• Question: If photosynthesis is the opposite of respiration, why arent humans adapted to photosynthesise in order to produce their own oxygen?

    Asked by Ellen to Ceri, Marikka, Matt, Rob, Sally on 11 Nov 2014.
    • Photo: Ceri Dare

      Ceri Dare answered on 11 Nov 2014:

      Gosh, this is a really complicated question to answer! Thank you, it has made me think and find out new things.

      2500 000 000 years ago (2.5 billion), there were no animals, just single cells a lot like bacteria. Some of those early single-celled creatures evolved chlorophyll, which gives plants the ability to photosynthesise and also makes them look green – and these eventually evolved into plants.

      When the early plants started making oxygen, at first it was poisonous to the other single-celled creatures which would eventually evolve into animals – it broke their DNA. But the early animal-type cells evolved ways to repair their DNA so they could survive.

      So when the animal-type cells evolved ways to break down some of the sorts of chemicals in food to make energy, they didn’t need to make their own oxygen – there was already far more oxygen about than they wanted!

    • Photo: Sally Cutler

      Sally Cutler answered on 11 Nov 2014:

      I imagine that humans have not really been around enough on the evolutionary timescale to need to have this skill.

    • Photo: Marikka Beecroft

      Marikka Beecroft answered on 12 Nov 2014:

      Interesting question! We evolved from a micro-organism that couldn’t photosynthesise and the ones that did produced oxygen which at that time was still incredibly toxic to us! As we have evolved we have made ways to use this oxygen so instead of it being toxic we now need it for life. We can’t photosynthesise now because we never had to as the plants and the algae have been doing it for us for so long that we have had no need to.

    • Photo: Robert Hampson

      Robert Hampson answered on 12 Nov 2014:

      Evolution proceeds in only one direction. This produces some results that don’t really make sense with the benefit of hindsight. For example, in the human eye, the blood supply runs across the front of the light sensing cells partially obscuring their view, the nerves which carry the signals from the light sensing cells also run across in front of them. Where the blood supply and optic nerve exit your eye you have a blind spot (hopefully you have heard of this).

      The reason you end up with these funny results is because the ability (to sense light in this instance) is very beneficial. Once the ability evolved it was preserved across the entire species very quickly even if it wasn’t the best, it was still better than not being able to do it at all (see light in this instance). It is better to be able to see badly than to be blind!

      So when the early ancestors of humans and other mammals were evolving, they evolved the ability to eat and digest things to provide food. The ancestors of plants evolved to be able to create food from carbon dioxide and oxygen and sunlight. Each branch of the evolutionary tree became better at these tasks. Even if we could produce chlorophyll we wouldn’t be very good at using it because we could not harvest much sunlight over the area of our skin. A tree however, is much better at this because it has many leaves to harvest the sunlight.

      It is much more beneficial in terms of evolution to specialise in some strategies than to try to do too many things. You couldn’t be efficient at finding and eating food whilst still being good at creating your own with chlorophyll. One needs muscle and moveability and a small mass to move around, the other requires a high surface area, low energy requirements, a sunny spot, etc….

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