Prehistoric Life 1

by Carl Strang

This year’s winter series is a review of the prehistoric life and geologic history of northeast Illinois. Each chapter will summarize current understanding, gleaned from the literature, of what was going on with life on Earth in a particular span of time, what we know about the local landscape, and what we can say about local life. The Earth is so old that every imaginable environment was here at some point, from ocean depths to mountaintops, from equatorial tropics to tundra, and from wetlands to desert.

Hadean Eon (4.6-4 billion years ago)

Earth’s history has been divided into four eons, the Hadean (4.6 to 4 billion years ago, though the recent discovery of rocks that may be 4.28 billion years old may push back the Hadean-Archean boundary [Science 321:1828-1831]), the Archean (4-2.5 billion years ago, though again the starting point may be revised), the Proterozoic (2.5 billion to 543 million years ago), and the Phanerozoic (the remaining time to the present).

The Hadean Eon began with the formation of the Earth, its end coincided with the formation of the crust. The eon was named in recent years for the molten planet Earth was then. Consequently there are no Hadean rocks on Earth, but rocks from the Moon are known from that time and help us in ageing these events (the Moon, being smaller, cooled more quickly).

Life on Earth. Conditions on Earth during the Hadean would not have permitted the existence of life. We must wait for the cooling that will come with the Archaean Eon to see the origin of life.

Local landscape. The Hadean eon is so called because the frequent collisions of comets and meteorites with the growing Earth kept it hot and molten. The largest of these involved a Mars-sized object called Theia by some, the core of which was absorbed into the Earth and some of the crust of which formed the bulk of the Moon (recent measurements and models indicate that one-fifth to one-third of the Moon’s material came from the Earth, and the impact happened 4.533 billion years ago [Science 301:84, 304:977]). The collision tilted the Earth on its axis, so that the Moon, tides and seasons all became possible during a few hours of violence. As the crust cooled, plate tectonics began to operate, and small areas of continental crust were rapidly forming and recycling into the mantle by 4.4-4.5 billion years ago (Science 310:1947; Science 315:1704).

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