请教语法，谢谢

vincentfjUID: 134258

新东方的黄皮语法教程74页26题：Like the giant reptiles, most lineages of organisms have
eventually become extinct; still, some exist that have changed very little in millions of year.
我知道应该把year改为years，但不太懂这个句子的结构，that引导的是什么从句？