in the records of an early mangrove plant. But the biggest rises were of two large extinct groups of dispersed pollen, the aquilapolles and the normapolles. They have been known for about fifty years and look quite different from any other pollen, fossil or modern. Their occurrences peaked 20 million years later and then slowly became extinct. We still don’t know anything about their parent plants’ structure or status but they do appear to have played an important role in early flowering plant evolution.

Why, you might ask, should I be so excited about a few graphs? For years my fellow scientists have been trying to solve what Darwin called the ‘abominable mystery’ of the origin and early evolution of the angiosperms, flowering plants. Many new discoveries of beautifully preserved flowers have pushed the date of the earliest known flower further back in time to the beginning of the Cretaceous. Arguments about the source of that origin and its subsequent evolutionary pathways are also raging between different specialists. They are all very lively topics.

The five curves show the first sign of a break in the monotonous vegetation cover that most land surfaces had supported since the beginning of the Jurassic 200 million years ago. It was dominated by the thick-skinned trees of cycads and conifers, Ginkgo, and ferns. Mostly it was a tropical climate and landscapes were unchanged through millions of years. The C–T eruptions were a major threat to their survival and mark a radical change in the way life was ordered. The onset of flowering plants as major components of the forests brought on more complex ecosystems as evolutionary rates increased for many groups of organisms.

There is another reason for my excitement on the train near Westbury. It was the first time I had cause to feel that my research group’s new approach might succeed. Who knows what patterns might emerge? What might we find in other large databases that I knew were being built? If this first set of results gives a significant surprise, what might we find at other moments in time, particularly at the boundaries between two periods? Are there statistically significant patterns lurking within the huge spreadsheets of data? What if the data show up groups of names with common attributes? Perhaps there are clues about evolution and taxonomy. It’s moments like this that make science one of the most satisfying things I can think of doing. It must be great to win a big race, score the winning goal, give a fine performance at a concert, cure a really ill patient. For me, the kicks come from having crazy ideas that may come together and make sense.

The realisation of a big expansion in occurrences of these plants 90 million years ago fits evidence from other sources. Around the Pacific rim there were a number of thin patches in the Earth’s crust covering deep hot spots waiting to blow up from inside. Most of the volcanoes all erupted at around the same time at the end of the Cenomanian 90 million years ago, and this drastically reduced the oxygen levels from the world’s oceans. It also led to high sulphur dioxide in the atmosphere, global warming and acid rain. This sequence of events killed the dominant trees of those forests, the conifers. Sure enough, the curves from our database show a huge drop in the records of pine just before the five flowering plant groups increased. The deforestation caused by the volcanic action was the chance for which the angiosperms had been prepared, with their much more sophisticated ecological tolerance and stronger reproductive abilities.

The five plant groups had originated many million years earlier. The genetic recombinations, the new biochemical pathways that they followed and the first physiological adaptations to the environment all happened long before. These early traits were becoming tried and tested on a very small scale. Modern inventors do the same with their prototype models, making sure the thing works in all circumstances and making adjustments when things do go wrong. Then, when the time is right, a full-scale sales campaign launches the new product into a new gap in the market. It was like this at the C–T boundary 90 million years ago. The five prototype groups of new angiosperms had been tried and tested for millions of years and they worked well, though opportunities were limited. Suddenly, lots of new space opened up where once there had been conifer forest. The explosion of numbers of individual angiosperms had begun.

When I first noticed this sudden fall in the number of records of pine, another thought about the potential value of database mining came into my head. It was an application of my inversion of Lyell’s principle of uniformitarianism, mentioned in chapter 1. If the fall in the occurrence of pine has such significance in the Cretaceous, could the present-day falls in occurrence of so many species have comparable consequences? If falls in occurrence in the fossil record show clear trends, do the same trends show up with Red list species today?

Meanwhile, there’s more excitement back on the train from Westbury, though by now I was just coming into Exeter. It was not until late in that same journey that I happened to glance at some of the other curves in my collection. Just at the beginning of the Tertiary, 65 million years ago, several of my curves showed sudden increases in the occurrence of flowering plant Families. Most of the temperate angiosperm groups which are known as the Arcto-Tertiary elements showed a clear response to the change: there was a massive increase in their diversity.