FACT
As you will learn in
the chapterHistory
of Life on Earth,
many of these
structural
adaptations allowed
animals to move
from water onto
land.- Endoskeleton: a cartilaginous or mineralized support structure inside the body, for
example, in humans and other vertebrates.
In this chapter we will be looking at support systems in animals and investigating the human
skeletal system in some depth.
The evolutionary development of the skeleton DUMMY
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 1000 times denser than air. The high density of water allows organisms
to float – a physical, upward force inherent in liquids known as buoyancy. Buoyancy allowed
organisms to grow and reach large sizes due to the force that supported 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 develop-
ment ofhydrostatic skeletons. Animals with this type of skeleton include jellyfish, octopus
and sea anemones. The changing shape of the animal reduces both friction and drag.
Over time, in order to refine movement and improve protection from predators, some or-
ganisms developed a hard chitinousexoskeleton. Exoskeletons first developed in the aquatic
environment in ancient arthropods. Animals with this type of skeleton include crustaceans
like crabs and lobsters.
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 anendoskeleton.
Initially, all endoskeletons were made of cartilage, which is a dense rubbery type of tissue.
Later, endoskeletons of bone evolved.
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 ani-
mals 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 en-
doskeletons). A major factor in their success was theexoskeletonwhich provides attachment
for muscles controllinglocomotion(movement of appendages). Exoskeletons also provided
some protection from dessication (water loss).
Amphibians withendoskeletons, like frogs and newts, live both on the land and in the wa-
ter. 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 flat-
tened, 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
Chapter 7. Support systems in animals 191