Geology Reference
In-Depth Information
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Figure 19.17
Outgassing and the Evolution of the Atmosphere
Escapes
Hydrogen
H
Water
H
2
O
Ultraviolet radiation from Sun
To atmosphere
Nitrogen
N
Escape to
space
Water
H
2
O
Carbon
dioxide
CO
2
Oxygen
O
2
Ozone
O
3
To oceans
Hydrogen
H
2
Oxygen
O
2
Water vapor
H
2
O
photochemical
dissociation
Volcano
Oxygen
O
2
To atmosphere
To atmosphere
Erosional debris
Carbon dioxide
CO
2
Water
H
2
O
Oxygen
O
2
Photosynthesis
Organic compounds
b
Photochemical dissociation and photosynthesis added free
oxygen to the atmosphere. Once free oxygen was present, an ozone
layer formed in the upper atmosphere and blocked most incoming
ultraviolet radiation.
a
Outgassing. Notice that the atmosphere contains several gasses
but no free oxygen.
not. A biologist might use several criteria to make the distinc-
tion, including growth and reaction to stimuli, but minimally,
a living thing must practice some kind of chemical activity
(metabolism) to maintain itself, and it must be capable of
reproduction to ensure the long-term survival of the group
to which it belongs. This metabolism-reproduction criterion
might seem suffi cient to decide if something is living or not,
and yet the distinction is not always easy to make.
Bacteria are living, but under some circumstances, they
can go for long periods during which they show no signs of
living and then go on living again. Are viruses living? They
behave like living organisms in the appropriate host cell, but
when outside a host cell, they neither metabolize nor repro-
duce. Some biologists think that viruses represent another
way of living, but others disagree. Comparatively simple
organic molecules called
microspheres
form spontaneously
and grow and divide in a somewhat organism-like manner,
but these processes are more like random chemical reactions,
so they are not living.
So what do viruses and microspheres have to do with the
origin of life? First, they show that the living versus nonliving
distinction is not always clear. And second, if life originated
by natural processes, it must have passed through prebiotic
stages—that is, stages in which the entities would have shown
signs of living organisms but were not truly living.
oceans reached chemical equilibrium and have remained in
near-equilibrium conditions ever since.
EARLY HISTORY
Scientists have found fossils in rocks as old as 3.5 billion years,
and chemical evidence in 3.8-billion-year-old rocks in Green-
land convince some investigators that organisms were present
by this early date. Today, the biosphere is made up of mil-
lions of species of organisms assigned to fi ve kingdoms—
animals, plants, protistans, fungi, and monera (bacteria and
archaea),*—whereas only bacteria and archaea are known
from Archean rocks. In Chapter 18, we discussed evolution,
which accounts for how organisms have changed through
time, but the theory of evolution does not address how life
originated in the first place. Here, we are concerned with
abiogenesis
,
that is, how life originated from nonliving
matter. Before proceeding, though, abiogenesis does not
hold that a living organism such as a bacterium, or even a
complex organic molecule, sprang fully developed from
nonliving material. Rather than one huge step, abiogenesis
holds that several small steps took place, each leading to an
increase in organization and complexity.
But what is living and nonliving? The distinction is clear
in most cases: dogs and trees are alive, rocks and water are
Investigators agree that for life to originate, an energy source
must have acted upon the appropriate chemical elements
from which organic molecules could synthesize. The early
*Archaea includes microscopic organisms that resemble bacteria, but differ
from them genetically and biochemically.
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