About 370 million years ago, in the Devonian Period, the Ichthyostega developed. It was no longer a fish. The four-legged salamander-like animal was one of the first terrestrial vertebrates. It was already capable of spending some time out of water. Its feet were still fin-like but were already attached to legs, although these were still fairly inflexible. That meant these prehistoric animals couldn’t walk yet. Instead, they used their limbs as crutches to pull their bodies through the mud.