Final answer:
Sharks have had their liver to assist with buoyancy for a long time. The liver contains squalene oil, which helps the shark to control its buoyancy and stay suspended in the water.
Step-by-step explanation:
Sharks have had their liver to assist with buoyancy for a very long time. The liver of a shark is large and contains a special oil called squalene, which is lighter than water. This oil helps the shark to control its buoyancy and stay suspended in the water without constantly swimming. The liver and squalene oil provide the shark with a way to float in the water without sinking.