Did you know that plants, animals, humans, and all my fun earth are related? You may wonder how such different things can all be related. All life on Earth shares a common ancestor. Small changes over billions and years have resulted in the wide variety of life you see today. Just like people can trace their roots back to their ancestors, all of life can be traced back to a common ancestor. Relationships in a family can be shown in the form of a tree, and scientists also use a tree shape to track relationships between organisms. This tree shows the relationships between fish and tetrapods, or animals with four limbs. Each place where two branches come together is called a node. The nodes are marked with red circles. Each node represents a common ancestor. For example, this node represents the common ancestor of mammals and lizards. If we look down at the root of the tree, we can see where all the branches come together. This is the common ancestor of fish and tetrapods. When we look at a fish and a tetrapod, we can see many differences. They do not look very similar to each other. This is because after the branches split off from each other, there have been many changes that form the fish and tetrapods we see today. The common ancestor of fish and tetrapods lived about 370 million years ago. There has been lots of time for changes to take place. In the fossil record, there are a lot of fossils that have characteristics of fish and tetrapods. Watch and see how a fish-like organism could have changed into a tetrapod over time. Small changes in organisms can cause new types of organisms to develop. The common ancestor of fish and tetrapods did not have limbs, but limbs evolved in the tetrapod lineage. As traits develop, they lead to new organisms that are all related by a common ancestor. But they are just changed a little by little. This is called descent with modification. The species of the fish and the tetrapods are now in the same species. They are also known as the species of the fish and the species of the fish. They are also known as the species of the fish and the tetrapods. They are known as the species of the fish and the tetrapods. They are known as the species of the fish and the tetrapods. They are known as the species of the fish and the tetrapods. They are known as the species of the fish and the tetrapods. They are known as the species of the fish and the tetrapods.