The women carried their babies much of the time (hey, no babysitters in those days), as well as bundles of firewood, or whatever they had gathered, foraged or scavenged. The men carried heavy spears or other tools, they dragged heavy carcasses of animals they had hunted, and they moved large boulders or logs to build shelters. They also lifted themselves into trees or up onto higher ground when escaping from danger or to scout a new route. The biochemical signals created by these very brief but intense muscle contractions generated a slight surge in growth hormone and a reduction in myostatin gene expression, prompting an increase in muscle size and power; particularly fast twitch fibers.
Go to the gym and lift weights for 30-45 minutes, 2-3 times a week. Focus on movements that involve the entire body and in wider ranges of motion – not just on isolating body parts. Emulate the movements of our ancestors: jumping, squatting, lunging, pushing, pulling, twisting, etc. This will stimulate your genes to increase muscle strength and power, increase bone density, improve insulin sensitivity, stimulate growth hormone secretion, and consume stored body fat.