The Evolution of Animals: Importance & Evolutionary History

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  • 0:07 Characteristics of Animals
  • 3:01 Evolution of Invertebrates
  • 4:09 Evolution of Vertebrates
  • 6:01 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

The animal kingdom ranges from simple organisms like sponges to complex organisms like humans. We will look at some defining characteristics of animals as well as examples of both invertebrates and vertebrates.

Characteristics of Animals

Out of all six kingdoms, the animal kingdom is most likely the one that you already know the most about. After all, we are a part of this vast group. While the characteristics of animals vary greatly, there are a few things that they all share. Animals are eukaryotic, multicellular organisms that do not have cell walls, get their nutrients by ingestion, and are capable of movement.

With so many shared qualifications, it may be hard to imagine all the possible differences between the more than 1.3 million living species of animals. Because they are made of eukaryotic cells - those with a nucleus and membrane-bound organelles - they are part of the Eukarya domain. 'Multicellular' means that they are made of many cells - not just one cell like their unicellular protozoan ancestors.

Unlike plant cells, animal cells do not have cell walls.
Animals and plant cells

Unlike the other kingdoms, animal cells do not have cell walls. While cell walls help protect cells, they also limit flexibility - something that is essential to the animal cells since these organisms are capable of movement. This movement is also permitted by the presence of both muscles and nerves.

As for reproduction, most animals reproduce sexually, though some species are capable of asexual reproduction. Sexual reproduction involves the formation of gametes - egg and sperm - as well as the union of these two cells to create genetically unique offspring. Most animals have sperm that are capable of movement due to the flagella, while the larger egg is not capable of movement.

Scientists are able to study embryos and have found that different animal species have very similar early stages of development - even though the mature organisms may be nothing alike. We can see below that the early embryos of several species look relatively the same but the mature organisms are completely different.

In the early stages of development, animals species look the same.
comparison of embryos

There are four main characteristics that are used to classify animals: symmetry, body cavity, tissue, and vertebral column. Let's quickly take a look at some of these before we look at some examples of animals. The first characteristic is symmetry. Animals generally fall into two categories - they either have radial or bilateral symmetry. Organisms like sea stars have radial symmetry, while organisms such as humans have bilateral symmetry - as we can see here:

The two categories of symmetry in animals are radial and bilateral.

The next thing is body tissue. The more complex an organism is, the more distinctive layers of tissue it has. There are three main layers:

  • Ectoderm - the outermost layer
  • Endoderm - the innermost layer
  • Mesoderm - the middle layer

As for vertebral column, there are two key groups: invertebrates and vertebrates. Invertebrates are animals without a backbone, while vertebrates are animals with a backbone. As we look at the evolution of animals, we will first look at the more simple invertebrates and then consider the more complex vertebrates.

Evolution of Invertebrates

Let's look at some key evolutionary characteristics of invertebrates. Most of the pattern of evolution is from simple to more complex. Examples of simple invertebrates include things like mollusks and nematodes. Mollusks have a soft body and protective shell, such as snails, sea slugs, oysters, and squids. Nematodes are simple worms - not earthworms. Nematodes are organisms like hookworms and roundworms that often cause illnesses in humans. Nematodes are actually the most widespread group of animals, meaning they live in many different places.

Arthropods are more complex invertebrates that have a segmented body, hard exoskeleton, and jointed appendages. This group is often called the 'insects' but contains more than just bugs. There are more than 1 million different identified species of arthropods ranging from horseshoe crabs to spiders to centipedes to butterflies.

Examples of arthropods

Another important group of invertebrates is the echinoderms. The name means 'spiny skin,' which is easy to remember due to some classic examples including starfish and sea urchins.

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