<P> The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth , about 4 billion years ago . For about 3 billion years , most organisms were microscopic , and bacteria and archaea were the dominant forms of life . Although bacterial fossils exist , such as stromatolites , their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution , or to date the time of origin of a particular bacterial species . However , gene sequences can be used to reconstruct the bacterial phylogeny , and these studies indicate that bacteria diverged first from the archaeal / eukaryotic lineage . The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion -- 3.2 billion years ago . </P>
<P> The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth , about 4 billion years ago . For about 3 billion years , most organisms were microscopic , and bacteria and archaea were the dominant forms of life . Although bacterial fossils exist , such as stromatolites , their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution , or to date the time of origin of a particular bacterial species . However , gene sequences can be used to reconstruct the bacterial phylogeny , and these studies indicate that bacteria diverged first from the archaeal / eukaryotic lineage . The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion -- 3.2 billion years ago . </P>
when did the first bacteria appear on earth
when did the first bacteria appear on earth
about 4 billion years ago
about 4 billion years ago