Dear Readers, this week my OU course was concentrating on the concept of Deep Time, and so I thought I’d share a bit of what I’ve learned with you, to see if I’ve understood it myself. Consider yourselves guinea pigs! And all questions and comments gratefully received, as always.
The earth has been in existence for approximately 4.6 billion years, and while humans are a very recent addition, the first bacterial life popped up as long ago as 3.5 billion years. Some tiny fossils have been discovered in rocks known as the Fig Tree chert in South Africa, and these appear to be caused by the first bacteria, who lived in those ancestral oceans. If you look closely, you can actually see those ancient life-forms dividing, just as they do now. Other fossils show lines of bacteria joined together in long filaments. It’s thought that these bacteria were the fore-runners of the cyanobacteria that came into their own a mere 3 billion years ago (see below), and like them, they lived in the oceans, where all life originated and where, for most of Earth’s history, it stayed.
Chert is a sedimentary rock, usually composed of siliceous ooze – this was the bacteria-rich deep ocean floor, where bacteria both lived and died. Flint nodules are a form of chert, as are agate, onyx, opal and jaspar. How easy it would have been to miss the tiny bacterial fossils, and indeed for a long time they were contentious. However, the finding of similar patterns in cherts in Greenland and Canada seems to have settled the issue – there were bacteria on earth, quietly dividing and getting on with their lives, a billion and a bit years after the earth first formed from space debris.
These tiny bacteria are already beginning to affect the atmosphere by producing nitrogen, but for oxygen to be produced, we have to wait for the appearance of the cyanobacteria 3 billion years ago. Also known as blue-green algae, these organisms are still with us today – they create long filaments that mix with sediment and form molehill-like structures called stromatolites. Because they photosynthesize, turning energy from the sun into oxygen, these creatures slowly begin to produce the key ingredient of the air that we breathe.
So, the cyanobacteria work away over millenia, releasing oxygen into the oceans. However, at first it doesn’t stay there. Oxygen is an extremely reactive gas, and at this point there are endless sulphur and iron-rich rocks laying about. These absorb the oxygen to start with, preventing it from becoming available to anything that might be able to breathe it. In the early years, iron-rich rocks are banded in green and white: this is thought to show that the iron has reacted with the oxygen released into the sea water, turning it green (all such banded rocks come from the ocean).
These banded iron deposits may show that, to begin with, the cyanobacteria could be poisoned by their own waste product (oxygen) when it built up too much, hence the periods when the iron was oxygenated, followed by periods when the algae died back and there was just sediment. Later, the bacteria evolved to be able to protect themselves from this poisoning, and this signalled the start of the Great Oxygenation Event of about 2 billion years ago. Nonetheless, the iron deposits that we mine today are largely banded iron formations (BIFs).
Once all the iron in the ocean has been used up, we start to see bright red, iron and oxygen-rich rocks instead, which shows that there was a constant supply of oxygen produced by the cyanobacteria. A tipping point has been reached, and the oxygen not only oxygenates the oceans, but starts to leak into the atmosphere as well.
The effect on the atmosphere is two-fold. Firstly, the oxygen displaces the carbon dioxide and methane, leading to cooling, and possibly the first period of global glaciation – some have called this ‘snowball earth’. Secondly, an oxygen-rich enviroment seems to make it possible for more complex forms of life to appear, and for cells who carry their genetic material in their nucleii to develop. Between 700 million and 545 million years ago, many forms of multicellular life appear for the very first time. The Earth is no longer ruled purely by bacteria. By about 440 million years ago we are gearing up for the First Mass Extinction event, but for now life is starting to diversify and become rich in a way never seen before. It’s the age of the trilobite, and of many other creatures besides. This is the era of the Burgess Shale, and it’s almost as if nature is trying out many forms of life that we can’t even begin to understand now. What a remarkable time period this would be to study! What I’m loving about my course is the sheer variety of paths that it explores. Who knows which of them I’ll decide to come back to?
Photo One from https://slideplayer.com/slide/8485989/
Photo Two By Minghong – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=509405
Photo Three By Paul Harrison – Photograph taken by Paul Harrison (Reading, UK) using a Sony CyberShot DSC-H1 digital camera., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=714512
Photo Four from http://jsjgeology.net/Banded-iron-formations_files/image016.jpg
Photo Seven from By James St. John – Agnostus pisiformis fossil trilobites (Alum Shale Formation, upper Middle Cambrian; Andrarum, Scania, Sweden) 5, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=40021575