Evolutionary Development of the Skeleton

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The Evolutionary Development of the Skeleton

Body Support Provided by Water

The earliest forms of life evolved in the oceans. The fact that this is an aquatic environment is key. Water is about \(\text{1 000}\) times denser than air. The high density of water allows organisms to float, due to a physical, upward force inherent in liquids known as buoyancy. Buoyancy allowed organisms to grow and reach large sizes because the buoyancy force supported the body weight of these animals. However, the density of water also provides resistance to movement, and animals had to adapt to ensure that they were able to move efficiently through water.

An early adaptation by organisms was the ability to change the hydrostatic pressure within different chambers of their bodies to enable quick movement. This resulted in the development of hydrostatic skeletons. Animals with this type of skeleton include jellyfish, octopus and sea anemones. The changing shape of the animal reduces both friction and drag.

Evolutionary Development of the Skeleton

The animal above is a jellyfish. It uses its muscles to contract against the hydrostatic skeleton to bring about movement.

Over time, in order to refine movement and improve protection from predators, some organisms developed a hard chitinous exoskeleton. Exoskeletons first developed in the aquatic environment in ancient arthropods. Animals with this type of skeleton include crustaceans like crabs and lobsters.

Evolutionary Development of the Skeleton

Crustaceans, such as this crab, developed a protective exoskeleton.

Eventually, there were some animals that developed a skeletal structure internal to the body, which would become the vertebrate group of animals. These animals have an endoskeleton. Initially, all endoskeletons were made of cartilage, which is a dense rubbery type of tissue. Later, endoskeletons of bone evolved.

Evolutionary Development of the Skeleton

The first vertebrates evolved in the oceans. This fish has an internal endoskeleton that makes it streamlined and allows it to move rapidly through water.

The adaptation of the skeleton to a terrestrial environment

The two major requirements for survival on land are the development of a suitable support system and an air breathing mechanism. One of the biggest problems encountered by animals moving from water to land was the loss of the effect of buoyancy. In order to counter this, animals needed to develop strong limbs and had to adapt the skeleton to support their body weight on land. Moving effectively on land is essential, particularly if one needs to avoid predators, catch prey, or adapt to a particular habitat. Different skeleton types have solved these problems in different ways.

Animals with exoskeletons like arthropods (a class of animals including insects, crustaceans and arachnids) transitioned from sea to land long before the vertebrates (organisms with endoskeletons). A major factor in their success was the exoskeleton which provides attachment for muscles controlling locomotion (movement of appendages). Exoskeletons also provided some protection from dessication (water loss).

Evolutionary Development of the Skeleton

This beetle is an example of a insect. Insects have a protective exoskeleton that made it possible for them to colonise land millions of years ago.

Amphibians with endoskeletons , like frogs and newts, live both on the land and in the water. Their skeletons have adapted to give advantages in both conditions. They have calcified bones to support their body weight under the force of gravity. Their skull is light and flattened, for both motility on land and a streamlined shape for moving easily in water. Their pectoral girdle is adapted to give support for the forelimbs, which absorb the body weight when landing after a jump.

Evolutionary Development of the Skeleton

Amphibians were the first vertebrates to colonise land. They begin their life-cycle in water, and emerge onto land as adults.

Depending on their means of locomotion, terrestrial animals needed to adapt their shapes and skeletons to overcome the effects of gravity. Limbless animals, such as snakes, had to overcome drag and friction. Flying animals such as birds and bats need light skeletons and very strong sternums for wing muscle attachment. Animals that support their bodies clear of the ground needed an energy efficient way of maintaining balance. For this reason, the leg bones of most animals are held directly underneath the body. In this position they act as props or struts and it is the bones rather than the muscles that take most of the strain of the body’s weight.

Evolutionary Development of the Skeleton

Land vertebrates often have legs placed directly beneath the body. The legs acts as struts, and are the most energy-efficient way to keep the body suspended above the ground.

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