In Earth's history, such periods of swift genetic change usually have resulted from life entering an otherwise empty niche (such as the explosion of multicellular life that marks the Cambrian or the rapid diversification of plant life forms that came about when plants first colonized land). Such periods inevitably lead to periods of mass extinction as the variety of forms compete with one another for limited resources.
So the gnomes could have diverged from humans after monkeys and apes did provided they settled a plane that had at least initially low selective pressures.
There's also always my initial convoluted proposal (arise on Jermyn, human ancestors warp to Teragard, and gnomes diverged from there as in the paragraph immediately prior to this one).
Last, but not least, there's always the possibility that the races in question aren't related at all, and that humans simply share more superficially similar traits with monkeys and apes than gnomes do (likely internal traits (ex: organs) due to external similarities) as a result of convergent evolution (ex: dolphins and sharks having a streamlined, torpedo-like body despite the different origins of their bodies' shape).
13-Mar-2018 17:33:40