6.1 Searching for Life’s Orgins
For example, a 3.5-billion-year-old fossil tells us that life on Earth arose before 3.5
billion years ago.
Because the geological record is incomplete and because we may not yet have
discovered the oldest intact fossils, we do not know exactly how long life has
existed on Earth.
3 Lines of Fossil Evidence
Stromatolites- from the Greek word for “rock beds”
-Characterized by a distinctive layered structure, in size, shape, and interior
structure, ancient stromatolites look virtually identical to sections of mats formed
today by colonies of microbes sometimes called “living stromatolites”
-Living stromatolites contain layers of sediment intermixed with different types of
microbes, microbes near the top generate energy through photosynthesis those
beneath use organic compounds left as waste products by the photosynthetic
-There is some controversy about the biological origin of stromatolites, because
geological processes of sedimentation can mimic their layering.
-If we are correct in concluding that stromatolites tell us that photosynthetic life
already existed some 3.5 billion years ago, then we can infer that more primitive
life must have existed even earlier, and that the origin of life itself substantially
Microfossils- Individual fossilized cells, Finding ancient microscopic fossils, or
microfossils, is quite challenging, both because rocks become increasingly rare
with age and because the oldest rocks have been altered by geological processes in
ways that tend to destroy microfossils within them.
-It can be quite difficult to determine whether an interesting-looking microscopic
structure is biological or mineral in origin (causes scientific controversy)
-Microfossils in rocks dating to between 2.7 and 3.0 billion years old show more
conclusive evidence for life: They contain particular molecules (such as a variety
of hydrocarbons) that almost certainly indicate biological origin. (as opposed to
those found from 3.5 billion years ago which have more questionable biological
origin, northwesternAustralia,Africa) Isotopic Evidence- The third line of evidence for an early origin of life comes
from isotopic analysis of some of the most ancient rocks on Earth. Living
organisms can change the ratios of isotopes from their background, nonliving
values (living organisms—and fossils of living organisms—always show a slightly
lower fraction of carbon-13 atoms than that found in inorganic material)
-On an island off the coast of Greenland, this lower carbon-13 ratio has been found
in rocks that are more than 3.85 billion years old
-The rocks are metamorphic, meaning they have been transformed substantially by
high pressure or heat, which would explain why no intact microfossils remain
within them. For these rocks to have contained life, we must presume that they
were sedimentary before they under- went the metamorphic transformation; if they
were volcanic (igneous) rocks, then it is much more difficult to see how they could
have been home to or preserved evidence of living microbes.
IMPLICATIONS OF THE EARLY ORIGIN OF LIFE
-By itself, this early origin of life proves nothing about life elsewhere, since it is
always possible that Earth was the lucky beneficiary of a highly improbable event.
However, if we assume that what happened here would be typical of what might
happen elsewhere, then the early origin of life is profoundly important: It suggests
that we could expect life to also arise rapidly on any other world with similar
conditions. Because we expect many other worlds to have conditions similar to
those that prevailed on the young Earth, this idea gives us reason to think that life
might be quite common in the universe.
MAPPING EVOLUTIONARY RELATIONSHIPS
-Because living species have evolved from common ancestors, the base sequence
in the DNA of living organisms provides a sort of map of the genetic changes that
have occurred through time. By comparing the genomes of different organisms,
we should be able to reconstruct the evolutionary history of much of life on Earth.
-Over millions and billions of years, continuing evolution led to new species with
DNA molecules increasingly different from the DNA of the common ancestor.
But, always, the new molecules were built by changes to the older ones so that, in
principle, the changes are traceable in the precise base sequences of living
-Two species with very similar DNA sequences probably diverged relatively
recently in evolutionary history, while two species with very different DNA
sequences probably diverged much longer ago. -These types of DNA sequence comparisons are what have enabled biologists to
map out the relationships shown in the tree of life. That is, DNA studies tell us that
life can be divided into the three domains (bacteria, archaea, and eukarya)
-For example, the fact that animals and plants represent two branches that split off
in about the same place from other eukarya tells us that all animals and plants are
quite similar genetically, at least in comparison to organisms on most other
branches. Moreover, organisms on branches located closer to the “root” of the tree
must contain DNA that is evolutionarily older, suggesting that they more closely
resemble the organisms that lived early in Earth’s history.
