Norwegian supermarkets: You mean your prices weren't already raised?

To what extent is it kept warm simply by heat retention from primordial times and the formation of the earth? As in, how does the thermal energy generated by the core/mantle over the past few billion years compare to the thermal energy lost and the thermal energy it started out with when the planet was a mostly molten blob?

I guess to simplify the question, I'm curious whether, in the absence of radioactive decay in the mantle, we'd be another Mars right now.

> we aren't fighting with arrows anymore

We are, however, still fighting with javelins.