Page 80 - Earth's Climate Past and Future

56 PART II • Tectonic-Scale Climate Change

FIGURE 3-11 Life-forms and weathering Over Earth’s 4.55-
Byr history, plants evolved toward more complicated forms
capable of playing a greater role in chemical weathering.
Primitive organisms similar to (A) the modern bacteria Oscillatoria
existed by 3.5 Byr ago. The first simple land plants with roots
and stems similar to those of (B) the modern plant Psilotum
appeared by 430 Myr ago. Increasingly complex treelike plants
similar to (C) modern tropical cycads appeared by 400 Myr ago
and led to today’s diversity of trees and shrubs. (A: Sinclair
Stammers/Science Photo Library/Photo Researchers. B: William
Ormoerod/Visuals Unlimited; C: Gerald Cubit.)

A

B C

algae in the ocean and microbes on land early in Earth’s roughly with the disappearance of previously widespread
history. They point to recent discoveries that modern minerals such as FeS (pyrite, or “fool’s gold”), which form
bacteria with similarities to early primitive life-forms only under reducing conditions (no oxygen). The only
are now thought to play a greater role in the weathering conceivable source of the oxygen that caused the wide-
process than has generally been recognized, and they spread change to oxidized forms of iron is photosynthesis
suggest that these organisms must also have been by marine organisms, implying an active global-scale role
more important than generally thought early in Earth’s for these organisms far back in Earth’s history.
history, when they were the only life-forms present Gaia supporters also point out that the general path
on land. of biological evolution matches Earth’s need for pro-
One indication that early life-forms were important at gressively greater chemical weathering through time.
a global scale is the first development of an oxygen-rich The more primitive organisms played a much smaller
atmosphere near 2.3 Byr ago, even before the first multi- role in accelerating the process of chemical weathering
celled algae (see Figure 3-10). Evidence for this impor- during a time when it was to Earth’s advantage to retain
tant event includes the first appearance of rocks that show CO in its atmosphere to counter the weakness of the
2
red staining (rusting) of iron (Fe) minerals. The appear- faint young Sun. Then, as the Sun strengthened and
ance of oxidized iron minerals at this time coincides provided more heat to Earth, more advanced organisms