How the Evolution of Animals Changed Planet Earth

Does the evolution of animals really affect planet Earth? Our current concern about climate change certainly seems to suggest so.

According to latest figures, global warming is increasing exponentially as a result of our production of excess greenhouse gases, particularly by burning fossil fuels. The evidence is that the increase in the rate of temperature rise has been going on since the Industrial Revolution. It is sobering to think that human beings are so ‘highly evolved’ that we can be a danger to the planet itself.  But this is not the first time that a species has had an effect on the planet’s atmosphere.

In general, for much of the history of life on Earth, it has been more a case of changes in the planet affecting the evolution of animals. Time after time over the 4,000 billion years since life mysteriously began on our planet, it has taken a beating. What with climate change beyond our imaginations, Ice Ages, vast movements of continents and the impact of at least 2 world shaking meteorites, it is a wonder that life continued. We all know about the Jurassic extinction of the dinosaurs, along with 90% of all other life on the planet. So how can we talk about the effects of animal evolution on the planet Earth?

The answer takes us back to a time when our beloved blue-green marble was a raging inferno. From around 4.6 – 3.9 billion years ago, much of the planet’s surface consisted of oceans of magma, boiling rock. The atmosphere had no oxygen at all, being instead predominantly CO2 and nitrogen. Miraculously, somehow, life began.

The earliest life forms we know about are the cyanobacteria, or ‘blue-green algae’. A form of these is still around today. These are very simple, single-celled bacteria, but they are capable of that most crucial of processes, photosynthesis. This is the process by which sunlight is converted into matter by chlorophyll. In the process, CO2 is metabolized and O2 is released. The oldest fossil evidence for this is 3.45 billion years old.

It could be argued that bacteria do not count as animals, being single-celled, rather than more complex organisms but they cannot be classed as plants either. In addition, they do have many of the basic characteristics of animals. They respond to stimuli; they can move themselves about via whip-like appendages or flagella; they respire in some form.

Without the cyanobacteria, there would have been no further evolution of animals on the Earth. Staggeringly, the increase and spread of these uni-cellular animals actually caused the evolution of the Earth itself, enabling it to support more and more life. They did this through the process of photosynthesis mentioned above. Over the next 2,000 billion years or so, this cumulatively changed the concentrations of these gases in the atmosphere.

The decrease in the concentration of C02 allowed the planet to cool down to a more habitable level. The higher concentration of 02 meant that life could expand into more complex organisms. By around 2,400 billion years ago we had an atmosphere which could be considered oxygenated.

This process continued, mostly in the sea. Later on, from around 1,800 billion years ago, multicellular creatures, particularly those with shells, absorbed more CO2 from the atmosphere, locking it up in their shells which sank to the bottom when they died. Over time these great layers of dead creatures were compressed and fossilized to form sedimentary stone such as limestone. Meanwhile more and more complex creatures were able to evolve.

So, on the whole, the evolution of animals has been shaped by changes in the planet. However, life is a very powerful force and from it’s very beginnings it began to shape the planet’s atmosphere to enable it to support more life.


‘The Earliest Life’ The Palaeobotanical Research Group, University of Munster 2003

‘Bacteria’ Wim van Egmond,

History of the

‘Earth in the Beginning’, Franz Lanting, National Geographic Dec 2006