Most of us live in a climate that has cyclical seasons. Summer is a time of heat and fun, being outdoors and enjoying fruit, getting lots of exercise and generally feeling good. Fall brings on changing leaves and with it a chill and a beautiful melancholy. Winter arrives and brings barren trees and snow, blistering winds and a desire to be sheltered as much as possible. Finally spring breaks the cold, melts away the snow and fills the trees with fresh new buds.
The possibility that humans adapted to this ever changing flow is quite probable. Humans lived in temperate regions even in stone ages - anyone descended from those early peoples is going to have a digestive system that is trained to exist within the boundary of those seasons.
In summer, everything was availible - meat, fruit, vegetables, nuts, berries. Early man could have his pick of the abundance and probably ate a good bit of meat. Fall would come and animals would start to fatten in anticipation of their hibernation, and for early man potatoes and starchier ground vegetables would come availible as the above ground fruit and vegetables were disappearing. Early man would have to make it through the winter mostly on what he could eat in fall and what he could hunt over winter, plus a few hardy plants that last over winter like those starches, some berries, apples, etc. Biologically it made sense that man would need to increase his fat stores before winter just as other animals were doing - food would become scarce and temperatures would drop, and having a thicker layer of fat on the body is a natural solution to both of those problems.
As evolution progressed, our bodies would have evolved to gain weight when winter was coming - and one of the better indicators for our body was the types of food we were eating as we had nothing but what was seasonally avalible. When fall arrived, humans ate starchier foods - which turned on insulin and caused the body to begin storing fat rather than burning it. Insulin shuts off the body's fat burning and it also begins to store sugar (and carbs) as fat. Eating sugar or carbohydrates triggers the pancreas to release insulin in several doses the last of which lasts for hours. Eating starches at every meal causes us to continuously have insluin in our system and to not burn the calories we have stored.
When winter finally arrived, early man was fattened up and ready with his extra warmth and stores of energy. As winter progressed, he could occasionally find more starches and winter fruits to keep up on said stores - every time he consumed starches it would shut down the consumption of his stored energy and start the engine of storing any excess from what he ate in those hours. With fewer animals to hunt (as most were hibernating), early man had less food availible in general. He slept far more hours of the day (as nature intended) because winter brought longer nights and shorter days with it. So in a mass effort to conserve energy, man has adapted to gain weight when eating foods availible right before and during winter and to sleep longer during the winter.
Upon the arrival of spring, animals come out of hiding and are in a bit of a frenzy over mating, so they are very availible and hunting commences. After the long winter, early man probably gorges himself on a variety of meats and as the temperature warms and he consumes mostly meats, his extra layer of winter fat disappears. Into spring he goes, eating meat and readily availible leafy greens, spring gives way to summer and he enjoys some fruits and lots of sun, and the cycle begins again.
It is all of the above that leads me to believe that modern humans who decended from humans adapted to a temperate climate share the same metabolic quality - consumption of starches causes fat storage to begin, fat burning to end, and tells our body it is time to put on our winter coat of fat. Consumption of meat and leafy greens and no starches tells our body it is time to lose that winter coat of fat and become a strong virile hunter again. Consumption of some meat, some leafy greens, some fruits tells our body we are in our ideal state, food is abundant and can be enjoyed but neither fat gain or fat loss is necessary - our maintenance state.
Not everyone has ancesters who evolved in this manner. Those who can trace their origins back to tropical climates will likely have a different metabolic setup.
Our bodies are impressive biological machines that we do not fully understand yet - in fact we may understand them far less than we think.