production would not have survived before 3.8 Gyr ago. We wish to point out that a different conclusion is possible, even without challenging the assumptions about impact processes.Microorganisms exist in very large numbers, and often show great resilie nce to environmental stress. Consequently, the likelihood of some individuals surviving global catastrophes through special and fortuitous combinations of circumstances can be underestimated by simple models.In the present case, we can envisage that photosynthetic prokaryotes , returned from deeper water to the new photic zone, could become associated with sea-floor sediments, or with rock surfaces, without reaching the unihabitable uppermost part of the photic zone. It need not be assumed that such organisms would have to occur exclusively in either a free-floating or in a benthic situation. Recolonization of the photic zone and re-establishment of an ecosystem based on photoautotrophy could follow . Photosynthetic metabolism could have been evolving since before 3.8 Gyr ago.

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