One often wonders where life began, how long life has been around for, and what the earth has gone through in its 4.6 billion years of existence. We soon realised where life and the planet were before the dinosaurs isn’t common knowledge, queue our latest blog post.  

The earth began around 4.6 billion years ago, before we go straight in, just a warning, timelines can be very complicated. So, to split them up nicely, we’ll be focusing on four Eons. These are Haden, Archean, Proterozoic and Phanerozoic. Within these Eons we have Eras that are then divided into periods, epochs and ages – but we’ll get in those later.  



Hadean – Eon


Starting from the beginning, quite literally. Hadean is measured from the formation of our earth until the formation of first life. Our world was lifeless for the Hadean Eon and early Archean Eon, with an almost entirely Oxygen-free atmosphere. This Eon lasted 0.6 billion years and in that time our earth was mostly formations of rock, preparing itself for what was to come…



Archean Eon


This occurred 4,000 million years ago and lasted roughly 1500 million years. At this point, the crust of the earth cooled enough to allow life to form. Our earth was very hot at the start of this Eon, mostly due to the formation of earth's metallic core and the heat from the planetary accretion. Biogenic rocks formed the earliest life, many scientists predict that first life may have started in undersea alkaline vents. As the earth was so hostile to life, only simple single-celled organisms could exist.




Four Eras existed within the Archean Eon:

  • Eoarchean - beginnings of bacterial life, (aka) the start of life.
  • Paleoarchean - oldest ascertained life formed.
  • Mesoarchean - stromatolites happened
  • Neoarchean - oxygenic photosynthesis evolved, releasing oxygen...

which quickly lead to the…


Proterozoic Eon


The Proterozoic Eon existed 2500–541 million years ago. Beginning when Oxygen first formed in our atmosphere. This was the longest Eon in the Earth’s geological time scale. Substantial life began to appear in the form of trilobites or corals and the Eon took its leave just before complex life appeared. This Eon was again split into Eras which very simply were:



Paleoproterozoic – the longest Era in Earth’s history, the period from 2,500 to 1,600 million years ago. Continents first stabilized in this era, there was a huge increase to the atmospheric oxygen meaning anaerobic lifeforms became extinct and an event coined as the ‘Great Oxidation Event’ occurred. We think this was caused by a build-up of poisonous waste produced the process of photosynthetic cyanobacteria (o2), this then began to rise to the atmosphere. The survivors were those who could resist the poisonous effects of Oxygen, this huge extinction was the first but not the last of the mass extinctions.


Mesoproterozoic It was in this Era that the Columbia supercontinent began to break up this then formed a new supercontinent, known as Rodinia. Another very important event also occurred – sexual reproduction is thought to have begun here.


Neoproterozoic The final Era of the Proterozoic Eon. Severe glaciation (ice ages) occurred here, forming the first 'Snowball Earth'. When we say Snowball earth, we mean just that. The earth's surface was thought to be almost entirely frozen. Many have predicted this was due to a lack of volcanic activity. When the ice finally began to melt more oxygen began to release into our atmosphere. In the time we saw Eukaryotic cells (cells that contain internal organs), fossil evidence of photosynthesis (there is evidence it appeared earlier, but this is still being questioned) and multicellular life.


Queue the Phanerozoic Eon, which believe it or not, is the Eon of today!



Phanerozoic Eon



We’ve reached the final Eon, our current Eon. This is the Eon of life. The Phanerozoic Eon is split up into three major Eras:


The Paleozoic Era, which covered 7 periods making it the longest era of the Earth's geological history spanning from 542-252 million years ago. This is where things got really interesting.


The Era began with multicellular animals and sponges, but the planet then froze over again in another ‘snowball earth’ wiping out all creations hard work and killing almost all life. When the ice began to melt sea levels rose with it. Fish were everywhere after this and it suddenly got very hot!




Fungi and plants, jellyfish and other cnidarians also formed. CO2 levels were very high in this Era, which was great news for plants. More soft-bodied animals and sea life soon bloomed, the first period of this Era is the Cambrian period, due to the expansion of life, scientist call this time the Cambrian explosion!

 Just when the fish were thriving, another mass extinction came about; this was mostly due to changes in the climate and sea levels, new soil and asteroid impacts. Eventually, life began again and ‘the age of oxygen’ started, plants died and turned into coal very quickly because there was not enough CO2 and with so many plants there was a huge increase in O2 levels; this oxygen cooled the Earth and it wasn’t long until another ice age began, which was followed by a devastating mass extinction, one of three that occurred in this area, the first is thought to have been caused by the major drop in sea levels after the glaciation of Africa, 6-% of marine invertebrates and 25% of other families became extinct. Around 100 million years later another 70% of all species became extinct and finally, in the Permian Period of the Phanerozoic Era, an event coined as ‘The Great Dying’ occurred. The land was very dry during this time – some creatures adapted to this, but many did not. At this point, life was very evolved, creatures walked the land, vertebrates were common, bony fish existed – the coelacanth appeared, insects and four-legged animals conquered the land which gave rise to reptiles, birds and mammals. When the greatest mass extinction occurred at 250 million years ago, 95% of life became extinct… meaning we evolved from the 4% that managed to survive, let that sink in.

