Living things like humans, animals and plants undergo an evolutionary process throughout history. In this process, living things adapt and specialize with new environments, while for living beings who fail to do so, they will slowly become extinct.

So the image that arises in some biologists, students and students who study biology is evolution is always moving forward and living things that experience it lose the abilities that are not used anymore that once belonged to the ancestral species of these living creatures. Through more careful observation, evolution can actually move backwards, this is called de-evolution or devolution.

Life on earth is believed to begin in the oceans, ranging from single-celled species which then develop into complex species. The fossil of the oldest land animal species is 428 million years old Pneumodesmus newmani. Newmani is a multi-legged insect that became the ancestor of centipedes.

This fossil is important because it shows the presence of respiratory organs for the first time.

While fossils of marine animals from the Cambrian period were recorded as 543 million years old. Many fossils have been found from the Cambrian period which show plural-celled species such as shellfish, vertebrate animals and soft-bodied animals (molluscs). So life begins in the ocean, when animal species fill the ocean, then there are species that find food by the beach. This species eats food on the beach and then returns to the sea, then returns to the coast to eat and the longer it evolves to stay longer on land in order to get more food until finally fully adapted to live on land without having to swim back to the water for to breathe.

From this came the assumption that evolution always came forward. A paleontologist (the study of fossils of organisms and their interactions with their environment) named Louis Dollo proposed a hypothesis in 1893 that 'one organism cannot return, even in part, to its former state like its ancestors'. In a glance, this hypothesis seems correct, but when conducting further research, it is found that this hypothesis is incorrect. A great example is dolphins and whales. Both types of animals live in water, but they have developed spinal structures as in land mammals that are different from the arrangement of skeletons in fish. When the fish flick their rear fins to the right and to the left so they can move forward, the dolphins and whales move their rear fins up and down. Fish cannot move their rear fins up and down because they are limited in the composition of the bones in their bodies, while dolphins and whales cannot move the rear fins to the right and to the left because they are constrained by their spine. So dolphins and whales are examples of land animals returning to life in the oceans like their ancestors in more than 500 million years ago living in the oceans.