There's "simplify for general consumption" and then there's "simplify to the point yr biases become the dominant message". Primitive/developed distinctions definitely fall in the latter category.

Everything evolves! Some species just refine or develop different traits from others, settle on traits that are more recognisably distinct from the rest of the group, or are more recognisably distinct from their ancestors.

Like, a rat 'looks' more like an early mammal than a bat or a kangaroo, because it fills a similar ecological niche to many of them. That doesn't make the rat more 'primitive' or the kangaroo more 'developed', and it doesn't mean the rat's been twiddling its genetic thumbs while the others evolved 'past it'.

It's a way of introducing evolutionary concepts that carries a whole lotta baggage that's false and encourages an outdated view of Natural Heirarchies and all that Aristotlean jazz.