WHERE DID LIFE BEGIN?
-It seems unlikely that life could have arisen on the land surface. The early
atmosphere contained practically no molecular oxygen, so our planet could not
have had a protective layer of ozone
-Before the ozone layer existed, any surface life would have been exposed to high
levels of this radiation. While we can’t rule out the possibility that life might have
arisen in such an environment—some organisms today (such as D. radiodurans)
can survive high-radiation conditions—the environment would have been much
more hospitable under water (because water also absorbs ultraviolet light) or in
rocks beneath the surface.
-One such possibility, first suggested by Darwin, is shallow ponds. Organic
compounds may have formed spontaneously in such ponds. Once the compounds
formed, tides or cycles of wetting and evaporation could have increased their
concentration near the pond edges, spurring reactions that might have led to life.
-However, while these factors suggest ponds could have been a good location for
an origin of life, the shallow water would not have offered much protection against
solar ultraviolet radiation.
-Abetter possibility might be deep-sea or underground environments, which
would have been protected from high-energy radiation. Deep-sea volcanic vents
offer plenty of chemical energy to fuel reactions that might have led to life, and
chemical energy is also available underground in reactions between water and
minerals in rock.
-Moreover, even if life first arose in ponds at the surface, the impacts of the late
heavy bombardment probably would have allowed the survival only of life that
had migrated to deep-sea or underground environments. For that reason, it now
seems likely that the common ancestor of all life on Earth today evolved from
organisms that lived near deep-sea vents or underground, even if the first origin of life occurred elsewhere.
6.2 The Origin of Life
-Over the past few decades, laboratory experiments have given us insights into the
chemical processes that likely occurred on the early Earth. While these
experiments have not yet told us precisely how life first arose—and it’s possible
that they never will—we’ll see in this section that they give us good reason to
think that life may have started through natural, chemical processes.
-The laboratory experiments generally try to re-create the chemical conditions that
should have prevailed on the early Earth, an assumption that makes sense if life
originated here. However, it is conceivable that life migrated to Earth from another
-Life today is based on the chemistry of organic molecules, making it logical to
assume that the first life was somehow assembled from organic molecules
produced by chemical reactions on the early Earth. Such reactions do not occur
naturally today, because Earth’s oxygen- rich atmosphere prevents complex
organic molecules from forming readily outside living cells.
-The oxygen in our atmosphere is a product of life, produced by photosynthesis,
which means it could not have been present before life arose
THE MILLER–UREY EXPERIMENT
-As early as the 1920s, some scientists recognized that Earth’s early atmosphere
should have been oxygen- free, and they hypothesized that sunlight-fueled
chemical reactions could have led to the spontaneous creation of organic
-This hypothesis was put to the test in the 1950s in a famous experiment credited
to Stanley Miller and Harold Urey, now known as the Miller–Urey experiment
-The original Miller–Urey experiment used small glass flasks to simulate chemical
conditions that scientists thought represented those on the early Earth.
-One flask was partially filled with water to represent the sea and heated to
produce water vapor. Gaseous methane and ammonia were added and mixed with
the water vapor to represent the atmosphere. These gases flowed into a second
flask, where electric sparks pro- vided energy for chemical reactions. Below this
flask, the gas was cooled so that it could condense to represent rain and then was
cycled back into the water flask. -The water soon began to turn a murky brown, and a chemical analysis (performed
after letting the experiment run for a week) showed that it contained many amino
acids and other organic molecules.
-We now know that the methane and ammonia mixture in the original Miller–Urey
experiment was not representative of Earth’s early atmosphere. Nevertheless, the
experiment demonstrated that, at least under some conditions, the building blocks
of life form naturally and abundantly.
OTHER SOURCES OF ORGANIC MOLECULES
-The first of these potential “other” sources are chemical reactions near deep-sea
vents.As these undersea volcanoes heat the surrounding water, a variety of
chemical reactions can occur between the water and the minerals. These chemical
reactions would have occurred spontaneously in the conditions thought to have
prevailed in the early oceans, and they should have resulted in the production of
the same types of organic molecules thought to have been necessary for the origin
-The other additional source of organic molecules may have been material from
space.Analysis of meteorites shows that they often contain organic molecules,
including complex molecules such as amino acids.
