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P.51 left column top paragraph: "Of the four billion species estimated to have evolved on the Earth over the last 3.5 billion years, some 99% are gone (primary source). That shows how very common extinction is, but normally it is balanced by speciation. The balance wavers such that at several times in life’s history extinction rates appear somewhat elevated, but only five times qualify for ‘mass extinction’ status: near the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods (refs 2, 3). These are the ‘Big Five’ mass extinctions (two are technically ‘mass depletions’)(ref 4). Different causes are thought to have precipitated the extinctions (Table 1 BNID 112753), and the extent of each extinction above the background level varies depending on analytical technique (refs 4, 5), but they all stand out in having extinction rates spiking higher than in any other geological interval of the last ~540 million years (ref 3) and exhibiting a loss of over 75% of estimated species (ref 2)." |