Theory: Kerbals are a partly plant-based species.

[Chakkoty]

Now I know realism and lore aren't exactly what KSP is about, but I gave it some thought and arrived at the following conclusion:

Kerbals are, at least in part, a plant-based species. How come, you ask? No? I'll tell you anyway!

Their skin is green. What causes green skin? Possibly copper-based blood, but the inside of their mouths are flesh-colored, so: Chlorophyll! Perhaps in the distant past, Kerbal ancestors had to adjust to scarce nutrients and evolved to use their skin for photosynthesis, just as humans (and other mammals) incorporated many helpful bacteria into their digestive system over the course of their evolution.  This would also explain why they can survive/breathe on Laythe, where the atmosphere (according to science data) is not entirely like that of Kerbin. Their lungs aren't the only thing they use to "breathe", they use photosynthesis as well.

Their heads are so large because that way, a lot of skin is exposed to sunlight - similar to how humans evolved to stand on all fours so they could see over the tall grass in the plains of Africa.

 

Total [snip]... of course, but it's a fun thing to think about.

 

 

Edited December 13, 2019 by Snark
Redacted by moderator