eliza, you're mixing your evolutionary timescales up. the advent of cooking oil didn't come until long after civilization had sprouted. the earliest cultivation of olive trees was probably around the 5th century BC, which would have had to occur first in order to get enough olives to make enough oil to make cooking in it practical. prior to that, oils would have been ritualistic and ceremonial.

besides... oils are fat, and even though they come from plant sources, they are still fats. i know i said it's easy to imagine a diet without plants, but i was referencing leaves, grasses, grains, or abundances of fruit and vegetables prior to the introduction of civlization through agriculture. anything after agriculture took hold is a whole 'nother ball game.

fiber isn't the only thing that aids in digestive elimination. fat will do the same, in sufficient quantities. enzymes come from meat, as protein is broken down. the development of herbs in tea form did start out as medicinal, but i'm not sure any of us would consider things we imbibe for medicinal (or ceremonial) purposes as part of our diets. besides, tea is the extraction of flavor from dried leaves using hot water. you drink the water, you don't eat the leaves afterward.

anything and everything that we've discovered about the whys and hows of food came about only after civilization established itself. cacao trees existed before the mayans, but chocolate didn't.

the one thing we all definitely need to keep in mind is that regardless of how we personally handle the varieties of food available to us now, there have been 10,000 years of evolutionary changes since agriculture came about, and hundreds of thousands of years of changes since the previous human physical incarnation. the problem we're running into is that our technology and our imagination is developing far faster than our physical evolution can keep up with. so if you really want to be truthful, it's not carbohydrates that are screwing us up, it's our technology.