Evolution is constant since environment we live in is always changing and we need to adapt to it. Extra artery is an anatomical variation[1] because in order to evolve random mutations are happening and whatever works is passed to offspring.

For example this article[2] explains Alan Turing 's Turing pattern[3] and pattern variation of denticles, the toothlike protrusions that cover the skin of sharks.

The article says "Nature tends to invent something once, and then plays variations on that theme." That's true because variation is a way for nature to trial and error certain solutions until good enough solution is found.

[1] https://en.wikipedia.org/wiki/Anatomical_variation

[2] https://nautil.us/issue/68/context/how-alan-turing-deciphere...

[3] https://en.wikipedia.org/wiki/Turing_pattern