In the animal kingdom, the majority of animals are classified as invertebrates. These are animals that lack a backbone. It is estimated that approximately 95% of all animal species are invertebrates. This includes organisms like insects, mollusks, and arthropods, which thrive in almost all environments, from land to sea. In contrast, vertebrates, which include animals like humans, birds, fish, and reptiles, make up only about 5% of the total animal kingdom.