21-Q38:

All Y chromosomes in existence today are descended from a single ancestor’s who is thought to have lived about 140,000 years ago.

1. a single ancestor’s who is thought to have lived
2. a single ancestor’s who was thought to live
3. a single ancestor whom it is thought had lived
4. the Y chromosome of a single ancestor who is thought to have lived
5. the Y chromosome of a single ancestor whom it was thought had lived

   Answer: D 为何是to have lived ,后面的ago不是说明应该用过去时态么

我也问过这个问题，可是没人回答我，可能NN都太忙了。但是我发现有很多be thought to have done

的例句,就当作固定搭配了吧

用排除法：

A/B， a single ancestor's + who， 所有格后面接定语从句引导词who错，应该是ancestor+who;

C/E， 用了whom，可是whom只有在介词后才用，所以错，

于是只有D，不过当时做题时没有想你提的问题.

about 140000 ago在修live, 不動式中沒有過去時態, 頂多用have verb-ed來表示這個動作比main verb更早發生...