as efficiently consumed when they decayed. Decomposition
is the exact converse of photosynthesis— the same chemical
reaction, but run in reverse: the sugars and other hydrogen-
carbon compounds built by organisms react with free oxygen
to yield carbon dioxide and water (burning hydrocarbons, a
favorite human activity, is just a speeded- up version of this).
So if photosynthesis and decay are perfectly balanced, there
will be no net accumulation of O2 in the air. This seems rather
unlikely, however, to have been true for 1.3 billion years, given
the tendency for at least some organic matter to be buried in
sediments without decomposing (and eventually become those
hydrocarbons we love to oxidize).
Another possibility is that for more than a billion years, any
oxygen produced through photosynthesis quickly reacted with
oxygen- hungry volcanic gases, especially hydrogen sulfide from
Changes in the air 107
seafloor volcanism. Then, around the end of the Archean, there
may have been a transition to a more modern tectonic regime,
with gases from subduction- related arc volcanism, which are
less reducing, gaining in importance.12 Some geologists, follow-
ing the inborn urge for uniformitarianism, interpret Archean
rocks like the Acasta gneisses and the Isua greenstones within
the framework of present- day plate tectonics. A few uniformi-
tarian zealots even argue for a modern- looking Earth back as
far as Hadean time based on tenuous circumstantial evidence
from the Jack Hills zircons. Others (full disclosure— I’m in this
group) think we need to suppress Charles Lyell’s voice in our
head and consider the possibility of a different tectonic mode
in Archean and Hadean times.
For one thing, the solid Earth was hotter (Lord Kelvin was
partly right), and efficient subduction of ocean crust would have
been unlikely. Also, while Archean rocks bear evidence of some
sort of jostling and crumpling atop a convecting mantle, they
don’t have the same structural styles as those deformed at today’s
well- defined boundaries between rigid plates. Hotter, weaker
slabs of crust might have piled up on each other and undergone
partial melting, extracting the constituents to form granitic con-
tinents, leaving a deep layer of dense residual rock that sank back
into the mantle in a process unappealingly called drip tectonics.13
But starting with rocks from the end of Archean time, we can
recognize the elements of modern crustal architecture: conti-
nental shelves, subduction zones, volcanic arcs, and full- fledged
mountain belts that suggest Earth had cooled enough to form a
brittle outer shell. So a nudge from a new tectonic system may
have been enough to give oxygen production a slight lead over
oxygen consumption. It seems entirely reasonable, in fact, that
the Earth’s tectonic coming of age would coincide with profound
changes in the chemistry of the surface environment.
108 Ch a pter 4
Although the Great Oxidation Event was a first- order disrup-
tion to the old geochemical establishment, its actual magnitude
was not as great as its name suggests. Certain metallic trace ele-
ments in the banded iron formations such as chromium have
stable isotopes whose behavior is highly sensitive to oxygen
levels— Precambrian canaries, perhaps, in equally anachronis-
tic coal mines. The ratios of these isotopes suggest that the Early
Proterozoic atmosphere probably had only a small fraction— less
than 0.1%— of the present level of oxygen (now 21% of the at-
mosphere by volume).14 We Phanerozoic organisms would not
have found this world hospitable. But the difference, in terms of
chemical possibilities, between no free oxygen and even a little
is greater than the difference between a little and a bit more.
O N E B I L L I O N Y E A R S O F L A S S I T U D E
After the upheavals of the GOE, Earth’s atmosphere seems
to have settled into a long period of geochemical stability. Al-
though the main period of iron formation deposition ended
around 1.8 billion years ago, oxygen levels seem to have re-
mained about constant, and far below the current value, for
another billion years after that.15 Such sustained equilibrium—
akin to a national economy that experiences no inflation, reces-
sions, or market turmoil for decades— points to a remarkably
fine- tuned balance between the oxygen supplied by hardy one-
celled photosynthesizers and oxygen consumed by covetous
metals, sulfurous volcanic gases, and decaying organic mat-
ter. This steady state may have been enforced by a regime of
austerity— in particular, severe limitation on the availability of
phosphorus, an essential nutrient for all life.
