Читать книгу Extinction: Evolution and the End of Man - Michael Boulter - Страница 7

To understand how this flash fits into geological time, it helps to take a virtual journey on a time machine in which one complete day represents the 65 million years that have passed since the last great environmental catastrophe when the dinosaurs became extinct – a journey in which today’s universe is not yet seven months old. Let’s go to the beginning of time, the Big Bang. Imagine it happening just after midnight on New Year’s Day. Within a fraction of that second, from infinite density, the universe begins. Time begins.

It’s intolerably hot and the thick atmosphere makes it difficult to see much. By 18 March of our virtual year the universe has reached 5,000 million years of age. There’s not a lot we know about what was happening then – exploding stars, balls of hot gases, no familiar planets to the suns. It’s getting cooler, or rather, it’s not so horribly hot. By 1 1 June one of the chunks of matter orbiting our sun breaks into three. Mars and the Earth are formed, and about 500 million years later, on 19 June, our moon. The temperatures gradually fall and by early July life begins on the Earth. (Some say it came from outer space, others that the necessary organic molecules lormed from the planet’s own inorganic chemistry.) There is no land cool enough for life until around 10 August, so until then early life is aquatic, often with a vast range of, to us, weird forms. Stephen Gould’s Wonderful Life describes some of these from the Burgess Shale, which Derek Briggs and scientists from the Smithsonian Institution have brilliantly described and illustrated.

Large numbers of these 54o-million-year-old fossils from British Columbia show that an enormous structural diversity was present early in the history of life. Because they are among the first non-microscopic organisms they have many unusual features, hard to find in fossils that lived since, and controversy continues to haunt our interpretations. The remains were discovered in 1909 by the paleontologist Charles Walcott, who explained them as ‘a sublime conception of God which is furnished by science.’ Their different shapes and structures show unusual variety, and many scientists have thought them to be unlike more recent animals that they have seen then representing extinct groups. They also seem to have diversified suddenly and become extinct just as quickly. A more recent approach has been to look at the similarities between the fossils’ characters. Links have been made to familiar groups like trilobites and sponges. The confusions should be no surprise, because most new things start off by looking strange.

Our 65-million-years-to-a-day journey is reaching familiar territory now. Through middle August life is evolving very fast, diversifying day by day – vertebrates, ferns, dinosaurs. Some groups become extinct in those early and mid-August days: trilobites and jawless vertebrates. Then, on 20 August, the dinosaurs become extinct as well. On this time scale, that happened yesterday.

On the chime of midnight for the start of this new day, 2 1 August, we wake from one of the planet’s most horrific nightmares. The northern hemisphere is completely blacked out with smoke and dust in the first milliseconds of our virtual day. The vegetation returns to normal after a few minutes, with only a few changes, while the oceans take longer to clear up the debris that has rained down from the dirty clouds. It has mixed into the sea, and robbed it of much oxygen, causing extensive extinctions of plankton and fish.

At an hour past midnight the noxious outfall from the nightmare has completely cleared and a bright new world is beginning to take advantage of the new opportunities. It is not unlike the upturn of the Western economies after the Second World War – hesitation to reacclimatise at the beginning, then a surge in diversification to reach new highs. This is a pattern I return to in chapter 5. Throughout this Paleocene period of geological time, up to about 3 am, the environment is establishing new ecological niches in the very warm climates. This allows the number of mammal species to peak by the 6 o’clock dawn, comfortable in the new reality that they arc no longer the prey of dinosaurs.

A very clear trend is developing which we will see characterising this whole last day of our journey. There are many new species ranked together in new large groups of genera and Families, but there are very few extinctions. Overall, there is a massive increase in biodiversity. In the early morning there are the first primates, the first horses, the first whales, whole new major groups of animals, each with hundreds of new species.

