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Asked by to Amelia, Izzy, Sarah on 18 Mar 2014. This question was also asked by .
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Isabel Webb answered on 18 Mar 2014:
Plants evolved before the modern animals we know today roamed on the earth. This evolution of photosynthetic organisms like plants caused a massive change in the amounts of gases in the air – the oxygen levels just kept increasing (before this there was loads of carbon dioxide but not much oxygen in the air). This then meant that animals could evolve faster because more oxygen allowed them to grow larger without suffocating. Evolution of animals whilst plants kept evolving meant that eventually the production of carbon dioxide and oxygen balanced out into the ratios of the gases we knew today. It was a joint effort between evolution of plants and animals as multi-celled organisms which were more demanding for oxygen and carbon dioxide.
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Sarah Harvey answered on 18 Mar 2014:
When the earth was ancient there were high levels of carbon dioxide which gradually increased due to photosynthetic organisms. Bacteria can live in the absence of Oxygen (these are called anaerobes) and we still see these today for example living in swamps and bogs where the water is not oxygenated (and also in our intestines!).
There is a really cool resource here (which I just got distracted and played with for ages!) if you’re interested in more about this topic 🙂 It shows the oxygen levels over time and also what organisms were on the planet.
http://www.hhmi.org/biointeractive/geological-history-oxygen
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Amelia Frizell-Armitage answered on 19 Mar 2014:
OK good question. This is true of a lot of things on earth now, that every species needs another to survive, so how can they have evolved separately?
The answer to your question is that plant cells evolved first. There was no oxygen, but there was already carbon dioxide in the earth’s atmosphere when the first cell evolved. After a lot of trial and error, and many hundreds or thousands of years, one cell actually managed to evolve the ability to do photosynthesis. It took carbon dioxide from the air and combined it with light to make energy. The oxygen produced was a waste product the cell didn’t need, so it put the oxygen back into the air. This cell did really well because it could produce its own energy and evolved into plants.
As soon as photosynthesis was invented there was oxygen in the air. Other cells that could not photosynthesise were then able to take this oxygen and respire to produce energy. These cells were to be the animal cells.
Now it is true that neither plants nor animals could survive without the other, because they have evolved over many millions of years to live together on the planet. But back when things first began it was very different.
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