In the late 20th century, the pioneering work of _____________and others compared sequences of small-subunit ribosomal (SSU rRNA), which resulted in a more fundamental way to group__________ on Earth. Based on differences in the structure of__________and in rRNA, Woese and his colleagues proposed that all life on Earth evolved along three lineages, called__________Two of the three domains____________ are prokaryotic. Prokaryotes were the first inhabitants on Earth, perhaps appearing approximately____________ billion years ago.