Precambrian and Cambrian
The barren Precambrian world The first life on Earth appeared not on land but in the oceans. There was not enough oxygen in the air to breathe and ultraviolet radiation, lethal to life forms, was still at dangerous levels. Neither problem affected life underwater. Life probably arose about 3800 million years ago. The span of time from the origin of the Earth, about 4600 million years ago, to the time when a wide variety of life forms first appeared, about 541 million years ago, is called the Precambrian Period. The Cambrian Period lasted from 541 to 485 million years ago.
Beginnings of life
No one knows how life began, but shallow, warm-water pools at the edges of the oceans might have been the kind of environment suitable for the formation of chemicals that would eventually become the building blocks of life. The sea contain all kinds of salts, minerals and other elements. Certain vital chemical reactions could have been triggered by lightning or the shock waves of a meteorite or asteroid collision.
In some places on the ocean bed, water seeping down into the rocks is heated by the magma that has worked its way up into the Earth's crust. As a result, hot water shoots back up through cracks in the ocean floor, known as hydrothermal vents. These water jets are rich in minerals from inside the Earth’s crust, especially sulphur. The minerals gradually build up around the vents, creating tall chimneys called "black smokers", after the colour of the water jets. Many scientists now think these vents provide the most likely conditions for the origins of life.
Earliest life forms
The fossil evidence shows that life first evolved at least 3500 million years ago—but probably hundreds of millions of years earlier. The earliest life forms were the very simplest kinds: bacteria. The oldest fossils are known as stromatolites, bands of cyanobacteria (sometimes called blue-green algae) that inhabited shallow water. Life stayed as simple microscopic organisms for another 2500 million years, before the first complex life forms, types of seaweed, first appeared.
An Ediacaran fossil
The Ediacaran Period (c.635–542 million years ago) is named after the Ediacara Hills, Australia. Fossils of soft-bodied sea creatures found here give evidence of life in the late Precambrian (already some five-sixths of the way through the story of the Earth). This period saw the first complex multicellular organisms (with more than one cell). The most common were shaped like fronds, disks, worms and bags.
Ediacaran life forms
Cambrian Period marine life
explosion of life
The “explosion” of life took place at the beginning of the Cambrian Period, 541 million years ago. Fossils discovered in the Burgess Shale of British Columbia, Canada, show what life in a warm, shallow sea in Cambrian times must have looked like. Among the life forms that we are familiar with today, there were some very strange-looking creatures. One, Opabinia, had five mushroom-like eyes and a long, clasping “nozzle” to catch prey.
Cambrian Period marine lifeThe largest and fiercest of all Cambrian creatures was the 60-centimetre (24-inch) Anomalocaris, a name meaning “odd shrimp”. It had a cloak-like body, two large eyes set on stalks and a pair of pincer-like arms. Other smaller marine life-forms had defences to repel such giant marine predators. Hallucigenia, which moved about the sea bed on seven pairs of stilts, had a row of defensive spines on its back. Wiwaxia was hat-shaped, complete with two rows of dagger-like blades.
Pikaia: our ancestor?
One of the Burgess Shale animals was of particular significance. It was Pikaia, named after nearby Mt Pika. This small, worm-like creature had a stiffening rod running the length of its body—not quite a backbone, but very similar to one. It also had muscles arranged in V-shaped segments, exactly the same as in modern fish. Pikaia may have been an early ancestor of the group of animals called vertebrates (animals with backbones) to which fish, reptiles, birds and mammals such as ourselves all belong.
Consultant: Chris Jarvis
See also in Prehistoric