What do birds, reptiles, amphibians, and fish all have in common? 

In this blog post, we will answer the question “What do birds, reptiles, amphibians, and fish all have in common?” and learn more about the evolution and some biological aspects of these groups.

What do birds, reptiles, amphibians, and fish all have in common? 

Birds, reptiles, amphibians, and fish are all vertebrates, which means they have an endoskeleton that provides body stability to move. Thus, they have notochord, dorsal nerve cord, pharyngeal slits, and postanal tail at some stage of development. Birds, reptiles, amphibians, and fish also have a common ancestor. 

Beginning of everything

The beginning of life on Earth occurred primarily in water, at first primitive beings, such as bacteria, algae and microorganisms, emerged, about 3.5 billion years ago.

After millions of years, the first marine organisms evolved, rising to other forms of life, such as the first aquatic invertebrates, which was approximately 400 million years ago. In addition, plants had their dispersion in different environments. 

With the origin of life on earth in the seas and the emergence of aquatic organisms, it is easy to understand that all animals share an ancestor in common. So what do birds, reptiles, amphibians, fish and mammals all have in common? We can start to respond to this question here.

From an evolutionary view, we can define characteristics that unite all these animals in a given classification.

Evolutionary classification

All living organisms are structured into a systematic organization of nomenclature and classification. Thanks to this method we can establish an order and group organisms that have the same specific characteristic.

There are thousands of fossil species that show a clear transition between reptile ancestors and modern reptiles. 

Well, the groups of birds, reptiles, amphibians, fish and mammals are all vertebrates. There is a more generalized classification that includes not only these five groups. The vertebrates group uses the presence of the vertebral column during the embryonic stage as the junction point of these groups. This group is called Phylum Chordata.


The chordates are animals that present notochord at least during their embryonic stage. In some cases, it remains permanent throughout the animal’s life. In others, it may disappear during embryological development.

It is very common to believe that chordates are vertebrate animals, with a vertebral column. Chordates are animals that have four outstanding characteristics at some stage of life:

·      Notochord;

·      Dorsal nerve cord;

·      Pharyngeal slits;

·      Post-anal muscle tail.


The notochord is a flexible rod that is present in all chordate embryos and remains in some adults. This cord runs along the longitudinal axis of the animal’s body and works as a support axis.

Dorsal nerve cord

The dorsal nerve cord develops into the animals’ central nervous system.

Pharyngeal slits

It is possible to observe the presence of pharyngeal creases that develop, in most chordates, into pharyngeal slits, which allow water to enter the mouth and leave the body without crossing the digestive system. In fish, these slits form the gills, in other vertebrates, the pharyngeal creases do not form slits. In these animals, the pharyngeal arches that surround the sulci form some structures of the head and neck.

Post-anal muscle tail

This is a tail that extends past the anus and contains elements that provide water propulsion in various aquatic species. In many species, this tail is quite short or vestigial.

Craniata subphylum

Craniates are vertebrate animals. A striking feature of craniates in the presence of an endoskeleton. This structure protects the central nervous system and allows the body to move integrated with the muscular system.


The term vertebrate comes from the Latin “vertebratus” and means “presence of vertebrae”. The number of existing vertebrate animals is approximately 50 thousand species.

Vertebrate animals are living individuals that have the most complex metabolism on the planet. Probably the origin of vertebrate animals was about 450 million years ago.

The main characteristic among vertebrate animals is the presence of the spinal cord and vertebral column, which is formed by several vertebrae. Vertebrates have muscles and endoskeleton, allowing them to perform more complex movements than invertebrate animals. Many vertebrate animals have a developed nervous system, with the central system composed of the brain and spinal cord.

All vertebrates have a head and skull surrounding the brain. In the embryonic stage, they have a neural crest, which creates structures such as the cephalic ones. The breathing of vertebrate animals is done through the gills, lungs or even through the skin.


Fish are vertebrate animals that first appeared on our planet. Fish are aquatic vertebrate animals. Usually, their skin is covered with scales and they have gill breathing. Which means they can breathe in the water.

There are about 28 thousand species of fish catalogued, their size can vary from 5 centimetres to 20 metres. Fish can be found in rivers, lakes, weirs, swamps, and in the seas and oceans. Their diet is based on algae, but some species also feed on other animals, especially molluscs, crustaceans, larvae, other fish, and in the case of large fish species, they can feed on mammals.