-Apparently, organic molecules can form under the conditions present in
interplanetary space and can survive the plunge to Earth.
-Moreover, recent research shows that ultraviolet light from the young Sun could
also have produced some of the building blocks of life. It would do this by causing
chemical reactions to occur on dust grains orbiting the Sun as part of the solar
nebula. This dust, laden with organic molecules, could have “rained down” on the
-The many impacts of the heavy bombardment could have brought additional
organic material from asteroids and comets. The heat and pressure generated by
the impacts may have further facilitated the production of organic molecules in
Earth’s atmosphere and oceans.
-It’s likely that all three sources of organic molecules—chemical reactions near the
ocean surface, chemical reactions near deep-sea vents, and material from space—
played a role in shaping the chemistry of the early Earth. THE TRANSITION FROM CHEMISTRY TO BIOLOGY
-There must have been at least a few intermediate steps—each involving a
chemical pathway with a relatively high probability of occurring—that eased the
transition from chemistry to biology.
- Early life must also have had a self-replicating molecule, but it probably was not
DNA: Double- stranded DNA seems far too complex, and its replication far too
inter- twined with RNA and proteins, to have been the genetic material of the first
living organisms. (more likely the step between is RNA)
-RNA is much simpler than DNA because it has only one strand rather than two
and its backbone structure requires fewer steps in its manufacture.
- In modern organisms, neither DNA nor RNA can replicate itself. Both require the
help of enzymes
- These enzymes are proteins that are made from genetic instructions contained in
DNA and carried out with the help of RNA.
-Dilemma is that RNA cannot replicate without enzymes, and the enzymes cannot
be made without RNA.
-Away around this dilemma was discovered in the early 1980s by Thomas Cech
and his colleagues at the University of Colorado, Boulder. They found that RNA
can catalyze biochemical reactions in much the same way as enzymes
-We now know that RNA molecules play this type of catalytic role in many
cellular functions, and we call such RNA catalysts ribozymes
-Follow-up work has shown that some RNA molecules can at least partially
catalyze their own replication. These discoveries have led biologists to envision
that modern, DNA-based life may have arisen from an earlier RNA world, in
which RNA molecules served both as genes and as chemical catalysts for copying
and expressing those genes.
- Experiments show that several types of inorganic minerals can facilitate the self-
assembly of complex, organic molecules. Minerals of the type that geologists call
clay* may have been especially important. Clay is extremely common on Earth
and in the oceans, where it forms through simple weathering of silicate minerals;
indeed, the oldest zircon grains suggest the widespread abundance of clays more
than 4.4 billion years ago, so we expect clay to have been common at the time of
the origin of life. More- over, clay minerals contain layers of molecules to which other molecules, including organic molecules, can adhere. When organic
molecules stick to the clay in this way, the mineral surface structure can force
them into such close proximity that they react with one another to form longer
- RNA strand only 5 bases long that can act as a ribozyme
Possible explanation for the origin of life:
1. Through some combination of atmospheric chemistry, chemistry near deep-sea
vents, and molecules brought to Earth from space, the early Earth had at least
localized areas with significant amounts of organic molecules that could serve as
building blocks for more complex organic molecules.
2. More complex molecules, including short strands of RNA, grew from the organic
building blocks, probably with the aid of reactions catalyzed by clay minerals. The
minerals also helped catalyze the production of microscopic pre-cells in which
RNA and other organic chemicals became enclosed.
3. The concentration of RNA molecules within pre-cells facilitated reactions that
eventually led to self-replicating RNA, at which point molecular natural selection
favored the spread of those RNA molecules that replicated most accurately and
4. Natural selection among the RNA molecules in pre-cells gradually led to an
increase in complexity, until eventually some of these structures became true
5. DNA evolved from RNA, and its advantages made it the preferred hereditary
molecule. Natural selection continued, enabling organisms to adapt to a great
many environmental niches on planet Earth.
THE POSSIBILITY OF MIGRATION
The idea that life could travel through space to land on Earth, some- times called
The presence of organic molecules in meteorites and comets tells us that the
building blocks of life can survive in the space environment, and we’ve already
discussed some Earth microbes that are capable of surviving at least moderate
periods of time in space Scientists have identified and cataloged more than 30,000 meteorites
They have found about three dozen with compositions that clearly suggest that
they came from Mars, even more have been found from the Moon
Examination of these meteorites suggests that over time the inner planets have
exchanged many tons of rock. In a sense, Earth, Venus, and Mars have been
“sneezing” on each other for billions of years, offering the possibility of
microscopic life hitchhiking between worlds on one of the meteorites.
