The Tree Of Life

Looking at the cells of any organism under a microscope, biologists can easily identify one of two fundamental types of cells. On this basis, biologists may divide all life into two large groups. One of these groups consists of eukaryotes, or organisms whose cells possess a membrane-enclosed nucleus and many other cell organelles. Eukaryotes include organisms such as pangolins, people, and amoebas. The second group consists of prokaryotes. Prokaryotic cells are smaller than eukaryotic cells and have neither a nucleus nor membrane-enclosed organelles. Prokaryotes include bacteria.

Revising the Tree

Although the differences between eukaryotes and prokaryotes are obvious, systematists cannot assume that all organisms in one of these groups are equally related. A systematist can draw inferences about evolutionary relationships only by identifying homologous features. Because all members of one of these groups have few other physical traits in common, it is difficult to make comparisons between them. However, eukaryotes and prokaryotes do contain homologous macromolecules, such as proteins and RNAs used for storing and translating genetic information. When methods became available to compare such macromolecules, biologists began to use macromolecules in phylogenetic analyses.

Such analyses led molecular microbiologist Carl Woese of the University of Illinois to propose a major revision of the system that had classified Earth's diversity into six kingdoms. In 1977, Woese began comparing ribosomal RNA (rRNA) sequences from many different organisms and then grouping these organisms according to their similarities. All of today's living organisms, even prokaryotes, have ribosomes. Furthermore, the nucleotide sequences of the genes that encode rRNA seem to have changed little throughout most of evolutionary history. Thus, rRNA genes are uniquely useful for studying the most basic evolutionary relationships between Earth's diverse organisms.

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