11 May 2010



‘To treat life as less than a miracle is to give up on it.’
Wendell Berry

By around 30 million years ago the new layout of continents and a change in Earth’s tilt were beginning to create the distinct ocean currents and climatic zones of today. The following set of six zones is a lay-summary of the zones used by geographers.

Tropical climates are either hot and wet all year round, or hot all year but with monsoonal rainfall that divides each year into a distinct dry and wet season. Humid middle-latitude climates have dry summers (either warm or hot) and wet winters (either cool or cold). ‘Continental' climates have a lower rainfall and a huge seasonal temperature range. Arid and semi-arid climates have very little rain and a huge daily temperature range. Cold dry climates have enduring ice, tundra and/or conifer forests, and a very short summer. Polar and alpine climates are bitterly cold all year round, and any rain falls as snow.

Biologists call these climates and the organisms within them biomes, or ‘zones of life’.

In huge Equatorial rainforests, with the Sun virtually overhead every day and rain falling all year round, there is a huge variety of easy food for all kinds of organisms. And at this stage it was a relatively safe habitat for the diversifying primates. Most still ate young juicy shoots but some were becoming skilful at finding insects, fruit, and large tasty seeds, full of proteins and vitamins, otherwise known as nuts. As these were full of proteins and vitamins, they didn’t need to spend so much time eating. That left more time for play - relaxing, experimenting and socializing.

Among these primates were early fast-thinking and moving monkeys; and an ancestor of apes, Dryopithecus. They were developing better hand-eye co-ordination, specialist colour vision to help them pick out ripe fruit, a good memory, and flexible communication and problem-solving skills.

All this meant proportionally bigger brains and bigger skulls to protect them, and these brains had an outer layer with more folds and nerve connections than before (a neo-cortex). But since a birth canal could widen only so far their infants were born with immature brains that kept developing after birth. This meant that mothers had to move around the trees with a very dependent, bulky baby attached.

Meanwhile away from the Equator in humid areas such as the eastern coasts of continents trees were becoming less densely grouped, as rainforests gave way to dry forests and woodlands. As the tree-cover declined, the giant browsers would be replaced by mammoths and mastodons - no longer gigantic, but still bigger than the largest land-mammals today.

And the ancestors of modern carnivores would join, and eventually replace, the sabre-tooths. In Eurasia and Africa more agile cats started to appear. They had shorter incisors that could puncture a jugular vein and molars to slice and grind flesh. And in North America there were very efficient wolves and other wild dogs, and also large dog-like carnivores, like bears. 

They were all still solitary hunters, and they preyed on the smaller browsers, like early deer. And they were developing more flexible bodies and brains that helped them crouch in ambush, stalk, chase, pounce, and then hold their prey as it tried to dislodge them. With front-facing eyes (but no need for colour vision) they could select a victim from a distance and keep focused on it as they edged their way into striking range - keeping downwind.

In their playing young carnivores had to develop these complex survival skills. But their rich meat diet meant that they had plenty of time for playful learning. A successful carnivore mother had to be a good teacher, gradually letting her offspring take more responsibility in a kill. Those who did not learn how to do this step by step went hungry. Each mother had to expect a high death rate in her litters.

As we have seen, coevolution is not a sentimental process, and the dynamic balance among plants, plant-eaters and carnivores in any ecosystem is based on fewer plant-eaters than plants, and fewer carnivores than plant-eaters.

Grasslands were also starting to form on most continents, usually inland or along the western coasts, in what climatologists call ‘continental’ climates. Grasslands in tropical and sub-tropical areas are called savannas, vast plains of simple low-growing plants that could cope with dry periods, and did not have exposed growing cells at their tips. So they could survive being eaten by the grass-eating mammals that were coevolving with them - as long as the grazers kept moving.

They were more subject to wildfires than rainforests, and this became a factor in their evolution. Many of their seeds survived such a fire, and with their short life-cycle, the savannas quickly recovered – as long as there was rain soon. On the slopes and plains of the Middle East and South Asia, the grasses included the ancestors of wheat, barley, millet and rice. Their annual life-cycle and rich plentiful seeds would be the foundation of early agriculture.

The Northern Hemisphere mammals that came from the woodlands out onto the savannas would evolve into a range of deer and gazelles. They were developing high-crowned teeth to protect themselves from the harsh protective silica in the grasses, which were also pretty poor food. To cope better with this and with drought, they developed complex stomachs to re-digest the grass. They needed these more than they needed complex brains.

