Terra Nove (New Earth) was a colony that was formed by people who decided to live away from earth. At some point, all signs of them ceased to exist. The Enterprise comes to the planet that 73 years before was the site of that colony. It has now become the home of tribal people who have moved beneath the surface. It was their belief that humans dropped toxic rain on them, killing most of the colonists and sending the young ones below ground, living in a series of caves and arteries. When Archer and the crew explore, Malcolm is captured and a series of maneuvers to show these people what actually happened to them takes place. This is all well and good. The problem is the time frame for the evolution of the culture is only 73 years. How could such a culture have mutated so incredibly in such a short time. They have become so different in such a short time. It just doesn't wash.