For a living microbe to reach earth after such a journey it would have to survive 3 lethal
The impact that blasts it off the surface of its home world, the time it spends in the
harsh environment of interplanetary space, and the fiery plunge through our
Examination of Martian meteorites suggests that neither the first nor last of these
events poses insurmountable obstacles, suggesting that microbes inside these rocks
could survive both the initial impact and the later fall to Earth
A few meteorites are likely to be launched into orbits that cause them to crash to
Earth during one of their first few trips around the Sun. For example, calculations
suggest that about 1 in 10,000 meteorites may travel from Mars to Earth in a
decade or less. Because experiments in Earth orbit have already shown that some
terrestrial microbes can survive at least 6 years in space, it seems quite reasonable
to imagine microbes from Mars arriving safely on Earth.
Considerations almost certainly rule out the possibility of migration from other
star systems. Under the best of circumstances, meteorites from planets around
other stars would spend millions or billions of years in space before reaching
Earth; any living organisms would almost surely be killed by exposure to cosmic
rays during this time, or simply die because of desiccation—the lack of water.
REASONS TO CONSIDER MIGRATION
The key question probably is not whether life could migrate through space but
whether we have any reason to suppose it originated elsewhere rather than right
here on Earth.
Today, most ideas about migrating life fall into one of two broad categories.
The first broad idea suggests that life does not form as easily as we have imagined,
at least under the conditions present on the early Earth. In this view, the only
explanation for life on Earth (other than invoking the supernatural) would be migration from elsewhere.
The second broad idea suggests that life forms so easily that we should expect to
find life originating on any planet with suitable conditions. In that case, the origin
of life in our solar system would have occurred on whichever planet got those
conditions first; for example, if the very early Venus or very early Mars had
suitable conditions for life before Earth, life from one of those worlds might have
migrated to Earth and taken hold on our planet as soon as conditions allowed.
IMPLICATIONS OF MIGRATION TO THE SEARCH FOR LIFE
While ideas about microbes migrating to Earth are speculative, it seems a near-
certainty that microbes from Earth have many times made the journey to Mercury,
the Moon, Venus, and Mars. After all, Earth has suffered plenty of impacts large
enough to blast rock into space during its long history, offering plenty of
opportunity for hitchhiking microbes.
Thus, if it were possible for Earth life to survive on any of these other worlds, we
should actually expect to find it there.
We can almost certainly rule out the possibility of survival on the Moon and
Mercury, and probably on Venus as well. Mars, however, may well have habitats
that could provide at least temporary refuge to terrestrial microbes, and Mars may
have been globally habitable in the distant past.
The likelihood of such interplanetary migration raises at least two important issues
in astrobiology. First, if we someday find life on Mars, we will have to wonder if it
is native or if it arrived there from Earth.
Second, the possibility of life migrating among the planets raises the question of
whether we could ever distinguish between an indigenous origin of life on Earth
and an origin based on migration from elsewhere.
The only way we may ever be confident that life elsewhere is not transplanted
Earth life will be if its biochemistry is too different from that of terrestrial life to
allow for a common ancestor.
It is possible that Venus once had oceans and a habitable climate in which life
might have arisen. However, Venus now is so hot that any fossil record would
almost certainly have been destroyed by the heat and subsequent geological
One way or another, life arose on Earth quite soon after conditions first allowed it,
and even if life migrated here from another world, we have good reason to think that it evolved naturally, through chemical processes that favor the creation of
complex, organic molecules and the subsequent molecular evolution of self-
6.3 The Evolution of Life
Regardless of how or where it originated, life on Earth has been evolving
throughout the 4 billion or so years
Careful studies of the geological record provide the key data with which we
attempt to re-create the evolutionary time scale, while genome comparisons offer
data that help us map relationships among species.