While shallow ocean waters had become oxygenated, there
is evidence that deeper reaches remained in the transitional
Changes in the air 109
state of the Early Proterozoic. In these stratified conditions,
precious phosphorus would have been continuously removed
from deeper waters, stolen away on the surfaces of iron min-
erals, like currency smuggled out of a poor country in the lin-
ings of pilferers’ coats. This in turn created chronic shortages
of phosphorus in the shallow ocean. Biological productivity
was thus kept in check, which limited organic carbon burial
and in turn prevented atmospheric oxygen levels from rising.16
This lean eon encouraged organisms to pursue low-
phosphorus lifestyles and new recycling strategies. In other ways,
however, evolution seemed to be biding its time. The biosphere
was diverse but still entirely unicellular; planktonic species—
including some eukaryotic giants called acritarchs, up to 0.8 cm
(0.3 in.) in diameter— proliferated in the oceans, and stromato-
lites quietly blanketed coastlines around the world. This peace-
ful stretch of the Proterozoic Eon has come to be known infor-
mally among geologists as the “Boring Billion.” But this Homer
Simpson– inspired designation is unfair, and misleading— akin
to history books that focus only on war and skip over the much
longer intervening periods of peace when “nothing happened.”
First, maintaining such long- term equipoise is something
that we humans in the Holocene might look to as a template for
amending our own biogeochemical habits, since our looming
environmental crises are the result of unchecked consumption
of scarce resources and an extreme imbalance between the pro-
duction and removal of an atmospheric gas. The Proterozoic
Earth somehow “understood” the fundamental principles of
sustainability; geochemical trading flourished, but all commod-
ities flowed in closed loops— the waste products of one group
of microbial manufacturers were the raw materials of another.
Second, the Boring Billion was the period when the durable
cores of the modern continents were assembled, as the new
110 Ch a pter 4
F I G U R E 1 1 . The Un
ited Plates of America— how North America was assembled plate tectonic system swept together pieces of Archean crust
and then constructed additions in the form of volcanic arcs. The
basement rocks beneath my feet here in Wisconsin— and buried
under younger sediments across the Midwest and Great Plains—
are almost entirely Proterozoic, formed by mountain- building
events during the Boring Billion, when vast areas of continental
crust were annexed to the old Canadian Shield (see figure 11).
Changes in the air 111
Boring, perhaps, but it was a productive time of infrastructure
development— another practice we modern Earth- dwellers
could profitably adopt.
Maybe because Proterozoic rocks and their stories are so fa-
miliar to me— the late great Penokee and Baraboo Ranges of the
Lake Superior Region, the violent hotspot volcanoes of central
Wisconsin, the immense Midcontinent Rift that nearly ripped
North America apart— the Boring Billion doesn’t seem that
long ago. Thus it saddens me, irrationally, to know that in the
equivalent amount of time into the future, about 1.5 billion years
from now, the window of habitability will have closed for the
Earth. The Sun, which is still getting brighter (at the very mod-
est rate of about 0.9% per 100 million years), will have grown
so luminous that the oceans will begin to vaporize, triggering a
“moist greenhouse runaway.”17 Solar radiation will then break
down water molecules into hydrogen and oxygen, which will
be lost to space. In other words, if life first became viable after
the early bombardment era ended 3.8 billion years ago, we are
now almost three- quarters of the way through Earth’s habitable
period. Nevertheless, we should be grateful for the great wealth
of time that this planet has had as a consequence of belonging to
a yellow dwarf star with a lifetime of 10 billion years. Stars just
50% larger than the Sun have a life expectancy of only 3 billion
years, which on Earth would be equivalent to the time span from
the formation of the planet to the middle of the Boring Billion.
At that point, Earth had so much more living to do.
T H E L O N G E S T W I N T E R
Things might have continued indefinitely in the monoto-
nous Proterozoic mode, except that by around 800 million
years ago, the new tectonic system had shepherded most of
112 Ch a pter 4
the continental crust into one large landmass that girdled the
equator. Geologists call this ancient supercontinent Rodinia,
from the Russian ródina, “motherland.” Like all continents,
Rodinia was only a temporary configuration, and it began to
break apart through rifting by about 750 million years ago, cre-
ating expansive new coastlines at tropical latitudes. Rivers fed
by heavy rains would have carried sediment and rock- derived
elements to the sea, and organisms would have thrived in these
comparatively nutrient- rich waters. High sedimentation rates
on the continental shelves allowed organic carbon to be bur-
ied in significant volumes for the first time, which drew down
atmospheric CO2 levels and set Earth on a cooling trend.
Perennial sea ice would have begun to accumulate in the
polar regions, increasing the albedo, or reflectivity, of the
Earth’s surface, which in turn led to further cooling— a classic
example of positive feedback. Even as the ice advanced far-
ther, carbon dioxide continued to be withdrawn from the at-
mosphere through both organic carbon burial and the intense
chemical weathering of rocks on the low- latitude fragments of
Rodinia (the mechanism by which the Himalaya drew down
CO2 and cooled the Earth in the Cenozoic). Once ice cover
reached a critical point, the albedo effect would have led the
planet into a “snowball” state— a perpetual snow day.
