To complete this kind of assignment, you may be helped by thinking about different aspects of the caveman's physical character. In describing him, you can consider not just how he looks but how he moves, sounds, smells, etc. 

Here are some questions that may help you determine what specific language or details to include in your description:

• How does the caveman walk? Does he crouch, shuffle his feet, move almost silently? Does he leave footprints on the ground and if so what do they look like - deep imprints, light scrapes across the ground? 
• How tall, short, skinny, muscular, top-heavy or bottom-heavy is the caveman? 
• Does he carry anything? Is there a stone tool in his hand? An animal or plant that he has brought home? 
• Is he dry and dusty? Is he wet and muddy? Is there mud crusted on his ankles? 
• Is he young or old? What does his skin look like? 

Once you get a sense of what your answers are to this question and get a specific caveman in mind, you can select language that fits him in particular (as opposed to a generalized/abstract caveman). 

Some ideas that come to mind for me when thinking about the appearance of a caveman include hard-soled feet, calloused hands, outward pointing ears, a musty smell, a heavy and stooped manner of walking and a tendency to constantly look around. These are not necessarily accurate details regarding cro-magnon man, but they are what come to mind for me.