Very little is known about prehistoric hominid cave dwellers. However, a recent study of skeletons of these hominids has revealed an important clue about their daily activities: skeletal fractures present are most like the type and distribution of fractures sustained by rodeo riders. Ttherefore, it is likely that these cave dwellers engaged in activities similar to rodeo riders -- chasing and tackling animals. Which one of the following principles, if valid, most helps to justify the argumentation above? ['The most important aspect of prehistoric life to be studied is how food was obtained.', 'The primary source of clues about the lives of prehistoric hominids is their skeletal remains.', 'If direct evidence as to the cause of a phenomenon is available, then indirect evidence should not be sought.', 'If there is a similarity between two effects, then there is probably a similarity between their causes.']