Multi-cellular Life

900 to 245

Million Years Ago

Trilobites.  Public Domain image from the US Dept. of the Interior

Pikaia

 Earth's Present Age

 

 

Earth's latest great age stretching into present time opens with three momentous developments.   Multi-cellular organisms evolved, fish developed as the first vertebrates, and plants and animals moved onto land.  Atmospheric oxygen had reached today's levels and an ozone shield protected land creatures from damaging sun rays.  For the first time life could safely move out of the water.  Yet despite stunning innovations, 250 million years ago an extinction event occurred that was so grim it came close to annihilating most of life more complex than microbes. 

This section identifies a small selection of significant events that occurred from 900 to 245 million years ago.

Evolutionary Explosion

570 million years ago

Following the extinction of the earliest multi-cellular organisms the Earth experienced a period now known as the Cambrian Explosion.  Millions of new species, apparently quite suddenly evolved in the sea.  Some were bizarre, even phantasmagorical, others are of critical ancestral interest to us.   All the major animal groups we know appeared.  The last group or phylum to develop was the chordate to which we belong.

 

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The Cambrian Explosion

First Mass Extinction of Our Age

445 million years ago

A fifty million year period of development saw plants move from the sea to the land and fish develop as vertebrates.  It was followed the first mass extinctionof our age.  A super-continent, Gondwana, composed of modern day South America, Africa, Antarctica, India, Australia and parts of North America and Europe drifted to the South Pole.  Huge glaciers formed and sea levels dropped causing the extinction of up to 85% of living species.

 

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Fish Adapt to Land

370 million years ago

300 million years ago, seed-bearing plants developed a way of reproducing without surface water by using the wind to spread pollen.  Plants began to colonise the land and the Earth began to turn green.  A home on land was enticing, and whether they were hardy, desperate or courageous, some fish crawled out of the water.  They evolved as amphibians, the first four-legged animals on land.

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The Age of Coal

360 million years ago

Despite extinctions the greening of the planet continued for another 75 million years.  Huge plants and tropical forests grew trees reaching up to 160 feet.  Supported by this explosion of photosynthesis oxygen levels were elevated to 35%.  A lot of plants grew very fast and very very big.  The big is what ultimately made so much coal.

 

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The Amniotic Egg

300 million years ago

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We don't usually think of the egg as an evolutionary masterpiece but that is what the amniotic egg was, an ingenious method of reproduction that protected and nourished newly-conceived young on land.  Birds today, the most well-known egg-layers, inherited their method of reproduction  from their reptile ancestors.

 

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Ancestral Reptiles

286 million years ago

 

The period of luxuriant vegetation was terminated by another ice age which lasted for 42 million years.  The damp and humid north was where many land animals and plants survived during this period.  One of our distant relatives from this time is the dimetrodon, an aggressive reptile with a sail on its back.  It was an early ancestor of the mammals, and one of the first land animals that could prey on animals its own size.

 

 

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The Great Dying

250 million years ago

About a quarter of a billion years go, Earth endured the most catastrophic extinction in history.  96% of all living species perished, 75% of all vertebrates.  The trilobites, survivors for close to 300 million years, all died.  Living things came close to being knocked back a billion years when Earth was colonized only by microbes.  It is called the Permian Extinction.  The planet has not seen death on this scale before or since.

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