Evolutionary tree of life Animation

An evolutionary tree of life that reflects mainstream thinking about the connections among major lineages (bya = billions of years ago, mya = millions of years ago). By about 3.8 billion years ago, chemical and molecular evolution had produced the first living cells. The first major divergence gave rise to eubacteria, and to the common ancestor of archaebacteria and eukaryotes. In a second major divergence, the ancestors of archaebacteria and eukaryotic cells parted ways and started down separate evolutionary roads. Between 3.5 billion and 3.2 billion years ago, the cyclic pathway of photosynthesis arose in some anaerobic eubacteria. By 2.5 billion years ago, the noncyclic pathway had emerged and oxygen began to accumulate in the atmosphere. This opened the way for the evolution of aerobic respiration in many bacterial groups. The first eukaryotes may have evolved before 1.2 billion years ago. Mitochondria arose when an aerobic species became an endosymbiont of the anaerobic forerunner of eukyarotes. Chloroplasts arose when an oxygen-producing eubacterium and early eukaryote became symbionts. Continued evolution and divergence gave rise to all the living taxa.

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