Until relatively recently humans were thought to be uniquely self-aware, (scientists now know that most chimpanzes and orangutans can recognize their own reflections.)
答案是把括号句子前加上一个but(即 but scientists now know that most chimpanzes and orangutans can recognize their own reflections)
我的疑问是，难道Until relatively recently humans were thought to be uniquely self-aware 能作为主句吗？
原句不改动是 run-on sentence 错误。
加上but 后也无主、从句之分。加上 but 后则是由连词 but 连接的并列句。
问题中的 until 是副词，不是从属连词，故全句无主、从句之说。
until 的词性有：
A. 副词
Until recently, sponges were believed to be modern descendants of the first animals.
Until recently I thought an armadillo was a pangolin, but now I know they are not quite the same.
B. 介词
Bring the liquid to a boil, then reduce heat until simmering.
Stay put until dusk.
But the others did not borrow in size until that fall.
C. 从属连词
Review of applications will begin immediately and continue until position is filled.
The wallpaper image will remain until the computer is restarted.
Steel cables stretched over the giant pylons provided support until the arch was completed.
Seeds cannot germinate until certain favorable environmental conditions are met.
They are only biding their time until political conditions allow them to once again cut taxes.
There they'll grow and bloom through winter until their crescendo in spring.
不是修饰
population，而是说明形容词
dense(密集的)的程度。因为令人吃惊的不是人口本身，而是密集度。修饰名词用形容词，修饰形容词则需要用副词，因此应该改为
astonishingly，意思是“密集度达到惊人程度的人口”。
