Historians have long been puzzled by dots, lines and symbols which have been engraved on walls, bones, and the ivory tusks of mammoths. The nomads who made these markings lived by hunting and fishing during the last Ice Age which began about 35,000 B.C. and ended about 10,000 B.C. By correlating markings made in various parts of the world, historians have been able to read this difficult code. They have found that it is connected with the passage of days and the phases of the moon. It is, in fact, a primitive type of calendar. It has long been known that the hunting scenes depicted on walls were not simply a form of artistic expression. They had a definite meaning, for they were as near as early man could get to writing. It is possible that there is a definite relation between these paintings and the markings that sometimes accompany them. It seems that man was making a real effort to understand the seasons 20,000 years earlier than has been supposed.
Why does the author use the simple present or present perfect in "have been engraved", "is connected" and "is a primitive type of calendar" as highlighted above, to describe the findings about the markings, while using the simple past tense in "were not simply", "had a definite meaning" and "were as near", to describe the findings about the hunting scenes?
Can we use them the other way around?
