My partner and I have been populating our house with plants. Replacing the plastic imitations that have the benefit of not requiring care but lack authenticity. The amount of effort to keep a plant alive varies greatly depending on the plant. Nevertheless, generally, the ingredients for survival are the right balance of water, nutrients (+ Ph) and sunlight. It got me thinking about photosynthesis. The amazing, perplexing process of taking water, carbon dioxide and energy from sunlight to create glucose. I imagined what an animal would be like if it could use this process as its primary energy source. Not a small slug like creature (which does exist and can photosynthesise: see below) but a large, moving vertebrate animal. Would this be theoretically possible? Could you get enough energy from photosynthesis to move? I have noticed that the plants in my house tend to stay where I put them. Lastly, I thought, could there (theoretically) be a world where no lifeforms are consumed by other lifeforms? Where all life survives on photosynthesis and absorbed nutrients in the environment for survival?
Feasibility: Could a sentient mobile animal survive on photosynthesis?
This was a tricky question to answer, and to be honest I am not best placed to answer it. However, I have tried my best. There are a few examples of animals that utilise photosynthesis, but all of them rely on obtaining chloroplasts from other organisms, such as consuming algae. An interesting example is the Easter Merald Elysia. This sea slug can absorb chloroplasts from algae and has genes in its DNA that it ‘lifted’ from the algae’s genome which are required for chloroplast function.
Imagine a planet where early on in the evolutionary process a cell evolves with the ability to photosynthesise and all complex life on the planet descends from that cell. You may have heard of Eukaryotic and Prokaryotic cell types. Your cells are Eukaryotic. Prokaryotic cells came first in evolution, are much simpler and lack organelles. Organelles are structures within a cell that have specific jobs. Imagine prokaryotic cells as open planned. Everything is happening in one room with less order, so the cell cannot perform as complex tasks. Eukaryotic cells on the other hand have organelles, including a nucleus. This allowed the genome of eukaryotic cells to become more complex as it was contained and regulated within the nucleus and the cells evolved other organelles that allow complex reactions to take place, such as photosynthesis in the case of cells that have chloroplasts.
How did plant cells evolve chloroplasts?
Endosymbiosis: A symbiotic relationship where one organism lives inside the other
It is believed that plant cells evolved to have chloroplasts by engulfing a prokaryotic cell, which could photosynthesize, and integrating that prokaryote into itself in a process called endosymbiosis. This symbiotic relationship is believed to be how cells evolved to have mitochondria too. In the world I am imagining, this event occurred early into eukaryotic cell evolution and all subsequent eukaryotic life had chloroplasts.
Could a plant move?
A typical tree consumes ~200 calories a day. A typical human consumes ~2000. With our big brains and mobile bodies, we just need a lot more energy. Let us assume a mobile, intelligent alien counterpart would need a similar amount of calories. Maybe it would not be feasible for something larger than the small translucent slug to survive on photosynthesis. Well, it is not inconceivable that life over time, with enough evolutionary pressure and the right environment (maybe rich in CO2) could evolve a more efficient form of photosynthesis. Currently leaves are only able to convert ~5-9% of sunlight into energy. Could an evolutionary breakthrough improve this? In addition, if a life form was fully translucent, maybe it could absorb more light than a typical leaf. I actually do not think (theoretically) getting enough energy from photosynthesis alone to move and evolve intelligence is the limiting factor here. If we are talking about the universe, with the trillions of planets then perhaps there is one with just the right environment for this to happen.
I think the limiting factors for a life form evolving that survives on photosynthesis, can move and is intelligent would be getting the right evolutionary pressure. There would need to be an evolutionary pressure to move but also the ‘animal’ would need to never evolve the ability to consume the plant life it would be surrounded by (which would effectively take away the need to photosynthesize in the first place).
All life on earth really survives because of photosynthesis. Some life (plants, algae, etc.) are able to do it, and the rest survive by consuming them (or consuming what consumes them, and so on). On a planet where all life was surviving on photosynthesis, any life that evolved the ability to consume the life around it and obtain their energy would have a huge advantage. I can’t conceive of a planet with any conditions which would allow life to develop but would prevent predation from evolving. It would already almost certainly be present at a microscopic level. And just like on earth, I think the ‘predators’ (herbivores and carnivores) that evolved would be the most mobile and the most likely to evolve intelligence due to the large energy demands which those things require. However, we are limited to only observing life evolve once. Maybe it is not completely impossible that a planet would evolve, with a vast collection of plant and animal life all of which survives on photosynthesis. There would be no herbivores or carnivores. It is an interesting concept, even if impossible. And imagine if one of those animals developed complex intelligence. Medicine, technology, literature, space travel. The things on earth limited to us. If an intelligent plant-based alien existed and could observe us and talk to us. How would we be perceived? Would they think our planet and our species is barbaric? Consuming life around us to survive. Even vegans would seem unethical to them.