Except that Chordata evolved from Arthropoda and we cannot reasonably be considered arthropods.
What? Arthropoda and Chordata are two separate Phylums. Chordata are a sub group of Dueterostomia which is a subgroup of Bilateria, arthropods are also in the Bilateria group but are protostomes. Chordata is not named as a subgroup of Arthropods, where did you hear this?
Sorry for the long pause. I forgot.
That's exactly my point. Early chordates evolved from some of the more advanced arthropods, but cannot be considered arthropods. Therefore, your claim that evolution can't cause something to become something completely different is false.
I'm not sure where you're getting the idea that chordates evolved from arthropods. All vertebrates are more closely related to star fish than any arthropod.
We don't have a great fossil record for the evolutionary history of chordates. What we do have shows jellyfish-like creatures becoming more slug-like/worm-like, and eventually developing a tadpole shape. Unlike arthropods, which have hard outer coverings, these creatures were quite soft and didn't get preserved as easily.
Somewhere along the line they developed notochords, which are simple, flexible rods that act like a backbone. All chordates, including humans, still have notochords at some point in their development. There's no a single arthropod, living or dead, which has anything similar.