EARLY MICROBIAL EVOLUTION
The earliest organisms must have been quite simple, but they undoubtedly had at
least a few enzymes and a rudimentary metabolism. Their cells probably looked
somewhat like those of the simplest modern bacteria or archaea, lacking cell nuclei
and other complex structures that we find in eukarya. Moreover, because the
atmosphere at that time was essentially oxygen-free, all early life must have been
anaerobic, meaning that it did not require molecular oxygen; by contrast, we are
aerobic organisms, because we cannot survive with- out molecular oxygen.
Both photosynthesis and the ability to digest other organisms probably evolved
much later, so we expect that the first microorganisms were chemoautotrophs
organisms that obtained their carbon from carbon dioxide dissolved in the oceans
and their energy from chemical reactions involving inorganic chemicals.
Natural selection probably caused rapid diversification among the early life-forms.
Modern DNA replication involves a variety of enzymes that help keep the
mutation rate low. Early organisms, with a much more limited set of enzymes,
probably experienced many more errors in DNA copying. Because more errors
mean a higher mutation rate, evolution would have been rapid among early
Fossil evidence supports the idea of rapid diversification. Recall that stromatolites
suggest the presence of organisms that obtained energy by
Photosynthesis some 3.5 billion years ago, and some of the oldest micro- fossils
also resemble modern photosynthetic organisms. Because photo- synthesis is a
complex metabolic pathway, its early emergence indicates that early evolution was
rapid. Photosynthesis developed through multiple steps:
First, some organisms developed light-absorbing pigments that absorb light
energy-these evolved to enabled the cell to make use of the absorbed solar energy
The rise of oxygen created a crisis for life, because oxygen attacks the bonds of
organic molecules. Many species of microbes probably went extinct
Some avoided these effects because they lived in (or migrated to) underground
locations where the oxygen did not reach them. We still find many anaerobic
microbes in such locales today, living in soil or deeper underground in rocks.
Others survived because the oxygen content of the atmosphere rose gradually,
allowing them time to evolve new metabolic processes and protective mechanisms
that enabled them to thrive rather than die in the presence of oxygen. Plants and
animals, including us, still use the metabolic processes that evolved in response to
the “oxygen crisis” faced by living organisms some 2 billion or more years ago.
THE EVOLUTION OF EUKARYA
The evolution of eukarya was the crucial first step in our own eventual evolution;
we are, after all, members of this domain. Even single-celled eukaryotes exhibit
much more diversity in cellular structure than exists among bacteria or archaea,
and multi-celled eukaryotes enjoy diversity far beyond that. Because more
variations are possible on complex structures than on simple ones.
When did eukarya arise? Despite the fact that modern eukarya have more complex
cellular structures than bacteria and archaea, genome studies do not suggest any
substantial differences in the evolutionary ages of the three domains. That is, it is
quite likely that members of all three domains—bacteria, archaea, and eukarya—
split from a common ancestor early in Earth’s history.
The oldest known fossils that clearly show cell nuclei date to about 2.1 billion
Modern, complex eukarya probably evolved through a combination of at least two
major adaptations that arose in their simpler ancestors.
First, some early species of eukarya may have developed specialized in folding’s
of their membranes that compartmentalized certain cell functions, ultimately
leading to the creation of a cell nucleus. Second, some relatively large ancestral
host cells absorbed small bacteria within them, creating a symbiotic relationship
in which both the invading organisms and the host organisms benefited from
living together Evidence for symbiosis comes from two structures in eukarya which appear to be
cells within cells (mitochondria).
Organs within a cell help produce energy (by making molecules ofATP and
chloroplasts, structures in plant cells that produce energy by photosynthesis).
Both also have their own DNA and reproduce themselves within their eukaryotic
homes. Moreover, sequencing of the DNA in mitochondria and chloroplasts
clearly groups them with domain bacteria, rather than with eukarya, making it a
near-certainty that they originated as free-living bacteria
THE CAMBRIAN EXPLOSION
We have seen that life on Earth existed at least 3.5 billion years ago (and perhaps
hundreds of millions of years before that), and all three domains of life were well
established by at least 2.1 billion years ago.
However, the fossil record tells us that all this life remained microscopic until
The earliest fossil evidence for complex, multicellular organisms—all of which are
eukarya—dates to only about 1.2 billion years ago. Thus, microbes had our planet
to themselves for more than 2 billion years after the origin of life.
