In the following items, each passage consists of six sentences. The first sentence (S1) and the final sentence (S6) are given. The middle four sentences in each have been removed and jumbled up. These are labelled P, Q, R and S. You are required to find out the proper sequence of the four sentences and mark accordingly on the Answer Sheet. 

S1: Before man settled down, his life was not governed by the seasons. 
P: So man began to measure time, and first he looked for convenient units of measurement, one of which was the day.
Q: This was done by making a mark for each day on a tree trunk.  
R: Once he settled down to the agricultural way of life, he had to be able to calculate the change of seasons on which agriculture depended.  
S: In order to calculate the passing of time, it was necessary for man to record the number of days in each season. 
S6: However, we do not know when man first began to divide the day into twenty-four hours, and the hour into sixty minutes.