[tamsinbell (OP)]

Vasundhara asks:

"Could humans evolve to eventually perform photosynthesis like photosynthetic animals such as the sea slug, spotted salamander, Elysia chlorotica etc? I know that complete photosynthetic evolution is not possible for humans yet, but I was wondering if smaller photosynthetic adaptations are possible?"

What do you think? Post below...

[Janus]

All instances of photosynthetic animals involve small creatures. The reason this is important is the the "square-cube law".
If you take an object and increase its dimensions by some factor, you increase it surface area by the square of that factor and its volume by the cube of that factor.  Double the dimensions and the surface area increases by 4 and the volume increases by a factor of 8.   
Photosynthesis requires surface area, and since surface area grows a  much slower rate than the mass of body tissue it would have to support, by the time you get up to something the size of a human, it just wouldn't produce a large enough fraction of the energy required to be worth it.
Just to get up our basic metabolic rate, we need ~18,300 J of energy/day.   
There is ~1 watt, or joule/sec of sunlight striking a square meter on a Sunny day.
An average person has ~2 square meters of skin. Less than half of that will ever be in direct sunlight, but we'll be generous and say 1 sq meter gets full sunlight.
Plant photosynthesis is between 0.1% and 2% efficient at turning sunlight into useful energy. We'll split the difference and say 1% for our photosynthetic human. 
That's equals 0.01 joules/sec of energy produced from sunlight by their skin if standing unclothed in direct sunlight.
This is 36 joules/hr, compared to the 762.5 joules/hr needed for running their metabolism.
And the Sun isn't up 24 hrs a day, so the total energy produced per day is going to be even less.
So the question is, would that little bit of extra energy provided be worth the disadvantages having photosynthetic skin would bring?

[syhprum]

There is at least 1 KW of energy per square meter in sunlight on a clear day