How did animals evolve from water to land?
Water to land evolution is a complex question, and one that is still actively researched today.
If we go back in time before the dinosaurs, before there was anything on land other than the initial starts of plants, we have organisms living in water that look just like fish today.
The ones we’re most interested in are lobe-finned fish, the Sarcopterygii. There are relatives of these fish alive today with a lineage that extends back to around 390 to 385 million years ago. During that time there was a split, a divergence between groups that became the ray-finned fish and those that were the lobe-finned fish.
At that split, the animal still looks more like a fish than it does a four-legged animal coming onto land. But if you look at the bones in its wrist or hands, they become more and more developed down the line for terrestrial, or land adaptation.
Originally, the limbs weren’t evolving into hands because the animals were trying to get on land. They were just living in environments where it was beneficial to have a solid pectoral girdle attached to their vertebrae, basically a stronger attachment for shoulders and hips. This gives them more stability and structure for moving in shallow water environments. We find a lot of these fossils in shallow marine environments.
As we continue to progress up the line of the evolving creature, we see more changes to the forelimbs and ribs.
Ribs get larger and stronger so they can support more weight. In the water, these creatures are buoyant—they don’t need as much structural stability because the water is holding them up. But as they start to get onto land, they need more rigid structures that help maintain the pressure of their body weight.
We don’t really know why they’re going onto land initially. It may have been to escape predators or at least lay eggs in a safer place. Maybe there were new food sources they were really trying to access. The land was a whole unexplored ecosystem at this time so there were many opportunities.