What if humans had photosynthetic skin?

The series "Imaginary Earths" speculates what the world might be like if one key aspect of life changed, whether related to the planet or humanity.

Green skin is common in science fiction, from little green men to Hera Syndulla from "Star Wars Rebels" to Gamora from "Guardians of the Galaxy." But what if green skin were not just for fictional aliens? If humans had green skin, for instance, what if it granted us the ability to perform photosynthesis, which plants use to live off of sunlight?

Let's analyze what science says about similar abilities in other animals and ask award-winning science-fiction author John Scalzi how he thinks humans might hypothetically benefit from photosynthetic skin.

Related: 10 things we learned about humans

Plant-like animals

Every animal consumes food to survive. In contrast, plants rely on photosynthesis to create their own energy. However, some animals do use sunlight for a range of capabilities. 

For example, a number of animals benefit from solar-powered molecules. The pea aphid produces pigments that, with the aid of light, generate adenosine triphosphate, or ATP, the compound that powers reactions with cells. In addition, a stripe of yellow pigment on the exoskeleton of the Oriental hornet (Vespa orientalis) converts light to electricity, which could help to explain why these insects become more active during the middle of the day.

Other animals make use of actual photosynthesis, using sunlight, water and carbon dioxide to produce sugars and other vital compounds. Plants and algae rely on chloroplasts, structures within their cells, to carry out photosynthesis, but Elysia sea slugs can steal chloroplasts from algae they graze on, to help them live solely on photosynthesis for months. .

Many other animals reap benefits from photosynthesis by forming partnerships instead. For instance, most corals partner with photosynthetic symbiotic microbes known as zooxanthellae, while the eggs of spotted salamanders receive valuable oxygen from algae.

Engineering photosynthesis?

If other animals can reap benefits from photosynthesis, could humans? Even if photosynthesis could work in humans, it remains uncertain how much of an advantage we could actually gain from it. 

Plants can live off of photosynthesis because they grow broad, flat leaves to harvest as much light as possible. They also need less energy because they are far less active than animals.

According to Lindsay Turnbull, a plant ecologist at the University of Oxford in England, if the skin of a typical adult woman were photosynthetic like a leaf, the amount of surface area she had would satisfy only 1% of her daily energy requirements to survive. For a photosynthesizing woman to meet her energy demands, she would need a lot more skin — about a tennis court's worth, Turnbull estimated.

In addition, photosynthesis requires carbon dioxide. Plants have pores called stomata that they use to supply the gas to their cells. Assuming that photosynthetic humans possessed chloroplasts, they might need porous skin to let in carbon dioxide, but such pores might let other things leak in or out — for instance, moisture — in ways that might prove detrimental to the human body.

Solar-powered humans

Still, if humans had photosynthetic skin, even a tiny benefit might prove useful. In John Scalzi's Hugo Award-nominated novel "Old Man's War," soldiers are equipped with genetically engineered bodies that not only possess cybernetic brain implants and enhanced strength, speed, senses, endurance and dexterity, but also green, photosynthetic skin.

Although the soldiers of "Old Man's War" cannot get all of the energy they need to survive from their photosynthetic skin, in the novel, they are told it can "provide your body with an extra source of energy and to optimize your body's use of both oxygen and carbon dioxide. The result: You'll feel fresher, longer — and better able to perform your duties."

"I was thinking about how, if you were to take the human body into the chop shop, so to speak, to bling it out, what would you do to it?" Scalzi said. Photosynthetic skin "would be a supplementary passive modification, as opposed to an active modification — you could just sit there and accrue benefits from it in terms of keeping energized. It might be a 3% to 5% advantage in the scope of things, but that's a margin you didn't have before, and you're getting it for free."

How might photosynthesizing feel? "I suspect it would feel like being caffeinated all the time," Scalzi told Live Science. "You would wake up, and just like you would say, 'I need my coffee,' you'd want to get some light."

Assuming humans could successfully become photosynthetic, how might this change the course of history if, say, "someone went back in time and gave Cro-Magnons access to a CRISPR machine?" Scalzi said.

Scalzi said he doesn't think human society would change radically if people could photosynthesize, given the marginal benefits it would provide. Still, the most energy-dependent part of the body is the human brain. "So I suspect that any surplus energy that photosynthesis might give is going to be taken up by the brain, because it's a hungry, hungry, hungry organ," Scalzi said "That might potentially mean societies might hit certain marks of progress a little bit faster, maybe have reached the Industrial Revolution in 10,000 BCE instead of 1800 CE."

One might wonder if photosynthetic people might prefer moving to sunny climes. Although such people might receive a marginal boost from photosynthesis if they moved to a desert area, they would likely have other resource issues to deal with, such as a lack of water, Scalzi said. "There's always going to be trade-offs," he noted

And would photosynthetic humans prefer little or no clothing, to absorb all those rays? In some photosynthetic societies, clothing might become a symbol of the elite — a sign they get enough energy from food to not need photosynthesis. "You can imagine them saying, 'I'm rich, so I can cover up,'" Scalzi said.

So, would Scalzi want photosynthetic skin for himself? "On the grand list of body modifications I would want, it's kind of in the lower middle," he said. "It wouldn't hurt, but I don't see the benefit from it being so substantial that I would completely change the way I'd look to benefit from it.

"But if someone else is like, 'I'm going to be photosynthetic,' then you do you," Scalzi said. "I'm glad you're happy."

Originally published on Live Science.