Animals | Basic Biology



Animals are the most advanced organisms on the planet. The first animals are thought to have evolved over 550 million years ago and they have since spread to all corners of the globe and diversified into millions of species.

Animals can be distinguished from other living things by the combination of three features: 1) they are unable to make their own food; 2) they have multiple cells; and each cell has a nucleus and other specialized structures called organelles. Scientifically put, animals are heterotrophic, multicellular organisms with eukaryotic cells.

Animal species are often split into vertebrates and invertebrates – two very broad categories of animals. Vertebrate species are animals that do have a backbone such as humans, birds, fish and reptiles and make up the majority of large land animals.

Invertebrate animals lack a backbone and often have a skeleton on the exterior of the body. The majority of animal species are classed as invertebrates and many you will be familiar with such as insects, jellyfish, worms and spiders.


Mammals are a group of warm-blooded vertebrates and include the largest and smartest animals on the planet. They are distinguished from other animals by having hair or fur and mammary glands for milk production.

Reptiles - Basic Biology

Reptiles are a group of animals that include snakes, lizards, crocodiles, turtles and tuatara. They are an ancient group of cold-blooded, egg laying vertebrates with scales or scutes rather than fur or feathers.

Birds - Basic Biology

Birds are a class of animals known as Aves and are easily distinguished by their feathers and wings. They have an incredible ability to fly and often perform amazing migrations over hundreds and even thousands of miles.

Fish - Basic Biology

Fish are a group of aquatic animals with skulls, gills and digit-less limbs. Fish were the first vertebrate animals to evolve and there is now more than 30,000 species living on Earth. The highest diversity of fish is found in tropical coral reefs.

Invertebrates - Basic Biology

An invertebrate is any animal without an internal backbone such as insects, shellfish, worms, and spiders. Invertebrates are incredibly diverse and make up roughly 95% of all animal species.