Carl Woese compared the genetic sequence of many different organisms, especially, microbes, to construct phlogenetic tree of life

Based on his en.wikipedia.org/wiki/Carl_Woese laborious phylogenetic analyses (which were more difficult in 1970s than they are today), Woese realized that animals, plants, fungi, and eukaryotes are remarkably similar in their biochemical characteristics and so constitute just one domain of life, the Eukarya. Prokaryotes, on the other hand, displayed astonishing chemical diversity and fell into two distinct kingdoms, which he called Bacteria and the Archaea, that are prokaryotes resemble other bacteria in many ways, constituted an unrecognized group of microbes. So contentious was this view of life that it tool almost two decades to gain general acceptance.

Woese's initial intention was to unravel aspects of evolution -- to establish a top-down family tree of life and infer the complex history of branching from parent to daughter species. This evolutionary pursuit led quickly to insights regarding the nature of the last common ancestor. He proposed, for example, that Archaea and Bacteria arose long before Eukarya. ~ Page 139

An important conclusion from these phylogenetic studies is that Earth's earliest cells were not so different from those of today. They probably relied on similar, though simpler, biochemical pathways. These similarities provide a clear focus for experimental studies of emergent biochemistry. ~ Page 141