The general consensus among the scientific community is that the Earth was formed some 4,550 million years ago. There is no fossil recording of the very origins of life, and we do not see any traces of life in the rocks before around 3,500 million years ago. The earliest known forms of life are the Prokaryotes - a very simple form of life which would today be referred to as bacteria. To the right is an image of the fossilized bacterium Primaevilium amoenum, claimed to be the oldest fossil on Earth.
The ancient bacteria were able to thrive in conditions that would be deadly to most living organisms, some had a metabolism that was based on sulphur - and can be found in hot springs and other such places to this day. Other forms of bacteria formed the ability to derive nutrients from the sun's energy via a process commonly referred to as photosynthesis. Cyanobacteria (photosynthesising bacteria sometimes referred to as algae) appear in fossilized sheets or mats, and they also appear in layered mounds or pillars known as stromatolites (pictured below). These were formed as the bacteria secrete calcium carbonate, creating these rock-like structures. Amazingly we have discovered living stromatolites - a truly ancient form of life, and a testament to it's longevity. These mats of algae and mounds of stromatolites dominated the planet alone for an incredible time span, it wasn't until 2,500 million years ago that the eukaryotes (organisms with a defined nucleus) appeared. For around 2,000 million years the prokaryotes dominated - a length of time that is impossible to comprehend.
Life remained within the confines of a single cell up until around 1,000 million years ago when the first multicellular organisms appeared. It is amazing to consider that over half of the planet's history was dominated by microbial life. The eukaryotes (fossil pictured to the right) were the branch in the evolutionary tree that eventually led to humans - and for our very existence we owe a lot to the photosynthesising bacteria. As a waste product the cyanobacteria produced vast quantities of oxygen, without which life would certainly not have progressed beyond its most simplistic forms.
We are often compelled to view single celled organisms as "primitive" and somehow feeble compared to complex life, however, in terms of sheer longevity these are some of the most amazing lifeforms to inhabit our world. They alone dominated our planet for a staggering length of time, far longer than any other form of life. If we were to award prizes for durability and longevity then surely our primitive bacterial ancestors would be well ahead of the competition. The next step in evolutionary history, however, was when things started to get rather interesting...
To be continued....