Even today, the total biomass of microbes far exceeds that of multicellular
organisms like fungi, plants, and animals
The fossil record suggests that animal evolution progressed slowly at first, with
relatively little change seen between fossils from 1.2 billion years ago and those
from a half-billion years later. But then something quite dramatic happened.
In the broadest sense, biologists classify animals according to their basic “body
plans.”Animals are grouped by body plan into what biologists call phyla
Mammals and reptiles both belong to the phylum Chordata which represents
animals with internal skeletons.
Insects, crabs, and spiders belong to the phylum Arthropoda, which represents
animals with body features such as jointed legs, an external skeleton, and
segmented body parts.
Remarkably, nearly all of these different body plans make their first known
appearance in the geological record during a period spanning only about 40
million years— less than about 1% of Earth’s history. This remarkable flowering of animal diversity appears to have begun about 542
million years ago, which corresponds to the start of the Cambrian period, called
the Cambrian explosion.
Presents us with two questions:
Why did the Cambrian explosion occur so suddenly, at least in geological terms,
yet so long after the origin of eukaryotes, and why hasn’t any similar
diversification happened since?
No one knows the answers to these questions, but we can identify at least four
possible contributing factors
1 Oxygen level in our atmosphere may have been well below its present level
until the time of the Cambrian explosion (change in animal life a result of oxygen
reaching a critical level for the survival of larger more energy-intensive forms)
2 Evolution of genetic complexity, as eukaryotes evolved they developed more
genetic variation in their DNA, opening more possibilities for further variation
(great diversity within short time).
3 Climate change (series of snowball Earth episodes ending prior to the
4 Absence of efficient predators, once predators were efficient and widespread, it
would have been much more difficult for entirely new body forms to find an
available environmental niche.
THE COLONIZATION OF LAND
Because most early microfossils are found in sediments that were originally
deposited in the oceans, it’s difficult to know when life first migrated onto land.
Plenty of such locations are available on land—including underground and
anyplace where water can pool under a shelter of over- hanging rock—so it is hard
to imagine reasons why microbial life would not have taken hold on land quite
For larger organisms, surviving on land was more difficult than surviving in the
oceans, primarily because it required evolving a means of obtaining water and
mineral nutrients without simply absorbing them from surroundings.
The timing of the development of the ozone layer may also have played a role in
the late colonization of land. Fossil evidence shows that plants (and perhaps fungi as well) were the first large
organisms to develop the means to live on the land. The colonization of land by
plants appears to have begun about 475 million years ago.
DNA evidence suggests that plants evolved from a type of algae.
Because such locales occasionally dry up, natural selection would have favored
adaptations, such as thick cell walls, that allowed the algae to survive during
periods of dryness. Cell walls would have given the organisms structure that
would have helped them survive on land.
Once plants moved onto the land, it was only a matter of time until animals
followed them out of the water. Within about 75 million years, amphibians and
insects were eating land plants. By the beginning of the Carboniferous period,
about 360 million years ago, vast forests and abundant insects thrived around the
THE ORIGIN OFATMOSPHERIC OXYGEN
Molecular oxygen is a highly reactive gas that would disappear from the
atmosphere in just a few million years if it were not continually resupplied by life.
Fire, rust, and the discoloration of freshly cut fruits and vegetables are everyday
examples of oxidation reactions—chemical reactions that remove oxygen from
Before oxygen-breathing organisms evolved, oxidation reactions involved
primarily volcanic gases, dissolved iron in the oceans, and surface minerals
(especially those containing iron) that could react with oxygen.
Remarkably, it seems that we owe our oxygen atmosphere to microscopic bacteria
sometimes called “blue-green algae” but more technically known as
cyanobacteria (atleast 2.7 billion yr ago, it took 2 billion years for oxygen to
build to present levels)
TIMING OF THE OXYGEN RISE
Careful study of rock chemistry offers even more clues.
Cyanobacteria split water and release oxygen in photosynthesis and are thought to
have been responsible for the rise of oxygen in Earth’s atmosphere.
Studies of rocks that are between about 2 and 3 billion years old, especially rocks
of a type called banded iron formations, show that the atmosphere during that time
contained less than 1% of the amount of oxygen it contains today. Very low oxygen in atmosphere until 2.35 billion years ago (great oxidation event began
to build up the atmosphere)
If we are correct in assuming that cyanobacteria began to produce oxygen at least
2.7 billion years ago—or at least 350 million years before the great oxidation
event at 2.35 billion years ago—what took so long? Our best guess is that
nonbiological processes, such as oxidation of surface rock and ocean minerals,
were at first able to remove oxygen from the atmosphere as rapidly as the
cyanobacteria could make it; only after the rock and ocean minerals were saturated
with oxygen could the atmospheric buildup begin.