But they had to spend most of the day eating and digesting, and they were very visible as they grazed on the wide plains. They became very good at picking up the presence of a carnivore by noticing the left/right symmetrical pattern that indicated an animal in the grass. And they also evolved into quite speedy runners.

So did the various predatory cats and dogs that were evolving. Among them were the ancestors of hyenas, who from this time until today have had the crucial job of finishing up the remains of another hunter’s kill – scavenging. They have to be able to pick up the scent of death, wait until any more dangerous carnivores are finished, and then strip the carcass as quickly as possible. And their brains have developed accordingly.

Drier areas were also increasing across Earth now: arid deserts, semiarid steppes, conifer forests and tundra.

In the vast deserts that were forming in Africa, Australia and the Americas reptiles were the most common vertebrates, burrowing underground to escape the extremes of the noonday heat and the nighttime cold. (In the deserts of today’s Central Australia is a unique little reptile, the Thorny Devil. They drink through their feet, collecting water from wet ground in the crevices between their scales. And then drops of water move through these crevices up to their mouth.)

Because monsoonal rains sweeping north from the Equator hit the ever-rising walls of the Himalayas and remained in today’s India and Bangladesh, vast cold steppes were now developing in Central Asia. They had hardy plants that provided food for hardy grazers like camels and little stocky horses, and they will play a very important part later in our story. Further north, in a forested band around Earth, hardy conifers provided a habitat for a few equally hardy cone-eaters and their predators, like ancestral squirrels and pine martins.

In a broad belt around the rim of the Arctic Circle, and on the Tibetan plateau, the tundra was evolving, with sparse and even hardier flowering grasses in summer and little except lichens in winter. Yaks and reindeer rely on them, having to cover huge distances to find enough. Alpine areas are even harsher – with a growing season of only 180 days, and bitterly cold winters. And then there were Low Arctic areas – with similarly cold winters but a growing season of only 50 days.

But the most severe climatic zone was at Earth’s Poles. The Sun had always been too low in the sky, even in midsummer’s six months of sunlight, to provide plants with a real growing season, and a Polar winter has six months without the sun. From now until today (except for occasional greenhouse eras) they have also had year-round ice. In response specially adapted mosses have emerged in the Antarctic continent and on the permanent ice-floes of the Arctic Ocean. And very few animals are able to live anywhere near the Poles.

The coastlines off today’s frozen Arctic Ocean have specially adapted flowering annuals, and over millions of years some bears and foxes would slowly make the necessary adaptations as they ventured further and further north, until polar bears and then Arctic foxes evolved. But at this point in our story they had not yet evolved.

Two former land-mammals would survive in the oceans by migrating to warmer climates during their breeding season - whales and seals. Of these whales are amazing not only because of their size and constant migrations. Like primates they are big-brained mammals and give birth to very dependent offspring, but because they evolved in an ocean environment their brain development differed in many ways. We are only now beginning to understand just how complex the differences are.

In and around the deeply frozen, dry and gale-swept continent of Antarctica virtually all birds survive winter today by adopting a way of group living that includes migrating together to warmer areas, where they breed and raise their infants. Of course these birds include penguins, whose other main adaptation was becoming swimmers rather than flyers.

Two of today’s iconic examples of bird adaptation in order to make it through Polar winters had both emerged by this time.

Intrepid Arctic Terns breed in the Arctic summer, fly south for the Antarctic summer, and then return north. In the course of a year they fly 60,000 kilometres, with very few dying. Like other migrating birds they use a variety of directional aids. At night they fly by the stars; by day they respond to sound waves from the landscape; and at all times of the day or night their internal compasses orient them to Earth’s magnetic fields. Perhaps most importantly they rotate times of leading the flock and times of resting in the slipstream.

Meanwhile Emperor Penguin fathers – stoic rather than intrepid – carry on through deepest winter on the barren Antarctic continent. They spend the twilight of early winter building up good fat reserves, and then fast for the two months of utter darkness, huddling together, and changing places so that they all have some time away from the freezing winds. Again and again they shuffle between places exposed to the freezing winds and more sheltered spots. Between his breast feathers and his feet each one is keeping an egg warm, and later a chick. Again the survival rate of infants to this stage is quite high.
Their partners are hunting in the fertile environment of Antarctic seas. Relatively comfortable under the thick ice-cover, they are building up their fat reserves, and pre-digesting food for their chicks. When the twilight of late winter returns they relieve the fathers, feed the hungry chicks and then prepare them for life in the sea.

