What characteristic can you use to separate all animals into two groups?

The most fundamental characteristic to separate all animals into two groups is the presence or absence of a backbone, also known as a spinal column. This leads to the classification of animals into:

* Vertebrates: Animals with a backbone. This group includes mammals, birds, reptiles, amphibians, and fish.

* Invertebrates: Animals without a backbone. This vast group includes insects, worms, mollusks, crustaceans, and many others.

This classification is significant because it reflects a fundamental difference in body structure and evolutionary history.