Amphibians’ main characteristic is that they live in transition between aquatic and terrestrial environments. Its dependence on water is related to its development and reproduction. 

They are considered ectothermic animals. Therefore, their body temperature is dependent on the external environment. Furthermore, the thin, permeable skin is used for cutaneous breathing, which is added to the pulmonary one. Finally, these animals undergo metamorphosis between the larval and adult stages.


Like amphibians, reptiles are ectodermal organisms, but their difference is that they are fully adapted to terrestrial life. That means they are completely independent of water for completing their lifecycle. 

Its skin is thick, formed by scales, plaques and/or carapaces made principally of keratin – a highly resistant protein. The respiratory system in these animals is exclusively pulmonary and their reproduction occurs with the development of eggs (oviparous).


Birds are easily identified by their adaptations to flight. They have skin covered with feathers, wings and pneumatic bones to help with flying. However all the birds have these adaptations, they can be flying or non-flying. As they are endothermic, they maintain a constant body temperature. All birds are oviparous. 


Mammals are the category that includes the human species. They are recognized by the mammary glands present in females, used to feed the young. Like birds, they are endothermic. Typically, their skin is covered with hair or fur and its limbs can be in a variety of shapes, adapted as paws, arms, wings, or fins. Unlike the other groups mentioned, they have many glands in the body, such as sweating and mammary glands. In reproductive terms. They are viviparous, that is the embryo develops inside the maternal body, in the uterus.

Same ancestor

Several characteristics unite such different groups. However, at this moment, we are willing to understand the differences to equalize the similarities.

Thus, detecting the similarities reinforces the theories that all animals descend from the same ancestor, and that life on Earth has been evolving and adapting for billions of years.


This post responded and explained about “What do birds, reptiles, amphibians, and fish all have in common?” and how a classification system manages to assemble all these groups of different animals in the same clade.

Frequently Asked Questions (FAQs): What do birds, reptiles, amphibians, and fish all have in common?


How did tetrapods evolve?

Tetrapods are from a lineage of vertebrates directly derived from pisciform ancestors (with a “fishy”-shaped body), which originated approximately 400 million years ago. 

How did the chordate animals evolve?

The probable origin of the chordates comes from primitive fish without jaws. Lampreys are believed to be between the first representatives of the phylum of chordates. Lampreys are aquatic animals and do not have a jaw.

How was the evolution of fish?

The first type of fish, the Ostracoderms, rose from primitive oceans of the Cambrian period, about 510 million years ago. They were fish that did not have mandibles and their bodies were covered by bony plates.

What was the first fish to exist?

ostracoderms. The first fish and certainly the first vertebrates were the ostracoderms which appeared in the Cambrian period, approximately 510 million years ago, and became extinct at the end of the Devonian, approximately 348 million years ago.

How did fish acquire jaws?

About 450 million years ago, with origin from some ostracoderms, placoderms emerged, vertebrates that had jaws. This characteristic allowed them to act as efficient predators. Current fish and all other vertebrates are descended from Placoderms.

Why were fish the first to appear on the planet?

Fish are our ancestors, because the first vertebrates to emerge on Earth were fish, more than 500 million years ago, in the Cambrian period – they are, in evolutionary terms, the ancestors of all animals with a backbone, including the human being.

When did water appear on Earth?

There are several theories. One of them indicates that water would have been formed along with the Earth or inside it and was expelled by volcanoes in the form of water steam over billions of years.

What is the oldest animal on the planet?

Sea sponges were known as the oldest animals in the world. Sea sponges are believed to have emerged 1 billion years ago.


Benton, M. J. (2009). Vertebrate palaeontology. John Wiley & Sons. 

Romer, A. S. (1970). Vertebrate body.

Hildebrand, M., Goslow, G. E., & Hildebrand, V. (1995). Analysis of vertebrate structure.

Devoto, S. H., Stoiber, W., Hammond, C. L., Steinbacher, P., Haslett, J. R., Barresi, M. J. F., … & Hughes, S. M. (2006). Generality of vertebrate developmental patterns: evidence for a dermomyotome in fish. Evolution & Development, 8(1), 101-1