Somewhere around 520 million years ago, there was an explosion of life known as the Cambrian Period.
It's pretty crazy to look at these renderings of what these animals may have looked like and think: "Wow, those look like aliens."
Because they're not alien… they were earthlings before humans were even a twinkle in mother nature's eye.
The Cambrian Period was a time when predators were getting deadlier and prey was getting better defended.
Everyone was evolving as fast as possible to try and stay alive and make some babies!
But not all of these creatures ended up surviving and some died off pretty quickly.
That's why certain creatures, like the one in the photo (above), don't seem to resemble anything alive today!
[Image via Quade Paul/National Geographic.]