The growth of grass: how life adapted to a cooler world
Tuesday 10 February 2009
Cooler temperatures tend to mean that less water evaporates from the seas, resulting in reduced rainfall in many inland areas. Such conditions gave birth to the great grasslands of today which replaced forested areas that need larger quantities of rainwater to sustain them. The prairies of North America, the pampas of Argentina and the grassland steppes of Europe and Central Asia all emerged during this Ice Age.
Animals adapted to these new conditions, evolving into larger species and more numerous herds as competition for survival increased in the wide-open spaces. The spacious grasslands offered feeding grounds for birds, which rose to the challenge of the conditions of an Ice Age by migrating in vast flocks to find the best (and avoid the worst) places to live. Grasslands rich with seeds provided convenient stopping-off points for feeding. Blackbirds, meadowlarks, sparrows, quails and hawks all thrived thanks to these new steppes which eventually came to cover more than 25 per cent of the world’s land surface.
The last continental dodgem of significance in the Earth’s giant fairground ride was South America. About 3 million years ago it collided with its North American neighbour, joining up via a thin sliver of land – today’s Panama. The effect of this collision was as cataclysmic as the rest. For the first time ever, mammals that had evolved in complete isolation could now walk between these two continents.
Opportunities for some spelled disaster for others. Llamas, dogs, cats, lions, bears, horses and rats roamed down to the south, while armadillos, opossums and sloths moved up to the north. New predators were introduced, competition for feeding grounds increased. This event, called the Great American Interchange, reached its peak 3 million years ago. The biggest casualties were the great marsupials of South America, the pouch-bearing lion, and the rhino and elephant lookalikes whose fossilised bones still stud the Earth.
They were no match for the violent invaders from the north – the giant cats, dogs and bears. Extinctions were the inevitable result. The collision of these two continents had its own dramatic effect on the Earth’s weather. Atlantic sea currents were forced northward, and were prevented from mixing with the waters of the Pacific because their path was now blocked by land.
A new weather system spluttered into life, as a result. The Gulf Stream began pumping warm air northward, heating up north-west Europe by at least 10C. It also brought more water vapour, freshly evaporated from the Atlantic. As its clouds blew northward towards the Arctic, rain turned to snow, which settled over time in layers on the cold seas, where it turned into thick packs of ice. The effect was that by 3 million years ago the Earth had donned a second ice cap, this time in the Arctic.
With ice caps now on both poles, the Earth grew colder still. Much of it was plunged, literally, into a deep freeze. Colossal ice sheets groped down from the poles, sometimes as far as modern-day London, Paris, Berlin and Moscow. They stretched past the plains of Canada, over the Great Lakes and as far as New York. Most of Russia and all of Greenland were iced over. For thousands of years the seas all around were thick packs of ice, many soaring skyward more than a mile high.
Sea levels plummeted because so much water was trapped as ice. It was possible to walk from where Dover is today across to Calais, France. There was no English Channel, just flat, desiccated tundra. Even beyond where these enormous walls of ice ended, it was still too cold for anything much to grow.
In northern Europe temperatures often fell as low as -80C, while winds whipped up to speeds of 200mph. Massive frozen rivers of ice were also on the move. They advanced like gigantic bulldozers, slowly but relentlessly scratching their way down the sides of the Earth and then gradually retreating as temperatures rose again, melting as they went and leaving freshwater lakes in their wake. These glaciers carved out many of the world’s most spectacular lakes and valleys – from the Lake District in England to the Swiss mountain valleys, and from the Great Lakes of North America to the fjords of Norway.
Conquests of ice like these have occurred as many as 30 times in the last 2 million years. On each occasion huge boulders of rock have been carried miles away from their mountains of origin by the ice, rockfaces have been squeezed and scratched into the most contorted shapes, and the ground has been pushed down – only to bob upwards again, slowly, after the massive packs of ice fell into retreat. The United Kingdom, for example, is still rising after the retreat of the last massive ice sheet 10,000 years ago – but only by about 1 millimetre a year.
Life thrived despite these enormous and dramatic events, because nature’s changes were slow enough for succeeding generations of species to adapt successfully. Not all life was affected. It was still warm in the tropics, and some rainforests remained, although there were far fewer than before the Ice Age began approximately 40 million years ago.
It was now a cooler, grassier age. One where the seas rose and fell, opening and closing causeways to other lands, and where relentless thick packs of ice came and went like clockwork. It was into this dramatic world that mankind was born.
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