Nothing looked exactly like you 555 million years ago, but that’s when life emerged that looked like you in one crucial respect. What’s the most basic thing that you share with some creatures and not others? Brains? Lungs? Red blood cells? Nope. It’s that you’re bilateral. You have the ability to be cut into two identical sections, along one and one line only.
And we know to within 3 million years when the first “bilateria” appeared. These creatures sported an amazing innovation – a differentiated mouth and anus. Even more impressively, they had three little nerve bumplets along the line between the mouth and anus which, eventually, turned into the forebrain, the midbrain, and the hindbrain. These are the first animals to set the bilateral trend which would eventually take over the macroscopic world. These guys are the reason you’re not a hexapod.
Which doesn’t mean these things look anything like you. There’s a lively debate going on about what the earliest common ancestor of every bilateral being looked like. It could have looked like a hairy little tube worm, or a flattened, arrow-head-shaped leech. Some people think it had mouth tentacles.
Artist’s Rendering Of Earliest Common Bilateria.
Whatever it was, it shared something common to all of us, but something so common and simple that we take it for granted. So blink your two eyes twice and clap your two hands together for our oldest relations.