If a permanent turns into a(nother) creature, has it entered the battlefield?
&quot;Enters the battlefield&quot; means moving from a zone besides the battlefield into the battlefield. As you say, it didn't actually enter the battlefield - it didn't change zones, it just changed from one thing to another within a zone. So no, it doesn't trigger evolve.
As for citations, well, there's a ruling explicitly for this land-becoming creature case on many of the lands that can become creatures (e.g. Celestial Colonnade):

When a land becomes a creature, that doesn't count as having a creature enter the battlefield. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters the battlefield won't trigger.

I don't think the comp rules really say this explicitly; they say the battlefield is a zone, and &quot;entering&quot; a zone clearly means coming from outside the zone into the zone. For example, there's stuff like this:

110.1. A permanent is a card or token on the battlefield. A permanent remains on the battlefield indefinitely. A card or token becomes a permanent as it enters the battlefield and it stops being a permanent as it's moved to another zone by an effect or rule.
603.6a Enters-the-battlefield abilities trigger when a permanent enters the battlefield. ...

Nothing explicit, but consistently &quot;entering&quot; a zone is used to mean changing zones into that zone.