The process by which humans and other animals with backbones (vertebrates) evolved from animals without backbones (invertebrates) is a result of millions of years of gradual changes through the process of biological evolution. The theory of evolution, first proposed by Charles Darwin, explains how species change over time through natural selection and other mechanisms.
The key steps in the evolution of vertebrates from invertebrates involved the following:
Common Ancestors: All animals with and without backbones share a common ancestor. This ancestor was likely a simple, single-celled organism that lived billions of years ago.
Emergence of Invertebrates: Over time, simple organisms evolved into more complex ones. Invertebrates, animals without backbones, appeared much earlier in the evolutionary timeline. They include creatures like insects, mollusks, and worms.
Evolution of Jawless Fish: Among the earliest vertebrates were jawless fish, such as the Agnatha, which appeared around 500 million years ago. These fish had a cartilaginous structure instead of a true backbone.
Development of Jawed Fish: Around 400 million years ago, jawed fish (Gnathostomata) evolved from their jawless ancestors. They had more advanced features, including true jaws and a vertebral column made of bone, marking a significant step toward modern vertebrates.
Development of Tetrapods: Approximately 360 million years ago, some jawed fish ventured onto land, adapting to terrestrial life and evolving into the first tetrapods (four-limbed vertebrates). Tetrapods were the ancestors of amphibians, reptiles, birds, and mammals.
Diversification of Tetrapods: Over millions of years, tetrapods diversified into various lineages, eventually leading to the evolution of modern animals, including mammals like humans.
It's essential to note that evolution is a gradual and continuous process that takes place over an incredibly long time. Small genetic changes accumulate over generations, leading to the development of new traits and species. The fossil record, comparative anatomy, and genetic evidence support the theory of evolution and the common ancestry of all living organisms, including humans.