A History Of Life On Earth: The Precambrian Era
The Blue Planet
Earth’s story begins with its birth and the start of its first geological period, the Precambrian, which is also incidentally the longest geological time period in Earth’s history, lasting nearly 4 billion years. It was during this time that Earth transformed from a lifeless ball floating in the vastness of the solar system to a blue water world, home to many types of life. However, much of the life at this time was what we call primitive and their fossils were rather more small and delicate, meaning that they were comparatively rare when compared to fossils from later periods.
How Did the Solar System and Earth Form?
The Geological Clock
The Moment Everything Changed
The Cradle of Life
Archaean Eon: 4600-2500 Million Years Ago
For centuries it was believed that our world and everything in it was at most only a few thousand years old and all life including man had been created within the space of just few days. Then, around 200 years ago, people practising the relatively new scientific discipline of geology began making sense of the strange fossilised plants and animals that were being brought to them quarries and rocky outcrops around the world. Thanks to these fossils, those pioneering scientists began to draw the conclusion that the world was in fact much older than mere millennia.
Detailed studies of rocks, fossils, meteorites and other objects tell us that the Earth was in fact formed around 4600 million years ago. Before the formation of Earth, there were no planets in our solar system; instead there was just a loose mass of rock and dust swirling about the relatively young sun. Eventually gravity drove the loose rock and dust together into large balls; these balls eventually formed the first planets, which included among them, Earth.
For the first few hundred million years of its existence the Earth was a hellish place, with a boiling hot surface, dominated by constantly erupting volcanoes. Moreover, Earth at this time was continuously being pelted by giant asteroids. The atmosphere would have been thick with poisonous gasses, leaving little chance for life of any kind to evolve.
But then something truly monumental happened, a turning point that occurred some 4 billion years ago. A planet roughly the size of Mars called Theia was hurtling wildly through the cosmos before smashing violently into our world. The collision was so violent that it almost smashed the planet completely in two. A part of Earth though, was thrown back into space where it gradually moulded itself into a natural satellite which we now call the Moon. The formation of the Moon had a profound effect on the development of life because it stabilised Earth’s orbit around the sun, and created the tides that we often take for granted today. The tides helped to aerate and cleanse the shallow seas where the first life forms evolved. Vast oceans of liquid water spread right around the world, transforming Earth into a blue planet. The oceans offered a constant and stable environment in which the slow process of life and evolution could begin.
The oldest sign of life on Earth is a chemical signature in rocks from Greenland that are around 3850 million years old. No one is entirely certain how life on Earth first evolved and there are a number of different theories that attempt to answer the question. One intriguing possibility is that the first organisms may have evolved in underwater hot springs, where there would’ve been an abundance of heat and the right sort of chemicals to permit life to evolve. For hundreds of millions of years life on Earth consisted of simple single celled bacteria that formed thin layers of slime across the ancient seabeds, hence where we get the term ‘primordial slime’ from. These first organisms lived by extracting energy from chemical reactions, but then one group known as the cyanobacteria evolved a way of extracting energy directly from sunlight, they were first organisms to use what we call photosynthesis. Bacteria soon formed vast mats on the ocean floor, sometimes creating layered mounds called stromatolites, the earliest fossils of which date from around 3500 million years ago. For hundreds of millions of years the slow pace of evolution meant that these comparatively simple organisms were the only life on Earth.
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Proterozoic Eon: 2500-543 Million Years Ago
If the Archaean eon marked the evolution of life on Earth, then the Proterozoic eon, which literally means ‘ahead of animal life’ saw it develop from bacterial slime into complex plants and animals of the sort that we would recognise today.
At the start of the Proterozoic eon, around 2500 million years ago, the oceans still harboured all life on Earth. The atmosphere contained significant quantities of carbon dioxide and possibly methane, but very little oxygen, which made the land simply too hostile for life. But this was soon to change; life would soon conquer the land.
The bacterial slime in Earth’s seas produced oxygen as a by-product of their growth. This excess oxygen was released into the environment and over millions of years built up into the atmosphere, until around 2000 million years ago, when the Earth finally achieved a permanently high level of the gas.
Oxygen is a highly reactive chemical and is an excellent fuel for life. Thus, the arrival of an oxygen rich atmosphere sparked the evolution of new, more complex single celled or organisms known as eukaryotes. Unlike bacteria (which are prokaryotes), each eukaryotic cell had a separate nucleus that contained DNA, the building blocks of life at its centre, it was another major turning point in the development of life on Earth. The first eukaryote fossils called acritarchs, are microscopic and date from around 1500 million years ago. Slowly these nucleated cells adapted to live together in colonies, eventually producing creatures similar to modern sponges and jellyfish.
So towards the end of the Proterozoic eon, the world was full of drifting jellyfishes and static filter feeders. But then, according to some, Earth encountered its first great disaster since the first appearance of life. Between 750 and 600 million years ago, the Earth became engulfed in a severe ice age, which sent temperatures plummeting to around -40 F and caused all of the world’s oceans to freeze over completely. This theory, which is called the ‘Snowball Earth’ theory is still rather controversial and has yet to be fully vindicated. But interestingly it was only after the period in which the Earth is said to have been completely frozen over that the first signs of truly complex animal life start to appear in the fossil record.
Fossils from 600 million years ago are very rare indeed, and they are often little more than faint impressions in the rock. Scientists find these fossils difficult to interpret, but it is thought they are remains of simple animals. They are often termed as ‘Ediacaran animals’ on account of the fact they were first found near Ediacara in Australia. They lived in warm, shallow coastal waters right across the world. Among their number was an intriguing creature known as Spriggina, a worm like animal that must have pushed its way forward through the mud. Creatures like Spriggina with a distinct head and rear end, were to lay the foundations for a spectacular explosion of life that has never been matched. It is also may represent the common ancestor to everything from earthworms, crabs, dinosaurs, whales and humans.
More to follow...