All the living things on earth can be classified into two groups; Vertebrates and invertebrates. Vertebrates can be divided into five groups; Fish, Amphibians, reptiles, birds, and mammals. A vertebrate is an animal with a back bone, just like we humans are. Vertebrates can be in the oceans, rivers, or land.

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The first and least complex vertebrate is a fish. Fish are very old animals which have lived in the oceans for thousands of years. They are the oldest vertebrates in the world. The first fish appeared in the oceans around 500 million years ago. Currently there are more than 24,000 different species of fish in all the world’s oceans. Fish use gills to breath instead of lungs. The gills obtain oxygen from the water and then it goes to the blood stream to be delivered to all the cells in their body. Some fish have bony skeletons while other fish like sharks have bones made up of cartilage. Most fish reproduce by laying eggs but some fish are live bearers meaning that when the offspring come out of the body it is fully developed and has a higher chance of survival.  All fish have a two chambered heart, one chamber receives deoxygenated blood while the other chamber pumps blood through the capillaries to all of the cells and picks up oxygen from the lungs. Fish can live in a variety of different aquatic environments. Some live in shallow warm waters while other can leave in the arctic deep freezing waters.

Our next and a bit more complex vertebrate is the amphibian.  Amphibians have an advantage other living things don’t have. Amphibians can live on land and in water. Amphibia in Latin means double life. Amphibians reproduce by the process of metamorphosis. The eggs hatch into tadpoles which is the stage in which the amphibians are still developing underwater. When amphibians are done developing they have legs, lungs, and a three chambered heart. Amphibians have moist and thin skin and they have no claws. Some amphibians don’t even have lungs but breath through their skin, when oxygen diffuses into their body and goes around in the blood stream. Amphibians are ectodermic which mean their body temperature changes with its surrounding. Amphibians are thought of have appeared 360 million years ago. For some time when the earth was warm and wet, amphibians were the dominant vertebrates on land.

Reptiles are vertebrates that are also not dependent on water. Reptiles complete their life cycles fully on land. This is what let reptiles evolve entirely into land animals, reptiles do not need to stay close to bodies of water and can live in many different places. Reptiles have scales and dry skin. This helps them hold in body moisture. Most reptiles have a three chambered heart but some unique reptiles have a four chambered heart.  This heart separates oxygenated blood and deoxygenated blood completely. This helps reptiles better adapt to land because more energy is needed to survive on land. Reptiles have legs that are under them which help them move on land easier. They catch prey on land easier then amphibians. Reptiles also have claws which is another advantage over amphibians because claws can be used for attack and protection. Reptiles lay egg on land so the offspring is not in risk of being eaten by a water dweller. The offspring looks like the adult but smaller. All reptiles are internally fertilized and lay their eggs under rocks, and grasses so the offspring is covered and in less risk of being eaten.

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Birds are the only living things with feathers. Scientists explain feathers as light weight scales that allow birds to take off land. Birds molt which means they shed their feathers for new ones to grow so the feathers are always ready for flight. Birds have a sternum which is a strong flight muscle that’s connected to the breastbone. Birds have four chambered hearts because a lot of energy is needed to support a body in the air. Birds are endothermic which means their body maintains a certain temperature itself and unlike other vertebrates they do not depend on the environment. Their feathers reduce heat loss in cold weather, when it is hot the feathers flatten and the bird keeps its wings further from its body than usual. Scientists have done much research about the ancestors birds are related to and they came up with an answer. Scientists say that birds came from an evolutionary line of dinosaurs that never went extinct. Birds do not only fly the penguins of Antarctica and the arctic can only swim. They go into the ocean to hunt for fish and later come back to shore. Penguins can swim but they cannot fly.

Mammals are the most complex vertebrates. A mammal is one of the most amazing vertebrates because it can survive in any environment on earth. Mammals are endothermic which allows them to survive in variety of conditions. Mammals are the only vertebrates that have hair. Scientists think that hair also evolved from scales just like feathers. Hair absorbs heat and acts as waterproofing. Mammals also have glands that secrete certain fluids. Mammals use sweat glands to cool off, the sweat ends up on the skin and when it evaporates it cools down the mammal’s body. Mammals also have a diaphragm which enables the lungs to take in more air than other organisms which provides the mammal with much more energy. Mammals also have different kinds of teeth. Other vertebrates have teeth that are made just for tearing things down but mammals teeth have a certain shape for what it eats. Mammals are also very protective over their young and they also teach them survival skills. Mammals feed their young. The female has mammary glands which have milk that lets the mother feed her offspring.  What makes mammals even more amazing is their intelligence. Mammals can be trained and thought to perform certain tasks, especially humans are intellectual mammals. Humans have so much intellect that just using it we are currently controlling all of earth and we can control animals that are far larger than us. Humans are not the only ones who develop technology. Monkeys have also been using stones and special sticks to find food and make food easier to reach. The first mammals appeared 200 million years ago, scientists believe that mammals evolved from a reptilian ancestor called therapsids. Mammals are also further classified. There are placental animals in which the offspring is born after developing in the mother’s uterus. This makes the offspring safer from the environment and its predators.  Ninety five percent of all mammals are placental. Then there is the marsupial mammal. At first the offspring develops inside the mother’s body and then it begins developing a pouch made up of skin and hair. Most of the marsupials are found in Australia and the islands surrounding it. The last classification of mammals is the monotreme, it reproduces by laying egg. They are only found in Australia, Tasmania, and New Guinea.