请教一个时态问题

merrylili

(The domesticated camel, which some scholars date) around the twelfth century B. c., was the key to teh development of the spice trade in the ancient world.

D. the domestcation of the camel, thought by some scholars to have occured

答案选D， 可是 around the twelfth century B.C. is past tension, why use have occured?

lyricling

在动词think,know,believe,find,allege之后的动词不定式经常使用完成体，表明动词不定式的动作发生在上述动作之前。

merrylili

thanks!