Page 79 - Earth's Climate Past and Future

CHAPTER 3 • CO and Long-Term Climate 55
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0 much more controversial: it goes far beyond Darwin’s
Increase in concept that evolution occurs to enhance the reproduc-
flowering plants
100 tive survival of each species.
Earth’s long history reveals a sequence of life-forms
different from those that exist now (see Figure 3-10).
Myr ago No record of life exists before 3.5 Byr ago, although it is
200
possible that primitive life-forms did exist and have sim-
300
ply escaped detection because the rock record is so
scarce and poorly preserved. By 3.5 Byr ago, primitive
400 single-celled marine algae capable of photosynthesis
First land plants
had developed (Figure 3-11A). Over the next 3 billion
500 First marine years, slightly more complex organisms evolved: by 2.9
organisms Byr ago, moundlike clumps of marine algae called stro-
with shells matolites that lived attached to the seafloor; by 2.5 Byr
ago, organisms that contained a cell nucleus; and by 2.1
More O in
2
atmosphere Byr ago, a variety of multicelled algae.
1
(increased Most of the more complex forms of life did not arrive
photosynthesis) until late in Earth’s history. Near 540 Myr ago, hard
shells of many kinds of organisms abruptly appear in the
fossil record. Before that time, the only fossilized records
of life consisted of ghost impressions left imprinted on
First known
2 the surfaces of soft sediment layers. The first primitive
multicelled algae
O in atmosphere land plants did not evolve until near 430 Myr ago (Figure
2
Byr ago (photosynthesis) 3–11B). These plants acquired the ability to survive
because of stems and roots that delivered water from the
First organisms
with cell nuclei
ago (Figure 3–11C). Trees and grasses are important in
3 ground. The first treelike plants appeared by 400 Myr
modern chemical weathering because they acidify
groundwater by adding carbon to soils as litter.
First known Critics of the Gaia hypothesis point out that many
single-celled
algae of the active roles played by organisms in the biosphere
today are a relatively recent development in Earth’s his-
4
tory and that the role of life in the distant past was
? ? ? ? probably negligible. In this view, early life-forms were
too primitive to have had much effect on chemical
weathering, and the delicate climatic balance main-
FIGURE 3-10 Gaia hypothesis Over time, life-forms tained through Earth’s history must have been achieved
gradually developed in complexity and played a progressively
greater role in chemical weathering and its control of Earth’s primarily by physical-chemical means (the effects of
climate. In the extreme form of the Gaia hypothesis, life temperature and precipitation on weathering rates)
evolved for the purpose of regulating Earth’s climate. rather than by biological intervention.
Critics also note that the very late appearance of
shell-bearing oceanic organisms near 540 Myr ago
means that life had played no obvious role in transfer-
carbonic acid that enhances chemical weathering; and ring the products of chemical weathering on land to the
(3) the role of shell-bearing ocean plankton in extract- seafloor for the preceding 4 billion years. Instead, most
ing CO from the ocean and storing it in their CaCO CaCO in the oceans was presumably deposited in
2 3 3
shells. These modern biological processes are, without warm, shallow tropical seas where concentrations of
question, important components of the processes of dissolved ions increased to levels that permitted chemi-
chemical weathering and carbon cycling. And by exten- cal precipitation, apparently with little or no biological
sion, they contribute to the thermostat that moderates intervention. Floating planktic plants capable of photo-
Earth’s climate today. synthesis (coccolithophorida) evolved still later, in the
In its more extreme form, the Gaia hypothesis states last 250 Myr.
that all evolution on Earth has occurred for the greater Supporters of the Gaia hypothesis respond with sev-
good of the planet by producing the succession of life- eral counterarguments. First, they claim that critics
forms needed to keep the planet habitable. This view is underestimate the role of primitive life-forms such as