There is greater biodiversity (in the senseof genetic distance among higher taxa) ofextant marine than of terrestrialO₂-evolvers. In addition tocontributing the genes from one group ofalgae (Class Charophyceae, DivisionChlorophyta) to produce by evolution thedominant terrestrial plants (Embryophyta),the early marine O₂-evolvers greatlymodified the atmosphere and hence the landsurface when the early terrestrialO₂-evolvers grew. The earliestterrestrial phototrophs (from geochemicalevidence) occurred 1.2 Ga ago, over 0.7 Gabefore the Embryophyta evolved, but wellafter the earliest marine (cyanobacterial)O₂ evolvers (3.45 Ga) and marineeukaryotic O₂ evolvers (2.1 Ga). Evenby the time of evolution of the earliestterrestrial O₂-evolvers the marineO₂-evolvers had modified the atmosphereand land environment in at least thefollowing five ways. Once photosyntheticO₂ paralleling organic C burial hadsatisfied marine (Fe²⁺, S^2-reductants, atmospheric O₂ built (1) upto a considerable fraction of the extantvalue (although some was consumed inoxidising terrestrial exposed Fe²⁺ and(2) provided stratospheric O₃ and thusa UV-screen. (3) CO₂ drawdown to20-30times the extant level is attributableto net production, and burial, of organic Cin the oceans (plus other geologicalprocesses). Furthermore, (4) theirproduction of volatile organic S compoundscould have helped to supply S to inland sitesbut also (5) delivered Cl and Br to thestratosphere thus lowering the O₃ leveland the extent of UV screening.