Reconstruction of the early terrestrial arthropod Archaeognatha, a flightless true insect.

we see a timetable: scorpions out first at 430 million years ago but of a kind that may have been still tied to water for reproduction and perhaps even respiration, followed by millipedes at 420 million years ago, and insects at 410. But common insects did not appear until 330 million years ago. How does this history relate to the atmospheric oxygen curve?

The Berner curve for this time interval (shown at the start of this chapter) indicates that the end of the Silurian was a time when oxygen had already reached very high levels—the highest that until then had ever been evolved on Earth—with a high-oxygen peak at about 410 million years ago, followed by a rapid fall, with a rise again from very low levels of perhaps 12 percent at the end of the Devonian (359 million years ago) to the highest levels in Earth’s history by somewhere in the Permian (299 to 251 million years ago). The Rhynie Chert, which yielded the first abundant insect/arachnid fauna, is right at the oxygen maximum in the Devonian. Insects are then rare in the record until the near 20 percent oxygen in the Mississippian-Pennsylvanian, the time interval from 330 million to 310 million years ago—the time of the diversification of winged insects. The correspondence to the Berner curve is remarkable.

Let’s formalize this relationship between oxygen levels and the first arthropod land life:

Hypothesis 5.1: The conquest of land by vertebrate groups was enabled by a rise in atmospheric oxygen levels during the Silurian time interval. Had atmospheric oxygen levels not risen, animals might never have colonized land.

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