system
Given the following post, respond as if you're commenting on Reddit:
human
[Sonic] Why is the Ultimate Lifeform... A hedgehog?

Like, Shadow was developed by humans, right? With the help of Black Doom. But what's the deal with him being a hedgehog? Why not a human? Did they experiment on hedgehogs? Why hedgehogs? Why are there Sapient two-legged animal people anyway?
gpt
Basically there was an ancient mural in some old ruins that Professor Gerald saw and he based the Bio-Lizard and Shadow off of those murals. Essentially it was a prophetic mural that depicted the final battle between Super Sonic and Perfect Chaos. Professor Gerald decided to base his Ultimate Lifeform off of those murals with Shadow being based off of Super Sonic and the Biolozard being based off of Perfect Chaos. 

This is why Shadow calls Sonic the “True Ultimate Lifeform” at the end of Sonic Adventure 2.

Basically the Ultimate Lifeform is a Hedgehog because Sonic happened to be a Hedgehog.