大学英语第六册三单元一个段落的理解。

大家好，我在看英语的时候，碰到了一个段落的几个句子，怎么也看不出其中的文法结构，希望能得到大家的帮助。谢谢。如下：
大学英语第六册（上海外语教育出版社，UNIT 3. THE QUEST FOR EXTRATERRESTRIAL INTELLIGENCE ）
这是四自然段，第三句话，也是我不明白的地方。即：They require assigning numerical values to quantities such as the numbers and ages of stars; the abundance of planetary systems and the likelihood of the origin of life, which we know less well; and the probability of the evolution of intelligent life and the lifetime of technical civilizations, about which we know very little indeed. 但是，为了大家理解的方便，我把整个第四自然段都粘贴了出来。同时，为了大家的理解，我给出了整个第四自然段的翻译。
我不理解的是，这个由两个分号连接的三个句子中的第二和第三个句子是什么结构。第一句显然有一套主谓结构，可是，第二和第三个句子，即：the abundance of planetary systems and the likelihood of the origin of life, which we know less well; and the probability of the evolution of intelligent life and the lifetime of technical civilizations, about which we know very little indeed.我就不明白了。他们没有主谓结构，没有谓语，显然就是一堆名字。我的猜测是省略吗，可又看不出来，从什么地方省略了。是悬垂结构吗，可是，它们又是和第一个句子并列的，它按道理上来讲，应该有一套主谓结构。我现在怀疑，这个地方，是不是英语里一种特殊的文法，希望能得到指点。不胜感激。
附录：
Some scientists working on the question of extraterrestrial intelligence, myself among them, have attempted to estimate the number of advanced technical civilizations -- defined operationally as societies capable of radio astronomy -- in the Milky Way Galaxy. Such estimates are little better than guesses. They require assigning numerical values to quantities such as the numbers and ages of stars; the abundance of planetary systems and the likelihood of the origin of life, which we know less well; and the probability of the evolution of intelligent life and the lifetime of technical civilizations, about which we know very little indeed.
翻译：
许多从事外行星人研究的科学家，包括本人在内，均在努力估算银河系中有多少个先进的技术文明社会----为了方便起见，我们给出这种文明社会的定义是：掌握了射电天文学的社会。这类估算无异于猜测。它要求将许多情况用具体的数字表达出来，诸如恒星的数目和年龄；行星系究竟多达多少和生命起源的可能性---这些，我们知识甚少，以及有智慧的生命进化的可能性和技术文明社会有多长的生命期，---对此，我们更是近乎一无所知。

看不明白，不过LZ潜心学英语的精神值得肯定，我个人的看法是英语嘛不要太钻牛角尖了，语法再厉害还是要会写，会说，个人看法……

我认为，两个分号隔开的内容the numbers and ages of stars; the abundance of planetary systems and the likelihood of the origin of life, which we know less well; and the probability of the evolution of intelligent life and the lifetime of technical civilizations, about which we know very little indeed.
都是such as后的内容，the abundance of planetary systems and the likelihood of the origin of life,这个名词词组又用了非限制定语从句which we know less well来修饰， and the probability of the evolution of intelligent life and the lifetime of technical civilizations, about which we know very little indeed.这个名词词组组合也是使用了非限制定语从句的结构，这样原句还是主谓结构，不知道我分析的对吗

你真得看看语法了  兄弟