40 LIFE FROM RNA WORLD
This chronology is consistent with apparent
isotope fossils at 3.85 Gya and implies that organisms were
abundant a maximum of a few hundred million years after the
primal bombardient of Earth slackened.
As we saw in Chapter 3, there are tgree broad styles of life on Earth-organism more similar to each other than they are to other styles of Organism: bacteria, archea, and eucaryotes. Bacteria and archea are mostly single-called and have no nuclei; the DNA genome swims, unrestricted by a menbrane boundary, in the cell’s interior. Eucarya have nuclei and other subcellular membrane-bounded compartments; they are the building materials for almost all complex, multicellular beings. Neverthless, all three styles of creature decorate the same tree (descend from a common ancestor) and all may be traced back similarly close to the beginnings of life.
Stromatolite formers were simple, presuambly bacterial creatures. Eucaryotic (nucleated) cells apparently as complex as our own do not appear obvious until 1.6-2.1 Gya. Thus life was simple and microbial, single-celled, through billions of
years of history, waiting for the invention of cells capable of a more complex style of life. This complexity appeared about