Gate Color Theory.

[Avalon]

There is a sprout, and they take either it or another seed to the Trann Dome (I'm assuming it was another seed, since sending your only food source across a waste land would be very stupid, but I never really checked).

As for the trees on death mountain, a few of the smaller ones do appear to have some stubborn range leaves attached to them so they may have died somewhat recently (I've heard that some people believe the mountain to be Lavos' shell, so that would help some to explain why they appear to be the only plant life obtaining nutrients).

And back on the topic of the gates, I always thought that the Red gate was important because it took Lucca to a very specific point in time, by contrast to my thought (I'm new so I'm a little hazt as to the standard belief in this) that the standard blue gates took you a set distance on the timeline, like 400 years to the minute (Till they start taking you to the end of time, where there really is no time.) I usually just chalked up the green gate to Lavos interference.

[Anacalius]

Yes it was a 2nd sprout.

Apparently, whatever happened killed most plant life, but not all. However, with such little plant life came the little knowledge to grow it, so humans over the years lost that knowledge altogether, and MUST have survived almost (if not solely) on meat. Perhaps cannibalism played a role in their lives?

[VincentGAU8]

Cannibalism would have even accelerated extinction of humans.. they had not only a broken wasteland to call home, but also they were killing themselves off..

Back to the gates:
Blue gates were of either Lavos' or Entity's influence, or both, for travel to different points of the Planet's history..
Red Lucca Gate's origin is unclear, but probably it is the work of Entity alone..
Green gates were for direct contact with Lavos....

although i am quite unsure of these things...

[Boo the Gentleman Caller]

Naw, there were plenty of giant rats and mutant creatures to eat - and that's just what we SEE.

[Thought]

Wasn't that what the Enertrons were for? Hence the entire "but you are still hungry" bit. Keeps humans alive even without food.

[Boo the Gentleman Caller]

I just always thought a gave you a full night's rest...

But seeing as how it replenishes your HP and MP, it must be able to keep human alive without actually eating.

[Thought]

And there was the entire comment by Lucca that the Enertrons wouldn't last forever, so that people would need the seedling. At least to me, this seems to indicate that food was hard to come by (but people knew what it was, so presumably it wasn't unheard of).

[V_Translanka]

Yeah, there must have been things for them to hunt (though I doubt any of them could have taken down one of the Mutants)...I mean, they had various weaponry that they sell to Crono & Co., right? Plus potions & whatever else too...

[Boo the Gentleman Caller]

I recall someone in one the domes saying that they were afraid to cross the ruins for fear of mutants...

But to note, they said "mutants" and not "Mutants", so it may have been a general use of the word to describe all the creatures found within the ruins.

Oh, and this is going on the ORIGINAL translation.

[VincentGAU8]

I cant imagine them eating freaky monsters for breakfast.. But without vegetation, the whole food chain would have collapsed, even if there were any critters left for them to hunt and eat, they would not have survived... the Enertrons would have been the only possible solution for that matter...

[V_Translanka]

Well, there are all those damnedable rats (with sticky fingers) everywhere...and talking frogs! Can't forget about the weird talking frogs that show you where that one switch is and then completely disappear from the game...And those weird octopus thingies that are shot out of those plant things...hey! There ya go! Those octopus shooting plant thingies are plants! Mystery solved.

[VincentGAU8]

Hahaha!! Really now... Still, the fact remains that man cannot live by genetically altered meat and plant thingies from monsters alone!! 

[Anacalius]

man cannot live by genetically altered meat

Why?
Please note that these mutants and whatnot are commonplace in the Chrono universe (And every other RPG, pretty much). That being said, who says that people cannot survive off of eating them? They are a regular part of the Chrono universe, so how could they not fit into the food chain?

[VincentGAU8]

man cannot live by genetically altered meat

Why?
Please note that these mutants and whatnot are commonplace in the Chrono universe (And every other RPG, pretty much). That being said, who says that people cannot survive off of eating them? They are a regular part of the Chrono universe, so how could they not fit into the food chain?

The mutants were only in evidence after the Apocalypse, which i suspect destroyed the food chain.. But desperation might have driven the humans into eating them. still, most of these mutants appear to be stronger than the ordinary human, so i find the notion of mankind living off them very unlikely..

[V_Translanka]

I think the Mutants may be part of an experiment from Lab 16...maybe that's also why there are 'smart rats'? The Shadows could be the ghosts of the people who worked there? Aren't there some Shadows that pop out of something that looks like a car? Well, whatever the case, the Mutants pallet swap from the Black Omen is the Metal Mute and may be descendants of the much powerful Zealian monsters on the Black Omen: Mega, Giga & Terra Mutant. Perhaps also Zombor & Retinite are Magus' (or the Mystic's) version of those monsters since they have similar constructions & attacks...