prep1-256 为什么要用现在完成时？

256.      (28362-!-item-!-188;#058&003003)       (T-4-Q1 TS-7-41)

Scientists have dated sharp-edged flakes of stone found in the fine-grained sediments of a dry riverbed in the Afar region of Ethiopia to between 2.52 and 2.60 million years ago, pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.

A. when it is known that humans made

B. at which it is known that humans C. at which humans are known to have made

D. that humans are known to be making

E. of humans who were known to make

正解为C，我选的B。

我觉得主句的谓语为 have dated 现在完成时，而 humans had made stone tools 肯定是发生在 have dated 之前，当让要用过去完成时啊。如果用了现在完成时则体现不出date 和 make的时间顺序啦啊。

“原句是想说human are known to have made stone tools的时间被pushing back；过去已经发生的事情，现在被发现也是现在完成时使用的情况之一。”

http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=389550&replyID=&skin=0

不明白为什么“过去已经发生的事情，现在被发现也是现在完成时使用的情况之一”。

望NN能够明晰一下~