Our mammalian ancestors were by now also experimenting with survival strategies based on group living, as we shall see in the next scene.

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‘South of my day’s circle
I know it dark against the stars, the high lean country
full of old stories that still go walking in my sleep.’
Judith Wright

As Australia’s tectonic plate collided with the Indian and Pacific Ocean plates violent volcanic activity began to push up the islands that form today’s Indonesian archipelago, and the long mountain range in Papua New Guinea. Over the next 25 million years there would also be volcanic activity along the eastern part of Australia, creating Mount Warning, the Warrumbungles, Mount Canobolas and Lord Howe Island.

Indonesia and Papua New Guinea cut off much of the monsoonal rain sweeping south from the Equator, but Australia was now near the Equator. So the steppes and deserts that evolved here, unlike those in northern Asia, would be very hot during the day and nowhere near as cold at night.

Trees would virtually disappear from central areas. And near the eastern coast rainforests, including the one at today’s fossil-rich area of Riversleigh, would be reduced to pockets. Elsewhere were dry forests that would be abundant until a few million years ago. They contained several trees that are today unique to Australia: conifers such as the Huon and Wollemi Pines; the huge range of acacias (wattles) that like African acacias (carobs) evolved from a common Gondwanan ancestor; and eucalypts, which also evolved in Gondwana but would soon survive only here.

In its isolation our continent was developing its own distinctive life zones. Today more than 80% of its plants and animals are unique. Along low-lying tropical and middle-latitude coastlines there are tidal mangroves or sand dunes, and on southeastern coasts with adjacent ranges and foothills there are dry forests. Inland and on the west coast there are intermittent rivers and lakes, semi-arid scrubland, tussock grasslands, salt marshes, gibber plains and deserts.

All vegetated areas are susceptible to wildfires, and so plants have a variety of fire-survival strategies. Thus eucalypts produce short-term epicormic growth to hasten photosynthesis after a fire, and banksias’ seedpods need a fire in order to split open.

At this stage in the dry forests were spiders that lived in the trees, diverging from the others as they tried out new ways to catch their prey among the branches: moths, flies and beetles. Some call them apprentice-weavers because they were the first ones to make silk-woven snares to catch prey. They also trailed silken safety lines wherever they travelled.

Among them was the Red-back Spider, which, like Sydney Funnel-webs, have adapted to live in built-up areas, and are often found close to our houses. But only the elegant females are dangerous as they protect their young with a venom that can kill humans, and are often picked up by accident. As well as their insect prey, females’ sticky tangled webs usually contain a number of diminutive suitors, several egg-sacs, or hundreds of spiderlings - depending on the season. When they are old enough the spiderlings disperse into the air in tiny silken balloons.

At the other end of the scale again, in demonstration of their fitness for Australia’s harsh environment, reptiles, flightless birds and marsupials were all multiplying and diversifying. And some were growing very much larger, particularly in the scrubland. One marsupial of these times is the diprotodont. They were like giant wombats, the largest of their species being the size of today’s rhinoceros.

But there were many other kinds of marsupials, adapted to the various ecosystems. In its isolation Australia was an excellent continent for convergent evolution to apply, with similar animals filling the same ecological niches in different places.

Soon there would be native-cats – in the same niche as ocelots in South America; grazing kangaroos – instead of Africa’s antelopes; and sugar-gliders – like the Northern Hemisphere’s flying squirrels. And there would be marsupial moles and wolves, and several agile tree-dwelling and cliff-dwelling marsupials, and of course the fiercely competitive, but also endangered, marsupial carnivore, the Tasmanian Devil. Even monotremes have their niche here – like echidnas, which are the equivalent of placental ant-eaters elsewhere.

But Australia’s most amazing mammal is another monotreme, today’s reclusive Duckbilled Platypus, with no equivalent on any other continent. And it appeared about this time. The females laid their eggs in a burrow on a creek-bank, and they spent most of their time in the water. By now, instead of teeth like their ancestors, they had a bill that looked a bit like that of a duck. But this was a special kind of bill. Among other things, it could sense the presence of their prey animals, as they swam along the bottom of the creek.

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1 comment:

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