At 9 o’clock in the morning it is 49 million years ago, during the period of geological time we call the Eocene (see figure 1.2). The planet is becoming quite a familiar place, with dense mixed woodlands and savanna grazed by herds, and there are even cocks crowing. Monkeys are one of the big new groups to originate and quickly diversify. Global temperature differences are much less than today’s, the tropics being about the same and the poles equably temperate. The shores of the Arctic Ocean, as well as the hills of the Antarctic continent, have warm tranquil climes with low sun in the long summer and little if any frost in the darkness of the long winter. If there had been humans and a travel industry then, it would have been a tourist’s paradise for half the year.

Just before lunchtime, 35 million years ago on the geological timescale, temperatures have peaked due to high carbon dioxide concentrations in the atmosphere. The greenhouse effect in these times is much stronger than now. Continental drift causes the North Atlantic to widen and at about the same time to open into the then temperate Arctic Ocean. Temperatures at the poles begin to fall. We’re not sure how this happened but perhaps it was caused by some astronomical phenomenon, or by the changing positions of the continents and oceans.

In the nowadays highly populated northern temperate regions of the world, tropical rainforest stretches from Asia, eastwards to Europe and the newly separated continent of North America. The very rich faunas and floras begin to take on a familiar look. The world is becoming more varied, with more species than ever before, and its ecology more diverse, with a wide variety of modern habitats. With the wider-ranging weather and climate, and the peak diversification of animals and plants, come maximum complexity and range.

By now it’s early afternoon and many familiar groups of animals and plants are making their first appearance. While only the odd species is becoming extinct, all Family groups continue to diversify. There are now apes in the warm forests, and other new Families including camels and deer in the shrub. There are also new grasslands, legumes and palms. Evolution continues at a high rate, though as yet, in the Early Miocene, there are few, if any, modern species. They will come later, in the evening, migrating south to the more restricted equatorial latitudes of the modern tropics, and evolving into different groups to populate the cooler forests, wetlands and grasslands of the more temperate regions.

Through the afternoon, temperatures become much lower, the weather much more extreme, the ocean currents developing trends familiar today. The changes are not smooth or simple, but occur as oscillations over different timescales. Present-day changes in weather show that these uncertain variations continue. Around teatime the poles become covered with permanent ice, which slowly spread towards the Equator, developing glaciers and icebergs cooling the oceans. Coldloving mammals grow large woolly coats and take advantage of the new niches in the freezing landscape. They diversify accordingly.

Early hominids first appear around 11 o’clock in the evening, and at 20 minutes to midnight Homo habilis is soon followed by Homo erectus. Neanderthals and then modern humans originate a few minutes before 12 o’clock, depending on your view about the precise time, place and definitions of the species. This happens during the Pleistocene, the period of the ice ages, which began nearly 2,000,000 years ago. The northern hemisphere continents become glaciated at five- to ten-minute intervals for half an hour or so before midnight, the end of our day. But then, at just two seconds to midnight Jesus Christ is born to begin the first millennium of the calendar we record today.

Into the future, no one’s too sure how long our virtual journey will last, how far away that elusive end-point really is. Even the dates I’ve given for the virtual journey so far are debatable, different specialists arguing about them all. But what does seem to be pretty well agreed now is the basic plot of the story. Our Earth really is on this kind of journey. These changes really arc happening.

Our planet isn’t the only part of the system that is following the sequence of origin, expansion, maximum diversity, then finally contraction. Species and groups of species do the same thing. I suspect that man-made institutions like governments and empires, businesses and fashion also follow the same patterns, but I have no data to test that hypothesis. If so, this raises an important universal issue: can all these apparently different systems be following a similar pattern?

But so many different things are influencing the Earth system itself that it’s hard for us to be sure of the full effect of each one. We are just beginning to be able to take those that we recognise together, only just beginning to think of our own role in the system of nature and the many different influences we are making. Can the behaviour of our ancestors building Tirefour Broch 2,100 years ago, or hunting mammals in Alaska 8,000 years ago, be seen as part of the same evolutionary process that I’ve painted for the last 6c million years? Even more basically, can the human mind, from our position inside the system, have the ability to interpret these complex patterns and explain the way life works on the whole Earth?