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On a fantasy-themed planet with Earth-like humans and plants, over a very long period of time, a dryad-like species has evolved. Dryads look mostly like regular humans, but they have a few qualities that differentiate them, like green hair, special reactions to sunlight (see Zotoh Zhaan from Farscape, whose skin changes color) and so on.

Clearly they have something in their genetic makeup that makes them closely related to both humans and plants. Is a species with both human and plant DNA plausible?

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    $\begingroup$ For those of us unfamiliar with Farscape, could you elaborate on the specific sunlight-reactive traits you had in mind and the degree to which you want your species to have them? $\endgroup$ – Dog Feb 14 '17 at 0:15
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    $\begingroup$ @Dog You're right. In Farscape, Delvians are a plant-like species who take sexual pleasure when exposed to a certain kind of radiation. What I'm after for the dryads when exposed to regular sunlight is perhaps pleasure in general (not necessarily sexual), physical or mental rejuvenation, a temporary boost in their abilities (like an adrenaline rush), maybe aid in reproduction, anything that could underline their plant-like nature. $\endgroup$ – Sigma Ori Feb 14 '17 at 0:18
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#1: Convergent evolution

Separate species are known to evolve to be similar if they live in similar conditions. We've already covered the evolution of intelligent plants here, and if those intelligences live in similar conditions to humans, it's possible for them to evolve similar characteristics. A stretch, but technically possible.

#2: Common ancestry

The dryads and humans are actually very closely related, and only recently have the groups split and evolved differently. Maybe humans lost their plantlike characteristics, or dryads evolved photosynthesis on their own.

#3: Mimicry

It can be evolutionarily advantageous to look like another species. Perhaps the more "people-like" plant-organisms scare off predators, and survive to reproduce through generations. Green hair and color change may be artifacts - technically, atavisms - from earlier forms.

#4: Symbiosis

Lichens are organisms consisting of both fungi and algae (or other plants). The fungi provide a structure while the plants produce energy, and together, they act as one organism. Perhaps one offshoot of humans developed a similar relationship.

#5 Horizontal gene transfer

Many organisms on Earth contain genes that they did not evolve - genes that were placed by viruses or bacteria. Although unlikely, it's possible that your dryads and humans evolved completely separately, but similar transfers began to blur the lines.

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Did the species evolve at all? Or are we looking at genetic engineering at work?

I find the latter hypothesis far more likely as it seems unlikely that photosynthesis is of any substantial value to a land animal. Average solar incident energy is ~1000W/m^2 not counting clouds, this averages down to 400W/m^2 over a 24 hour cycle. Unfortunately, I'm not having much luck on finding the vertical cross section of a human but I did stumble upon .2 m^2 and this is a bit above what I get trying to ballpark my own body. This means there is about 80W of incident energy. The best we see from plants is about a 2% conversion efficiency--said dryad will only get 1.6 watts of power from the photosynthesis. Said human has a resting energy use of about 100W. Photosynthesis thus will provide only 1.6% of the body's energy if the sun is overhead.

Facing directly towards (or away from) the sun will raise this to 3.2% of the body's resting energy.

Note that this only applies in a near ideal climate where the body does not need to expend energy on heating or cooling. As soon as you leave such an environment you either expend energy on temperature control or you cover up--cutting the photosynthetic intake.

Given the limited value I do not believe such an evolution is possible.

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  • $\begingroup$ Genetic engineering in their origin is not out of the question. But I'm curious if such a species is scientifically plausible, even if it's engineered. $\endgroup$ – Sigma Ori Feb 14 '17 at 13:02

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