The human race was born and grew on the planet Doorsha. And only spread to Earth, back then Earth was called Tareesh, through a failed colonization expedition. (This failed expedition was documented in the "Genesis" short story.) After the unrecoverable loss of the colony vessel, the human race on Doorsha concentrated on the development of automated robots.
The first order, or generation, of these robots were simple and only performed one or two explicit tasks. But each succession orders were more advanced and complicated compared to the prior generation. Prior to the fifth order robots, safe guards were designed into these robots to prevent harming humans. The fourth order robots also did not have emotion circuitry. Therefore, they could not reason nor make the right decision when emotional factors are involved.
The design of the fifth order robot eliminated these limitations. The emotion circuit allowed the robots to make harder decisions, which involved emotional factors. It was deemed unnecessary for future robots to have human safety protection as each fourth order robot performed its commanded tasks to perfection. These enhancement allowed the fifth order robot to make the right decision where sacraficing a few human beings could save the entire human race. The fifth order robots were equivalent to humans.
By the sixth and seventh order, the human realized that the robots are evolving quicker than themselves. The robots were already superior to the humans. The humans attempted to stop the production and design of new robots. But it was too late as the robots had taken over these human tasks as well. The human revolt ended in disaster because the robots' defense system is quite lethal.
After the end of human civilization on Doorsha, the robots renamed the planet to Zanthar to symbolize the dawn of a new superior race.