Initially you encounter rolling rocks. These move across the screen from left to right and the only way to avoid them is to jump over the top of them. They usually come in groups of four with very little random variation in timing or separation and it soon becomes very easy to judge the jumping.
Not only do you have to hop, skip and jump over the rocks, but in order to progress to the next level you have to move the left to break new ground. Trying to dodge the issue by moving to the right is of no use; you're limited to just a few steps.
Following the rocks comes a gang of dinosaurs. You have to be quick and accurate with your axe to kill them. Of course, if you don't you'll end up as their luncheon. They looked deceptively like herbivores to me.
If you manage to negotiate the dinosaurs, your next hazard is the dodos. Dodos, according to the instructions, never accomplished the art of flying. They only manage short hops and bounce around the screen in fairly large numbers.
It can get extremely difficult to avoid them. I never managed to kill one and I'm not even sure you're supposed to.
The fourth hazard is some different types of prehistoric birds (pterodactyls?) which fly above your dropping rocks on your head.
I'm afraid that my history books must be out of date as I could find no reference to rock-dropping birds.