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The Evolution of Animals

Chelsie Wheeler, Danielle Weber
  • Author
    Chelsie Wheeler

    Chelsie has been a private tutor and a peer mentor throughout her ten-year career as an environmental and fisheries biologist. She also has experience educating the public through various programs via outreach with presentations, festivals, and set courses. She has a bachelor of science degree in environmental management from Columbia Southern University and a master of science degree in coastal and marine environments from the University of Ireland, Galway.

  • Instructor
    Danielle Weber

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

Explore animal evolution. Learn about the characteristics of animals, stages of how animals evolve and have evolved, and view examples of evolution in animals. Updated: 11/19/2021

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Frequently Asked Questions

What are examples of animal evolution?

Some examples of evolution in animals can be seen in humans, fish, and birds. The evolution of humans began in Africa around 8 million years ago with one common apelike ancestor.

What are examples of evolutionary adaptations in animals?

Animals have many different adaptations that were caused through speciation and as a response to environmental changes. Adaptations include the development of lungs, appendages, loss of hair, formation of the spinal column, and many more.

What caused animals to evolve?

Animals evolved through a process known as evolution which is the change in a species' characteristics over several generations. Most likely changing environmental conditions caused animals to evolve.

The beginning of life on Earth began when it was formed, around 4.5 billion years ago. Fossilized records of organisms have helped scientists to link and date when life evolved. At that time life consisted only of single-celled organisms. A single-celled organism or unicellular organism is an organism that is made up of only one cell, this cell provides all necessary processes needed for life.

Characteristics of Animals

Animals are organisms from the Kingdom Animalia which are multicellular eukaryotes. The eight characteristics used to classify animals are:

  1. Eukaryotic cells: nucleus-centered cells
  2. Multicellular: having more than one cell
  3. Heterotrophic: they cannot produce their own food
  4. Ability to move: they can move freely on land or through water
  5. Symmetry- being the same on both sides divided by an axis, which can be bilateral or radial
  6. Body cavity: contain and protect vital organs, usually surrounded by fluids
  7. Tissue: grouping of cells to provide one function
  8. Vertebral Column: spine of an animal which begins at the base of the neck and extends to the tailbone

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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.
Symmetry

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.

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How did animals evolve? Animals evolved through a process known as evolution which is the change in a species' characteristics over several generations. Evolution can be caused by:

• Mutation: a change in the genetic material in an organism

• Migration: when organisms move or relocate to a different geographic area

• Natural selection: organisms who are best suited to their environment survive and pass their genetic material on to their offspring

Genetic drift: traits of species are more or less prominent, known as allele frequency

• Non-random mating: species choose mates based on specific traits

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The evolution of animals, and all living species, is a complex process that took billions of years. Simply speaking, modern animals have evolved from various lineages of simplistic aquatic species starting around 2.5 billion years ago.

The oldest known ancestors of animals are the metazoans, about which not much is known except that their shape was similar to worms. Second, came the Ediacaran animals which were a similar mix of jellyfish with tougher outer walls. Finally, during the Cambrian period, most of the ancestors of modern animals like insects, jellyfish, aquatic bivalves, and many more simplistic animal forms emerged. Once the evolution of fish began and began to become more complex, amphibians allowed a transition between water and land. Mammal-like reptiles formed around the same time that dinosaurs came into existence. The removal of dinosaurs allowed many different animal groups to flourish such as mammals and vertebrates such as fish.

Vertebrates are animals that have a spinal column and the presence of a notochord. Invertebrates are animals that do not have a backbone, they make up most of the species within the animal kingdom.

Evolution of Invertebrates

Invertebrates evolved during the first era in Earth's long history and are among some of the first life forms. They predate vertebrates and contain most of the species within the animal kingdom. Invertebrates can be simplistic with no real appendages or organs while others are highly specialized with a head, legs, and wings.

During the evolution of invertebrates three main traits were formed, these are:

  1. Specialization
  2. Symmetry
  3. Cephalization

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The gradual progression of life on Earth can be divided into four eras these are:

  1. Archeozoic
  2. Paleozoic
  3. Mesazoic
  4. Cenozoic

The Archeozoic era began when Earth was formed and lasted until about 550 million years ago. This is the longest and most significant era because the first life forms arose during this time, these life forms are the basis for life on Earth from which all things have evolved. Some examples of organisms that evolved from unicellular organisms during this time include bacteria, invertebrates, amoeba, and sponges.

The Paleozoic era began at the end of the Archeozoic era around 550 million years ago and lasted until around 248 million years ago. This period is broken into six separate periods which are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Some examples of organisms that evolved during this time include gastropods, fish, sharks, plants, insects, reptiles, amphibians, and some mammals.

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Some examples of evolution in animals can be seen in humans, fish, and birds. Animals have many different adaptations that were caused through speciation and as a response to environmental changes. Adaptations include the development of lungs, appendages, loss of hair, formation of the spinal column, and many more.

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The beginning of life on Earth began when it was formed, around 4.5 billion years ago. A single-celled organism or unicellular organism is an organism that is made up of only one cell, this cell provides all necessary processes needed for life. Multicellular organisms contain many cells, usually, this means that different cells become specialized to perform specific functions like in animals. The domain Eukarya contains organisms with eukaryotic cells, these eukaryotes are defined by their many cells that contain a nucleus and membrane-bound organelles. Animals evolved through a process known as evolution which is the change in a species' characteristics over several generations. Evolution can be caused by mutation, migration, natural selection, genetic drift, and non-random mating.

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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
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.

Video Transcript

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.
Symmetry

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
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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