Exactly what happened during this Snowball Earth time—
also called the Cryogenian Period, one of the few named di-
visions of the Proterozoic in common use— generates a lot of
heat in the geologic literature. There is no disagreement that
something went haywire for a time with the climate system.
The rock record makes that clear: on almost every continent,
rocks of this age are glacial deposits— either unsorted mixes of
boulders and clay laid down directly by ice on land, or finely lay-
ered marine sediments punctuated by iceberg- rafted cobbles.
Changes in the air 113
With much of the Earth’s water locked up in glacial ice, sea
level would have been lower by hundreds of feet, exposing large
areas of the continents to erosion, at least until the deep ice
age began and surface processes ground to a halt. The Great
Unconformity in the Grand Canyon, between metamorphosed
Proterozoic rocks like the Vishnu Schist and the first stratified
unit, the Cambrian Tapeats Sandstone, is a record of Snowball
Earth time in absentia. So while there is no question that an
exceptional cold snap occurred at the end of the Proterozoic,
specifics like how deep the freeze was, how the biosphere sur-
vived, and how the Earth emerged from its hypothermic state
stoke the fires of academic debate.
S P R I N G O F L I F E
But, clearly, the Earth did warm up again. Maybe the breath of
volcanoes, which would have continued to erupt while other
geologic processes had stopped, gradually coaxed Earth back
from its cold coma over many thousands of years. Or perhaps
a sudden, rude belch of long- sequestered biogenic methane
from the seafloor transformed the icy planet into a hothouse in
a matter of months or years. The resolution of the rock record
and the precision of our dating methods are not fine enough
for us to distinguish between these possibilities.
In any case, the end of Snowball Earth marks what could be
called the Great Aeration, the second big step in free oxygen
levels and the emergence of Earth’s fourth, and current, atmo-
sphere. Oxygen- sensitive trace elements in sedimentary rocks
finally started behaving in the modern way, indicating that O2
levels jumped from a fraction of a percent to something close
to the present value. But the details of how the long- reigning
quasi- oxygenated realm of the Proterozoic was overthrown
114 Ch a pter 4
are not known. Perhaps it was a large influx into the oceans
of phosphorus, from rock powder ground up by Snowball
glaciers, that kick- started marine life.18 Or it might have been
the energetic mixing of shallow and deep- ocean waters in the
transition between ice- bound and greenhouse worlds that fi-
nally broke the geochemical stratification that had prevailed
for 1.5 billion years.
Once oxygen levels rose even a bit higher, organisms that
evolved to use it in their metabolic processes were significantly
more efficient in extracting energy from the environment and
were able to grow larger than any had before. Within a million
years of the end of Snowball Earth, a strange new macroscop
ic
ecosystem of puffy organisms called the Ediacaran fauna ap-
pears in the fossil record at sites around the world, including
southern Australia, the White Sea region of Russia, Leicester-
shire in England, and Newfoundland in Canada. These bizarre
down- parka- like organisms were shaped like Frisbees and fern
fronds, the latter up to 1 m (3 ft) high, with holdfasts to anchor
them to the seafloor. They had neither guts nor mineralized
shells, suggesting their world was a peaceable kingdom of os-
motic nutrition without threat of predation. Some may have
been precursors to later, more familiar marine lineages such as
the brachiopods, or lampshells. But others seem to have been
early evolutionary experiments in building bigger life- forms
that left no modern descendants.
The Ediacarans’ moment in the avant- garde was brief, how-
ever. Within about 40 million years, the seafloor had become
the venue for the period of frenzied anatomical tinkering called
the Cambrian explosion, when the first carnivores set off an
arms race between predator and prey. Like Wile E. Coyote
and the Road Runner, they’ve been trying to outwit each other
ever since. Hard protective shells of calcium carbonate became
Changes in the air 115
obligatory for bite- sized organisms; specialized swimming gear
and killing apparatuses de rigueur for the big meat eaters.
The pace of evolution in the Cambrian explosion continues
to be a topic of some controversy, pitting paleontologists
against biologists who use genomic approaches to determine
when different branches of the tree of life first emerged. The
fossil record suggests that the interval between about 540
and 520 million years ago was a time of unprecedented, and
never to be repeated, biological innovation. But this is at odds
with various molecular clocks, which are based on the assump-
tion that protein- coding genes accumulate substitutions at a
constant rate in evolutionary lineages. Most of the molecular
analyses suggest that Kingdom Animalia, whose first members
were probably sponges, was founded in the late Proterozoic,
750– 800 million years ago and that the Cambrian “Explosion”
may instead have been a slow- burning fuse.19 This, however,
puts our infancy in the bleak and frigid time of Snowball Earth,
which would seem an unlikely nursery. The disagreement re-
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