Why do scientists think that all animals come from a common ancestor?

Answer 1

All animals share the same genetic code, the same, or very similar biochemical pathways. All animals share physiological similarities from the cell to the epidermis. All animals share behavioral similarities.

Answer

All animals have similar traits. All have the same sorts of cells (no cell walls, centrioles, only small vacuoles if any) for example. This is evidence that all animals have a common ancestor. It fits the model of animal common ancestry, it fits the hypothesis. Since animals breed using DNA as a hereditary material, then their offspring are capable of inheriting traits. If the common ancestor of all of a group has a trait, then we can reasonably assume that all its descendents should inherit that trait. And that works in reverse too. If all members of a vast group share a character, then it is violently possible that they all inherited it from a common ancestor. Take the homology of limb bones of mammals to illustrate this principle in a tiny subset of mammals or the shell-possession of molluscs as another homology (yes, all molluscs have shells - even squid). It is easy to see that mammals should all have a common ancestor. It is (relatively) easy to hypothesise that all molluscs have a common ancestor. What about all animals, ALL animals? We have forgotten, by the way, the most modern way to examine relationships - DNA analysis. DNA analysis shall confirm that all animals form a clade, a monophyly, a single group arising from a common ancestor. That must be the greatest vindication of the animalian hypothesis for biologists - that DNA can confirm that all animals have a common ancestor.