4.6B Describe how the anatomy of the pentadactyl limb provides scientists…

The idea

The pentadactyl limb is a limb with five digits (pentadactyl means 'five-fingered'). In this specification it is studied as a homologous structure: the same basic arrangement of bones appears in many vertebrates, even where the limb is used very differently. A typical pattern runs from the shoulder region through one upper bone, then two bones, then a group of small bones, then long hand or foot bones, then the digits. A human arm, a bat wing, a whale flipper and a horse leg all show this shared plan but differ in the relative sizes and shapes of the bones. Scientists interpret this shared anatomy as evidence that these animals inherited the plan from a common ancestor; later natural selection modified the limb for different functions such as grasping, flying, swimming or running. Comparing the anatomy therefore supports the idea of evolution from common ancestors.

What earns the marks

• Identifies the pentadactyl limb as a limb with five digits and a shared basic bone arrangement.
• States that the same basic bone plan is found in many different vertebrates, such as a human arm, bat wing, whale flipper and horse leg.
• Explains that the limbs differ in the relative size and shape of the bones because they are adapted for different functions.
• Concludes that the shared anatomy indicates inheritance from a common ancestor, which is evidence for evolution.
• Recognises that the pentadactyl limb is described as a homologous structure showing a common evolutionary origin.

Common mistakes and how to put them right

• Thinking the pentadactyl limb means all these animals have identical limbs.

  Put it right by explaining the basic bone plan is shared but the sizes and shapes differ for different functions.

• Believing the shared limb proves the animals live in the same habitat.

  Put it right by linking the shared anatomy to a common ancestor, not to habitat.

• Assuming the pentadactyl limb is only found in humans.

  Put it right by naming several vertebrates such as bats, whales and horses that also show the pattern.

Exam tips

• Learn the basic sequence of bones in the pentadactyl limb so you can describe the shared plan accurately.
• Use named examples such as a human arm, bat wing, whale flipper and horse leg to show the same plan adapted for different functions.
• Always finish by stating that the shared anatomy is evidence of a common ancestor and therefore of evolution.