The first stirrings of humanity
Tuesday 10 February 2009
Following the demise of the dinosaurs, life on Earth repeatedly underwent huge, convulsive changes.
Alterations in climate – caused by massive collisions of the Earth's continental crusts – created ice caps that plunged much of the planet into brilliant white winters lasting thousands of years. Then, as the temperatures fell and rains reduced, grasslands replaced trees and forests, birds evolved new patterns for migration, and animals either adapted or died. About 11 million years ago, gorillas – a species of ape – began to experiment with a new way of life, venturing into the new grassy open spaces; from about 4 million years ago, some of their descendants mastered the trick of walking on two feet rather than four. In short, the stage was gradually being set for the first appearance of our own species, Homo sapiens – the subject of tomorrow's booklet.
The great cooling: how climate change created expansive grasslands and caps of polar ice
Remember those old-fashioned rides at traditional funfairs? Carousels, the helter-skelter, and, best of all, the dodgems or "bumper cars"? Bizarre as it sounds, we are all riding on top of the Earth's very own natural set of dodgems. They aren't quite like those in man-made funfairs, because the Earth's bumper-car ride travels extremely slowly. Also, whenever its dodgems crash into each other they seem to have a rather dramatic impact on the world's weather.
Welcome to nature's fairground. Gradually, over the course of the last 200 million years, the Earth's restless crusts of land have broken up to form today's giant continents. Actually, this was quite good for life on Earth, because it made it much harder for any single type of living thing, such as the dinosaurs, to dominate the land. Seas and oceans forged natural barriers as the crusts split apart. New species evolved, many learning to thrive in different habitats with less competition. Scattered continents also increased the number of beaches, sea shores, wetlands and salt pans – all serving to bolster life on Earth.
Since the extinction of the dinosaurs 65.5 million years ago, the amount of carbon dioxide gas (CO2) in the Earth's atmosphere has plummeted from nearly 3,000 parts per million (ppm) to just 284ppm by 1832 (although they have, as we will see in the final episode of our pocket history, risen significantly in the past 175 years as a result of human activities). Less carbon dioxide reduced global temperatures significantly during the same period, despite the Sun warming by as much as 30 per cent since the beginning of our 24-hour journey. Such changes in the atmosphere and the temperature have, in large part, been caused by the constant collisions of the Earth's crusts, which, although they move slowly in human timescales, have had dramatic consequences for life on Earth.
About 90 million years ago, the Indian continent sheared away from Africa, becoming, in effect, a giant dodgem on the loose. It raced north at the unprecedented speed of 15 centimetres a year, swivelling upwards and crashing into Asia about 40 million years ago, having travelled 3,000km. Scientists believe it moved at such a speed because this plate is very thin compared with most others.
When it collided with the larger, slower-moving Asian continent, it created the biggest mountain range on Earth. The Himalayas, and the enormously high plateau of Tibet, are thought to have been responsible for reducing global temperatures in a dramatic way, by removing large amounts of CO2 from the atmosphere.
Air cooled by the high Himalayan peaks condensed huge volumes of water vapour from the warm Indian Ocean into massive monsoons, which tipped their loads on to India and the southern part of the Tibetan plateau. As the rains fell, carbon dioxide in the air dissolved into the water, eventually to wash off via rivers and streams and to settle as sediment on the sea floor. With the CO2 now safely removed from the atmosphere, the Earth got cooler still.
Meanwhile, other landmasses were slowly but chaotically making their way across the globe, crashing together with no less spectacular results. At about the same time that India rammed into Asia, Africa pushed up against the seas that separated it from the same continent. The ocean floor buckled upwards, forming a series of land bridges across the ancient Tethys Sea, a stretch of water that once connected today's Middle East to the Indian Ocean.
These bridges are what probably gave the African monkeys their land route into Asia, where they then evolved into the first members of our family – the apes ( see Part 3). The first horses and camels came across from North America into the grasslands of Asia, via another land bridge that connected Alaska to the eastern tip of Russia, allowing them to settle finally in the deserts of the Middle East.
Africa bounced off from this collision, pushing northward into Europe – a process that threw up the European Alps, which stretch from France through Switzerland, Italy and Austria. Then, in the middle of this multiple vehicle pile-up, which began about 20 million years ago, Africa crunched up further into the Middle East, closing up the Tethys once and for all. (It was eventually reconnected by the opening of the Suez Canal in 1869.)
By 6 million years ago, Africa had come so close to what is now southern Spain that its enormous weight threw up a new range of mountains that encircled the Mediterranean Sea by land on all sides. With no connection to the Atlantic Ocean, the water in this enormous lake gradually dried up, leaving layer upon layer of dirty white sea salt on its bed. In some places today, these salt deposits are more than a mile thick, leading experts to believe that this basin may have dried up and refilled as many as 40 times over the course of more than a million years. Here the Earth's natural process for removing massive quantities of salt from the sea was working overtime, powered by the movement of the Earth's crusts in a fairground ride for giants.
Finally, about 5.3 million years ago, water burst for the last time through the chain of mountains which once joined Spain and North Africa. Colossal cliffs plunged downward – perhaps as much as 3,000m to the valley floor. That's more than 50 times higher than the Niagara Falls.
Just 100 years later, the whole Mediter-ranean basin was filled with water once again. With 170 cubic kilometres of water cascading down the huge waterfall every day, this must have been one of the most extraordinary and dramatic episodes in the entire natural history of the world.
The Earth had already slipped into an Ice Age thanks to another landmass on the loose, on the other side of the world. When Antarctica split away from South America, about 40 million years ago, it slid towards the South Pole.
Cold water from the Southern Ocean was now forced to circulate around Antarctica, instead of moving north to mix with the warmer waters of the Pacific and Indian Oceans. As the region cooled, a massive ice sheet developed over this once tropical land.
Today's Antarctic ice sheets are more than a mile thick and stretch across a land mass 50 times larger than the United Kingdom. Such an enormous ice desert caused sea temperatures to plunge by as much as 10C, its bright white ice reflecting the Sun's rays back into space, causing temperatures to cool even more. With a gleaming ice cap on its South Pole, the Earth now entered a new Ice Age – its first in at least 250 million years – all due to the chaotic journeys of its out-of-control landmasses.
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