Hardgainers will rule the world
The environment for which we are genetically adapted is one that required hard physical work to procure uncertain sources of food.
An adaptation that would benefit an organism in such an environment would be the capacity to easily store excess energy in the form of fat and muscle.
However, today we find ourselves in a completely different environment. One in which food is not scarce and no physical work whatsoever need to be done in order to procure it. Unlike our environment, our genome has remained relatively stable. The result is skyrocketing levels of obesity.
But not everyone has remained true to their genetic roots. The "hardgainer" has adapted. Hardgainers are uniquely adapted in that their inability to gain weight makes them uniquely suited to survive in this brave new world.
That's what a hardgainer is. The next branch in human evolution.