Dawn is partly or wholly Neanderthal, while all of the other people around are Cro-Mangon.
The anachronistic humans in the Mesozoic are the result of Time Travel events in the post-victorian future
- While the time machines used in the story are lost, the fact they existed at all and functioned means that at some point they could be reinvented. sometime in the future time travel is reinvented, and it is used either for time-tourism and ends up leaving a lot of people stranded in the past, or was used as a dumping ground for criminals, political prisoners, and other people a nation deems undesirable. in either case, the survivors go on to breed and build a society of their own in the past. the more modern seeming groups (like dawn's adoptive parents and their farm) would represent more recent arrivals. The "cavemen" Dawn originally grew up with were less fortunate survivors that didn't (or couldn't) develop a more complex society and technology than stone age tribalism.
- though now that i think about it on rereading, this doesn't explain the various Tertiary critters like mammoths, sabertoothed cats, or megaloceros seen running around, or the fact you have creatures from all periods of the mesozoic mixed together.
