Q How did animals evolve from water to land?
— Ava Sainsbury, Madison, Wis.
A Dave Lovelace, vertebrate paleontologist and scientist at the University of Wisconsin-Madison Geology Museum:
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, there were organisms living in water that looked 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 about 390 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 got larger and stronger so they could support more weight.
In the water, these creatures were buoyant. They didn’t need as much structural stability because the water was holding them up. But as they started to get onto land, they needed more rigid structures to help maintain the pressure of their body weight.
We don’t really know why they were 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 an entirely unexplored ecosystem at this time so there were many opportunities.