Social anthropologists attempt to explain the social emergence, evolution and development of the human race. They also try to determine differences between human social organization and other primate beings. One of their attempts is note dissimilarity betyween various human societies. _____, all classifications of human socities and cultures may be arbitrary.

1. As a result
2. As usual
3. Moreover
4. However

(What is the most suitable expression in the blank in the passage?)

The most suitable expression to fill in the blank in the passage is "However."

The sentence structure indicates that the statement after the blank will present a contrast or counterpoint to the previous information. The phrase "all classifications of human societies and cultures may be arbitrary" provides a contrasting view to the earlier mention of social anthropologists attempting to determine differences between human social organization and other primate beings. The word "however" is commonly used to introduce a contrast or contradiction in a sentence, making it the most appropriate choice in this context.