It took a while, but our ecosystem did start to recover and sauropsids became the dominant race – we can call some of these, dinosaurs. Ammonites came about and reptiles took to the seas – these creatures would eventually evolve to become the huge marine reptiles of the Mesozoic Era. Mammals still existed, evolving into smaller nocturnal animals. In case you hadn’t realised, we’re now in the Triassic period, the last period of this Era, which quickly led to…


The Mesozoic Era, or as others called it, The Age of Reptiles! This Era started off with a mass extinction, this was due to climate changes, basalt eruptions (caused in most part by the Pangaea shift), methane from the seafloor and asteroid impacts. It was also the only known mass extinction of insects ever! Where was this in the movies?

This however made room for dinosaurs to take over from the sauropsids and the mammals that remained evolved warmblood, so they could now regulate their internal temperature despite the outside conditions.

At the end of the Triassic period, it was very hot and very empty –


Pangaea also famously began to break apart.


Life began to flourish again and before long, the world was full of the most diverse organisms we’ve ever known. Egg-laying animals (monotremes) split from their cousins that gave birth to live young and soon after the famous, Epidexipteryx lives, our first example of ornamental feathers and the Archaeopteryx or 'first bird'. This period was famously referred to as ‘The Age of the Dinosaurs’ or The Jurassic Period – there’s a reason why they called Jurassic Park after that particular period, though… weirdly enough the T. Rex didn’t actually live in the Jurassic period. It lived in the late Cretaceous period. (Getcha facts right Spielberg) speaking of which, we’re about to get into the Cretaceous period… Marsupials then split from the placental mammals and flowing plants soon appeared. There was very little life to begin with so there was little competition among species – lots of food and space, so they survived very easily. Our current ocean floor was also formed.

 The warm Cretaceous period had no ice in the polar regions, with much of the world underwater. More dinosaurs began forming, and the big boys came out of the forests – the largest dinosaur we know of, arrived at this time, why were they so big, you ask? This is a heated debate for many scientists, the size is thought to be a cause of many things. One is that unlike warm-blooded animals who spend a lot of energy on their metabolism, dinosaurs living in a very warm and pleasant climate (for the most part) their food could go direct to their size as it was not needed anywhere else. They also had never little competition, after the Permian-Triassic extinction, many animals were wipes off the earth. Many also argue that increased oxygen due to the spread of plants, was another contributing factor – they argue that oxygen is fuel to many animals – but as we said, many of this is speculation. The largest land animal ever recorded, the giant sauropod \Argentinosaurus, is thought to have lived around this time, 100 million years ago.




Towards the end of this Era, what we think are the ancestors of modern primates split from ancestors of modern rodents and grasses began to appear. Then it happened, the famous death of dinosaurs. The decline began with falling sea levels and basalt eruptions and was finished off by the huge asteroid. Around ¾ of life became extinct -including almost all dinosaurs making way for mammals. Thankfully this was the last mass extinctions that Earth has experienced so far.



After this, the Cenozoic era began. When the reptiles died, mammals dominated. Lots of new creatures appeared over time such as dry nosed and wet-nosed primates and haplorrhines develop into apes, monkeys and humans! Interestingly enough, another extinction occurred just 10 million years after the last. This was caused by a rise in CO2 levels, which caused the earth’s temperature to soar. Many deep-sea creatures became extinct whilst shallow sea and land creatures remained.



Ancestors of whales and dolphins appeared and higher primates split from old-world monkeys. The Himalayas were formed as India pushed against the Eurasian continent. Global cooling soon began and the climate shifted, fewer forests and more open areas spread around the world; Kelp forests now appeared – an extremely productive ecosystem. 6 million years ago, was where we diverged from chimpanzees and apes and before long, hominins began walking on two legs. The next period, 2.6 million years ago, brought multiple ice ages and whilst the ice was forming, we were evolving.



And that’s where we are now! It began when the glacier’s diminished, so while we’re technically still in an ice age, it’s just in an interglacial phase. To quote the Guardian’s article based on information from a study by Nature Climate Change: We’re already warming the Earth about 20 times faster than during the ice age transition, and over the next century that rate could increase to 50 times faster or more. We’re in the process of destabilizing the global climate far more quickly than happens even in some of the most severe natural climate change events.


Anyway, on a lighter note, to think us, as homo sapiens, have only been around for 200,000 years. Compare that to the 4.6 billion years the earth has existed! Science is wonderful.


Speaking of science, we have a lovely collection of science books, check them out here!