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Is it theoretically possible to genetically alter humans so that they could live off solar energy like plants? What biological and physiological changes would have to be made?

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    $\begingroup$ They's still need to eat, I mean, even plants draw nutrients from the ground, and humans don't have roots. $\endgroup$ – Xandar The Zenon Mar 2 '16 at 21:36
  • $\begingroup$ As haemoglobin developed as a "mutation" from chlorophyll it is unlikely that an organism uses both molecules $\endgroup$ – user38973 Jun 3 '17 at 13:19
  • $\begingroup$ "Could a human-plant hybrid exist?" evokes memories of things I previously saw on the internet. $\endgroup$ – Gene Jun 22 '17 at 14:50
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The basic problem is explained in http://what-if.xkcd.com/17/; there is simply not enough surface area on the average human (or cow) to provide the energy and nutrition needed for a high energy mammalian lifestyle.

This can be addressed in two ways:

  1. Plants can be made more efficient. Current plant life uses solar energy relatively inefficiently, with only a small percentage being converted into actual usable sugars or creating oxygen. While there would be a lot of obstacles to creating plants with efficiencies rivalling high end solar cells (mostly to do with heat rejection), the idea of plants which can convert 5% or more of the incoming solar energy would have lots of advantages, even in traditional applications like agriculture.

  2. Become a symbiont in free space. Since plants are limited in their collecting area by gravity, the ability of a human to carry an acre of photosynthesizing surface will be quite limited on Earth. IF the human is symbiotically paired with a "hyperplant", the plant part can shelter the person inside, and extend its leaves into free space to collect energy, as well as circulate sap for heat control like a giant radiator

Hyperplant in orbit

Free floating human plant symbionts in orbit would make a great far future setting. Just add water....

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    $\begingroup$ From that image, I wonder if it would be possible to make the plant into a solar sail. Then people could get solar energy and wander the local solar system (or farther). $\endgroup$ – Frostfyre Mar 2 '16 at 19:08
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    $\begingroup$ This would solve a lot of different issues at once, although the "pilot" would have to be very careful not to get too far from the Sun and not have enough energy to live. $\endgroup$ – Thucydides Mar 2 '16 at 19:24
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    $\begingroup$ Have mans would still need to eat, just as plants need to draw nutrients. That's how we construct sugars and things. Unless you convert the solar energy directly to energy for your body, although I'm not sure that's possible in current science. You'd also have to breathe in the over things you need. It's probably worth addressing. $\endgroup$ – Xandar The Zenon Mar 2 '16 at 21:58
  • $\begingroup$ As a symbiont, your wastes would go to the hyperplant while the "waste" products of the plant (like oxygen) would go to you. Landing on the occasional asteroid and putting down roots to draw additional nutrients and water would probably be part of the lifestyle of a symbiotic plant-human being. $\endgroup$ – Thucydides Mar 2 '16 at 23:58
  • $\begingroup$ It should be mentioned that you might need some method of propulsion other than solar sails to be safe, after all if you rely on the sun, a method of propulsion that lets you go away from it but not towards it can't be your sole engine. $\endgroup$ – Vakus Drake Mar 4 '16 at 4:12
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Yes, it exists in the fantasy world. Or something close to it. In the Wings Series by Aprilynne Pike, the main character is sort of a human-plant hybrid, though she is called a faerie. Females of her species bloom for one quarter a year (spring faeries bloom in spring, etc), where a flower blossoms on their backs, and if a male of that species is around a blooming female, his body produces pollen on his hands. Faeries need to be in the sun as much as possible, and they only eat fruits and vegetables.

I suppose if you want it to be purely science-fiction, you could try splicing human and plant DNA? Biologically, the change in DNA would do your work for you, but physiologically, you might have to deal with flowers and pollen (how would such hybrids mate) and more basically, the fact that chlorophyll may be produced by the body, causing certain parts of the body to turn green. Then, you'd have to see if the chlorophyll in the body is enough to power the person completely with just water, sunlight and CO2, or if he needs extra nourishment, in which case, a non-vegetarian diet might not agree with him.

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    $\begingroup$ "you could try splicing human and plant DNA? Biologically, the change in DNA would do your work for you". Sorry but no, that's not even remotely true. The biology/genes of plants and humans are so vastly different that a cell with a simple DNA combination would not be able survive (and the science needed to make it work is nowhere within our reaach). $\endgroup$ – Nicolai Jun 1 '17 at 6:08
  • $\begingroup$ With respect, this first paragraph doesn't really belong in a "reality-check" answer - the fact that it's been done in a fantasy settings with faeries can't really support the concept of genetically altering humans to achieve the same. $\endgroup$ – G0BLiN Dec 16 '18 at 9:21
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Turning green is fine, mating could be pollen replacing sperm in normal human mating, the alternative would be sperm being passed around the air or by bees and appled by touch, that would be too wierd. If sun was that was needed then no more world hunger...

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  • $\begingroup$ Plants do not live by sunlight alone. What about other nutrients? Where will the energy to extract those come from? Consider the active capturing area of any existing plant: is it enough to support the respiration of an existing human? How much more might be required, and what would be the cost of getting it? $\endgroup$ – nzaman Dec 16 '18 at 8:46
  • $\begingroup$ Your answer is missing more than half of what plants are. $\endgroup$ – L.Dutch Dec 16 '18 at 9:27
  • $\begingroup$ This doesn't answer the main question, which is how humans could be genetically altered to live off solar energy. You've addressed two completely different aspects of the human/plant hybrids instead. $\endgroup$ – F1Krazy Dec 16 '18 at 9:39

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