IMPLICATIONS FOR LIFE ELSEWHERE
Our study of the origin of life gives us reason to think that life might be common
on worlds with conditions like those of the early Earth.
But the fact that it took so long for oxygen to build up in the atmosphere on Earth
should make us wonder about the likelihood of getting oxygen-breathing life on
other worlds. Could Earth have been “lucky” to get conditions that allowed the
buildup of oxygen?
If so, perhaps life on most other worlds would never evolve past microscopic
forms; life might then be common, but advanced or intelligent life quite rare
Alternatively, maybe Earth was “unlucky” in having conditions that prevented the
oxygen buildup for so long. In that case, other worlds might have complex plants
and animals by the time they are just 1 to 2 billion years old, instead of having to
wait until they are 4 billion years old.
We will therefore continue our study with the assumption that Earth has been
“typical,” until and unless we learn otherwise.
6.4 Impacts and Extinctions
Reptiles evolved from amphibians; by about 245 million years ago, dinosaurs and
mammals followed. But the fossil record shows that the path was not smooth.
In particular, there is evidence for a number of striking transitions in the nature of
living organisms. The most famous of these defines the boundary between the
Cretaceous and Tertiary periods, which dates to about 65 million years ago.
THE K–T BOUNDARY LAYER The thin layer of sediments that marks the Cretaceous–Tertiary boundary, called
the K–T boundary for short (the K comes from the German word for
“Cretaceous,” Kreide), is unusually rich in iridium, an element that is rare on
Earth’s surface (because Earth’s iridium sank to the core when our planet
underwent differentiation but common in meteorites).
They calculated that it would have taken an asteroid about 10–15 kilometers in
diameter to deposit the iridium distributed worldwide in the K–T boundary layer.
Biological devastation (meteorite) occurred 65 million years ago.
EVIDENCE FOR THE IMPACT
Besides being unusually rich in iridium, this layer contains four other unusual
(1) High abundances of several other metals, including osmium, gold, and
(2) Grains of “shocked quartz,” quartz crystals with a distinctive structure that
indicates they experienced the high-temperature and high-pressure conditions of
(3) Spherical rock “droplets” of a type known to form when drops of molten rock
cool and solidifies in the air
(4) Soot (at some sites) that appears to have been produced by widespread forest
In addition to the evidence within the sediments, scientists have identified a large,
buried impact crater that appears to match the age of the sediment layer. The
crater, about 200 kilometers across, is located on the coast of Mexico’s Yucatán
Peninsula, about half on land and half underwater
If the impact was indeed the cause of the mass extinction, here’s how it probably
happened: On that fateful day some 65 million years ago, the asteroid or comet
slammed into Mexico with the force of a hundred million hydrogen bombs.
It apparently hit at an angle, sending a shower of red-hot debris across the
continent of NorthAmerica.Ahuge tsunami sloshed more than 1000 kilometers
Much of NorthAmerican life may have been wiped out almost immediately. Not long after, the hot debris raining around the rest of the world-ignited fires that
killed many other living organisms. Indeed, the entire sky may have been bright
enough to roast most life on land.
Dust and smoke remained in the atmosphere for weeks or months, blocking
sunlight and causing temperatures to fall as if Earth were experiencing a global
and extremely harsh winter. The reduced sunlight would have stopped
photosynthesis for up to a year, killing large numbers of species throughout the
Perhaps the most astonishing fact is not that up to 75% of all plant and animal
species died but that some 25% survived.Among the survivors were a few small
mammals. These mammals may have survived in part because they lived in
underground burrows and managed to store enough food to outlast the global
winter that immediately followed the impact.
For 180 million years, dinosaurs had diversified into a great many species large
and small, while most mammals (which had arisen at almost the same time as the
dinosaurs) had remained small and rodent like.
With the dinosaurs gone, mammals became the new animal kings of the planet.
Over the next 65 million years, the small mammals rapidly evolved into an
assortment of much larger mammals—ultimately including us.
