When you think about it, nearly half our body construction have come into being just to lug around a huge baggage of a digestive tract – which itself is just sloppily packed into our belly. Not to mention the inefficiency (waste vs energy and nutrients extracted).
Compare that with the efficiency of plants – light-energy directly converted (with help of water and CO2) and stored away as simple carbs. And the only waste being simple compounds like O2. Well I guess the photosynthesis can be ruled out as it won’t be able to produce enough energy at the rate we consume (being mobile). But who knows, a slight deviation in evolution could have evolved us a totally different mechanism — like the creatures in Gregory Benford’s novel that have evolved organic electro-chemical capacitive containers to store their energy.
In the krta yuga, man was pure conscious based sentient beings. Then in the treta yuga (when conscious differentiated into matter and energy) he degenerated to energy based sentient beings. In the dvapara-yuga, he degenerated further into matter based sentient beings. And finally in the kali-yuga he has just become lame “meat beings”.
As another example of degeneration, in krta yuga one could move from one place to another just through “thought” (like teleportation). Then in the treta yuga transport was through vimanas (flying machines) which were energized and powered by mantras (thought-vibration-engines). In the dvapara yuga, the vimanas had engines powered by mantras and yantras (mechanics). In the kali yuga, our mechanism of transportation is through only yantras (i.e. purely mechanical).
Today’s finding in modern physics has opened up science-fiction writers to invent space-time travel possibilities as described in the Puranas. Like in Star Trek, where the Enterprise ship is driven to “warp speed”, opening up a fissure in the fabric of space and time, allowing one to go from point A to point B via a short cut in higher dimensional space.