Evidence from the British Isles suggests that during 3 intervals in the early Palaeozoic conditions approximating to those of a non-uniformitarian ‘greenhouse (G)’ state Earth were realized: middle–early Upper Cambrian (St. Davids–Merioneth ages, ~30 Ma), mid-Ordovician (Llandeilo–Caradoc ages, ~32 Ma) and early Silurian (Llandovery–early Wenlock ages, ~12 Ma). During these times sea levels were high and ocean waters below the zone of surface overturning, poorly aerated and, at times, euxinic; pelagic biotas (graptoloids) were diverse, and in aerated portions of the highly productive shelf seas, benthic biotas (brachiopods) flourished. Opposing conditions, marking a well oxygenated (‘O’ state) Earth, occurred to varying degrees in the intervening intervals: early Cambrian (Comley age, ~20 Ma), late Cambrian–mid-Ordovician (Tremadoc–late Llandeilo ages, ~46 Ma), late Ordovician (Ashgill age, ~4 Ma) and mid to late Silurian (post early Wenlock age, >15 Ma). Within the constraints of the present data, perturbations in the early Palaeozoic marine realm do not appear to follow the same regular periodicity as in the Mesozoic and Caenozoic. Among a number of likely causes for this, the greater tendency for the early Palaeozoic seas to become anoxic seems the most important.