Extinction: Evolution and the End of Man. Michael Boulter
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.
The K–T catastrophe
For the dinosaurs, ammonites, and lots of other groups, life ended after an event that happened in Mexico one warm spring day. We know it was a spring day because a magnolia flower was found in the wreckage. A chunk of rock from outer space, 20km in diameter, had hit the earth off the Yucatán peninsula in south-east Mexico. If the meteorite had splashed in deep sea perhaps it would have been different, but the shallow-water impact made it one of the most severe physical crises in Earth’s history. The explosion shook the planet and the firestorm quickly spread up across most of North America. The fall-out of dust and smoke was blown east to Europe and beyond. We know that the projectile itself had come from a westerly direction because bits that broke off during the descent through the atmosphere have been found in sediments of the Pacific Ocean.
Flames and heat fanned around the planet. Rock and soil, steam and vapour, charred splinters, roast meat, were scattered into the acrid atmosphere right around the northern hemisphere and most of the south. Weather patterns changed, there was no light on the surface of the Earth. Life stopped or went into hiding; nothing thrived. The thick clouds of debris took many years to clear; the burnt vegetation on land, the acid and ash raining into the sea, halted the majority of life.
There are no graves like Pompeii’s, no swamps for easy preservation, so most of the remnants are gone and we are left to speculate. We don’t really know how long the fires lasted, their power, their full geographical extent. Like accident investigators or forensic scientists, paleontologists sift through the wreckage of 65 million years ago, looking for evidence. But in this case, we found the evidence before we understood the cause. To make matters worse there were very few rcfugia, places to hide, that survived the inferno, and of those many have broken up through the intervening 6 5 million years. Many are yet to be discovered as the fossil record unfolds.
After the explosion the most vulnerable passengers on Earth were the largest animals, the ammonites and dinosaurs, and none survived. The oceans lost much of their dissolved oxygen, so most marine species died within weeks of the impact, and became extinct. Once the black clouds and the fires had subsided new life came to the planet. Some of the carnage itself, both on the land and in the sea, remains as sediment for us to explore and thereby to understand more of what went on through those terrible times. It is at the Cretaceous-Tertiary boundary 65 million years ago, commonly known as the K-T boundary (the K coming from the German ‘Kreide’, meaning chalk), that we see remains of the catastrophe preserved in rock outcrops from many parts of the northern hemisphere, especially north of Mexico, to the east of the Rocky Mountains.
The thin band of sediments is rich in iridium, an clement rare on Earth but common in some asteroids. There are glass globules, fractions of a millimetre in diameter, the relics of molten silicates after the explosion. It also contains burnt vegetation, as well as the rare spring flower. We can even detect remnants from the lump of rock itself, several kilometres beneath the surface of the Chicxulub crater off Yucátan.
It’s only in the last few years that pretty incontrovertible evidence for all this has come together. Scientists from very different disciplines have become involved, from all over the world, bashing the story into shape. The idea of the K–T impact event began in 1977 from a coincidental conversation between two people from different disciplines. Walter was a young field geologist and he was talking about a new specimen with a famous Nobel physicist called Luis. It was strange that it should happen at all: geologists don’t usually talk to physicists. But in this case they were father and son, the Alvarezes. So the contact was accidental, not part of a planned experiment.
Walter Alvarez had collected a chunk of rock from near the mountains of Umbria, north of Rome. It had three layers: a chunk of white limestone, a thin layer of clay, and then more limestone, red this time with no fossils. The lower white layer was full of microscopic seashells well known to be Late Cretaceous in age. The red rock was like that found all over the region, known to be Early Tertiary. The clay lay between these two differently dated rocks. Did it come from the Cretaceous or the Tertiary? Were all three layers laid down in one continuous sequence? Or docs the clay layer represent some kind of break in the process, a gap?
Walter