Taiyou no Shippo - Wild, Pure, Simple Life [PSOne Books] Japan

In Tail of the Sun, you are a caveman in prehistoric times. Your mission: to build an ivory tower of mammoth tusks tall enough to reach the tail of the sun. You are to develop your caveman tribe by hunting, swimming, running, climbing, and taking on other activities to raise your physical or mental attributes. You must venture off into a vast, untamed world to explore harsh terrains that consist of deserts, caves, forests, and ocean floors.