The primary problem is that identifying the extinction of a species requires finding
its last occurrence in the fossil record, which means we can be misled if we’ve yet
to find a more recent occurrence of the species.
OTHER MEANS OF MASS EXTERMINATION
High levels of volcanic activity can lead to climate change and high extinction rates (sick
Mutation rate, copying errors within the cells, can be caused by external factors
(ultraviolet light, solar wind)
Supernovae, death by cosmic rays or gamma ray bursts Chapter 7
When we discuss habitability, we generally mean an environment in which life of
some kind might survive, not necessarily human life.
Where can we expect to find building blocks of life?
Life on Earth uses about 25 of the 92 naturally occurring chemical elements,
although just four of these elements—oxygen, carbon, hydrogen, and nitrogen—
make up about 96% of the mass of living organisms. Thus, a first requirement
might be the presence of most or all of the elements used by life.
Moreover, the elements oxygen, carbon, and nitrogen—arguably the most crucial
elements for life—are the third-, fourth-, and sixth-most-abundant elements in the
The nebular theory of solar system formation gives reason to expect the elements of life
to be common on other worlds. The first step, condensationaffects only the heavier
elements or hydrogen compounds containing heavy elements, because pure
hydrogen and helium always remain gaseous.As long as condensation and
accretion can occur, we expect the resulting worlds to contain the elements needed
Life on Earth is carbon based and we have good reason to think that life elsewhere would
be carbon based as well.
A somewhat stricter requirement is the presence of these elements in molecules
that can be used as ready-made building blocks for life, just as the early Earth
probably had at least moderate abundances of amino acids and other complex
Recall that Earth’s organic molecules likely came from some combination of three
sources: chemical reactions in the atmosphere, chemical reactions near deep-sea
vents in the oceans, and molecules brought to Earth from space.
Thus, we should concentrate on worlds that have either an atmosphere or a surface
or subsurface liquid medium, such as water, or both.
The first two sources can occur only on worlds with atmospheres or oceans,
respectively. But the third source should have brought similar molecules to nearly
all worlds in our solar system.
Studies of meteorites and comets suggest that organic molecules are widespread
among both asteroids and comets. Every world should have received at least some organic molecules
However, organic molecules tend to be destroyed by solar radiation on surfaces
unprotected by atmospheres.
Molecules probably cannot react with each other unless some kind of liquid or gas
is available to move them about.
Where can we expect to find energy for life?
In addition to a source of molecular building blocks, life requires an energy source
to fuel metabolism.
Some organisms get energy directly from sunlight through photosynthesis. Others
get energy by consuming organic molecules (for example, by eating
photosynthetic organisms) or through chemical reactions with inorganic
compounds of iron, sulfur, or hydrogen.
The energy available in sunlight decreases with the square of the distance from the
Sun, example twice as far from the sun as earth = 1/4 the amount of energy
Does life need liquid water?
3 vital roles for life on eIt dissolves organic molecules, making them available
for chemical reactions within cells; it allows for the transport of chemicals into
and out of cells; and it is involved directly in many of the metabolic reactions that
occur in cells.
But can other liquids do this?
No one knows whether other liquids could support life in the absence of liquid
Potential Candidates:Ammonia, Methane and Ethane
ADVANTAGES OF WATER
3 Advantages: Water remains liquid over a wider and higher range of temperatures (can
stay liquid through changes in weather or climate, higher range facilitates chemical
reactions, faster moving molecules=faster chemical reactions)
Typically, the rate of a given chemical reaction doubles with each 10°C increase in
temperature. Life using these alternative liquids would probably have a much slower
metabolism than life on Earth.
SecondAdvantage- they way water freezes, water is the only liquid that is less
dense as a solid than a liquid, allows ice to float and ice to insulate water below
This is important to long term climate stability (ice ages, ice prevents all liquid
from turning to ice, as if ice was more dense than water it would sink)
ThirdAdvantage- the way electrical charge is distributed within water molecules
the electrons tend to be distributed in a way that makes one side have a net
positive charge and the other side have a net negative charge
This charge separation affects the way in which water dissolves other substances.
Living cells have membranes that do not dissolve in water, so the membranes
effectively protect the interior contents of cells. If we place living cells in liquid
ethane, methane, or ammonia—molecules with less charge separa