In the long, slow history of lifeÂ’s development on Earth, Man is as recent as to seem a mere footnote. If the history of the world were a feature film, Man would make an appearance just as the words Â‘The EndÂ’ were appearing on screen.
From about 4 billion years ago, microscopic, primitive single cells, called prokaryotes, held sway in the Earth’s oceans. Larger, more complex eukaryotes began to appear 1.45 billion years ago as the oxygen level of the atmosphere build up. One billion years ago, the oxygen level was around six per cent of what it is now: 400 million years later, it had reached 50 per cent. Meanwhile, evolution accelerated.
Fossils from South Australia’s Ediacara hills show that by 670 million years ago, there were a number of soft-bodies creatures living in oceans that we would recognize today. They included jellyfish, worms and sea-pens. As the weird and wonderful variety of life beneath the sea multiplied, some forms began to prey on others. As a result, new adaptations with protective hard shells to ward off voracious neighbours, emerged over million of years. Another survival skill was the ability to move. There were crawlers such as the woodlouse-shaped trilobites, sliders such as snails, walkers such as starfish, jet-propelled squirters such as tentacles, shell-protected cephalopods and swimmers.
The first fish appeared at some time between 440 and 395 million years ago. They were very primitive, jawless creatures that nosed, heavy-headed, along the sea-bed. They were also the first creatures with real backbones. When fierce predatory fish with proper jaws and teeth evolved in the next 50 or 60 million years, they devoured the jawless fish, thought some remained as lampreys. The backboned killers flourished, growing in some cases to huge sizes, ten metres and more in length.
With increased oxygen came the first land life: plants at first, algae, then reeds, then bushes and trees. 345 million years ago, tall, scaly plants covered land surfaces in the first forests. They were the habitat of insects that evolved from spineless sea-life. As periods of drought dried up seas and lakes, some fish developed the ability to gulp air above the water. Lobe-finned fish evolved the skills of walking from pool to pool. They eventually became the first amphibians, equally at home in water or on land.
The ago of plants lasted 75 million years, laying down the fossil beds that are today’s coalfields. Reptiles, evolving from amphibians, stayed permanently on land. Flying insects developed, including giant dragon-flies, and the cockroaches that have hardly changed in the 300 million years till today.
The first mammals
Closer to our own era, the speed of the multiplication of life forms increased, as the huge pool of genetic information increased with each cell division. The dinosaurs ruled for over 200 million years. In that time, birds emerged and so did warm-blooded mammals – the next inheritors of the Earth.
Then as the dinosaurs vanished perhaps as the climate became too cold for them. Dust from a meteorite bombardment may have shielded the Earth from the sun’s rays. From the sudden extinction of the dinosaurs 65 million years ago, the great diversity of mammal life – from tiny shrews to huge elephant like mastodons – colonized the planet. Each species was subject to its environment and pressures of finding foods, escaping predators, and successfully multiplying. Some remain almost unchanged after millions of years, others fell victims to the relentless logic of ‘adapt or die’. It has been calculated that 99.999 per cent of all the life forms that have ever existed are now extinct. Yet today, Man shares the planet with up to ten million other species.