A fish may not look much like a human being, but it has most of our key body parts, systems and organs: It has a skull, jaw, spine and fins.

 
The evolution of a bony skull and movable jaws was a huge step on the road to becoming both a modern fish and, eventually, a human.

 
The appearance of paired fins, which eventually became our arms and legs, represented a huge advance in swimming.

 
Gills, and the structures that support them, eventually became key parts of our head, including our voice box and ears.

 
The bones in human fins have evolved so they are sturdy enough and shaped to support and enable them to move around on land.

 
Moving around on land rather than cruising through the water was a big challenge, unlike water, land is much harder, there's mud and rocks to get around.

 
Cold-blooded animals like reptiles have to rely on the sun to raise their body temperature enough to give them the energy to hunt for food.

 
Mammals, on the other hand, can produce their own heat, which means they can function in an greater range of environments.

 
The longer a baby stays with its parents, the more time it has to develop, and learn how to use, a larger brain.