We have a gene that stops us building muscle. Racing dogs without the gene are champion sprinters. Cattle without it are jacked. The MSTN gene produces myostatin, which tells muscles to not grow. So, why would evolution not want us to build muscle? Because more muscle is liability. I believe myostatin is a famine adaptation. Muscle burns energy just sitting there and a gene that prevents you from building more will get you through the winter. The thing is, we don't live in famine anymore. The world we live in today is the world of abundance. Food is unlimited and movement is optional. Mechanisms that evolved to constrain muscle may be maladaptive today and contributing to the obesity epidemic. Which is why the people born with myostatin deficiency have less fat, more muscle, stronger grip, their hearts are fine as well as fertility. I think myostatin inhibitor will be huge because in today's world the brake on muscle isn't needed anymore.