It wasn't so arid in the Pleistocene but a pine forest(https://www.sciencedirect.com/science/article/abs/pii/S0033589401922872). That's back when the glaciers were still around and height of hominid impact was small hunter gatherer groups with some sharp rocks at best.

There was a recent Baja grizzly up to the 1800s but this map is hard to interpret on time period because the population goes very far south and east into the great plains. It's likely pre westward expansion, but without being explicit it's hard to tell.