Download 1 Evolution is a dynamic and unpredictable process. Life on Earth

HOW HAS OCEAN LIFE CHANGED OVER TIME?
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volution is a dynamic and unpredictable process. Life on Earth probably began as
single-celled organisms in the ocean over 3.5 billion years ago. For almost the next 3
billion years, only single-celled organisms such as algae and bacteria lived on Earth,
then ocean life began to diversify. Life did not move out of the oceans to land until about
425 million years ago. Significant changes in marine species typically occur over timescales
measured in millions of years. The environment in which ocean life exists, however, changes
both rapidly and slowly, in ways that affect the nature and distribution of life in the ocean.
This theme (Life - Process and Change) discusses several methods used to study and evaluate
changes in ocean life.
Related Themes:
• A discussion of today’s life in the ocean and abiotic factors that affect life can be
found in Life - Scale and Structure.
• Ocean upwelling and hydrothermal vents are addressed in Life - Energy.
• The ways in which humans use the oceans and El Niño’s affect on fisheries are
presented in Life - Human Interactions.
• How physical factors that affect ocean life are measured is discussed in Life - Measurements.
• El Niño’s effect on ocean current systems is covered in Life - Systems and Interactions.
• Detailed information about the El Niño phenomenon can be found in Oceans - Process and Change.
• Energy associated with hydrothermal vents is addressed in Oceans - Energy.
• Past climates and climate change are presented in Climate - Process and Change.
• How we measure past climates is covered in Climate - Measurement.
Related Activity:
• Biogeography or “What Happens to Fish Populations When El Niño Arrives?”
INTRODUCTION
For most of the more than 3.8 billion year history of life on Earth, life was exclusively an
ocean phenomenon. The migration of ocean life onto land began relatively recently, less than
half a billion years ago. Marine life has changed substantially during its 3.8 billion year history as
some species have proliferated while other species have become extinct.
CYCLES OF MATTER CAUSING SHORT-TERM CHANGES
Marine life is highly dependent on a number of non-living, or inorganic, factors in the ocean
environment. Sunlight, oxygen, nitrogen, water, and a wide range of salts and minerals are
necessary for the survival of ocean life forms. The pressure and density of seawater also influence the kinds of marine life forms that will thrive in different ocean environments.
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Figure 1. Global seafloor topography. This map was made by combining ship depth soundings with
gravity data from Earth-orbiting satellites. Relatively shallow continental shelf and slope areas are colored pink and yellow, as are undersea island chains. Mid-ocean ridge systems are generally yellow-togreen. The deeper ocean, such as trenches, is colored blue and purple. The topography of landmasses is
also shown.
Hydrothermal Vents
The inorganic components of seawater are in
constant flux. Perhaps the most spectacular example of physical change in the ocean environment
is seafloor spreading. New layers of crust continually form along mid-ocean ridges that run like seams
along the ocean floor [Fig. 1].
Hot magma, driven upward through volcanic
vents on the seafloor, slowly spreads out, cools, and
eventually creates new crust. Spreading occurs at
rates comparable to the rate of growth of your fingernails, about 6 cm (2.4 in) to 12 cm (4.8 in) per
year.
Figure 2. Tube worms near a hydrothermal
Strange and unexpected life forms have been vent. They are nourished by chemosynthetic
recently discovered near many such volcanic vents, bacteria that live in their tissues.
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including sulfur-eating bacteria and giant tube worms [Fig. 2]. These are the most complex
communities whose original energy source is not the Sun. Before this discovery, all other known
communities which included multicellular organisms relied on photosynthesis rather than chemosynthesis.
Upwelling and El Niño
In any given ocean location, the amount of dissolved minerals, salts, and gases will change
constantly over time, but will generally remain close to long-term average values. Under some
conditions, high concentrations of inorganic nutrients are carried from depth to the surface in a
process called upwelling. Because of the high nutrient concentrations, tiny algae grow, which are
eaten by microscopic crustaceans, and these in turn become food for fish. Thus, these upwelling
regions are often the location of important fisheries [Fig. 3].
El Niño conditions bring unseasonably warm water to the eastern equatorial Pacific Ocean.
This raises the temperature of the typically cold waters off the west coasts of Chile and Peru. In
Figure 3. Map of global ocean fisheries. Over 90% of fisheries are found in coastal waters less than 200
km (124 mi) from shore. Many fisheries (e.g., along the west coasts of North and South Americas, Africa)
are associated with vigorous coastal upwelling, which provides the nutrients needed to sustain large populations of fish.
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addition to altering weather patterns, El Niño often causes major reductions in marine life populations off the west coasts of North and South America by displacing nutrient-rich cold water. In
1972, an El Niño, combined with excessive fishing pressure, led to the collapse of the Peruvian
anchovy fishery.
THE LONG-TERM HISTORY OF OCEAN LIFE
First let us consider how we know about the distant past history of ocean life, and then we
will discuss in general what that history has been.
Most of the species that ever lived on Earth are now extinct. For example, many species of
dinosaurs lived on land and in the seas over a period of about 180 million years. But no dinosaurs exist today. To learn about the distant past, paleontologists search ancient rocks for fossils
of land-dwelling plants and animals. Similarly, ocean scientists study rocks from the sea and
ocean sediments to find evidence of ancient ocean life forms. By reading the fossil record, the
history of life on land and in the oceans can be reconstructed.
One of the principles that scientists use to guide their understanding of long-term geologic
change is the principle of superposition. This is based on the principle that, in most cases, geological strata found above other layers are younger. Conversely, older strata are deeper than younger
ones. For example, in Arizona’s distinctly layered Grand Canyon [Fig. 4], rocks found at high
elevation are younger than those near the canyon’s bottom. The superposition principle is a
very powerful technique for developing an understanding of
the evolution of life over time in
stratigraphic sequences. Suppose
you find one type of fossil in
older rock strata but not in
younger ones. You then expand
your study across the globe and
always find the same result.
From this, you may reasonably
conclude that plant or animal became extinct, and you can begin
to set some boundaries on when
the extinction occurred.
Working in the opposite direction is also a powerful tool for
dating rocks. Suppose you
know when a certain species
lived, then finding a member of
that species in rock strata tells
you the age of that layer. The
geologic time scale defines various
time periods in Earth’s history
Figure 4. Example of stratigraphy (layering). These rock layers
exposed along the Colorado river were formed at the bottom of the
ocean, and are now visible on land due to millions of years of tectonic processes.
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based upon plant and animal fossils
from the stratigraphic record.
As opposed to dating rock layers
relative to each other, radiometric dating can determine rock ages in an absolute way. Some heavy elements,
such as uranium, decay into lighter elements through a process called
nuclear fission. A given heavy element
will have several isotopes that undergo
radioactive decay. Virtually all rock
samples contain at least minute
amounts of radioactive isotopes. An
isotope that decays is called the “parent” isotope, whereas the isotope it
decays into is called the “daughter”
isotope. The rates at which various
radioactive isotopes decay is known.
Some decay in a matter of days and
others in millions or billions of years.
By measuring the ratio of parent to
daughter isotopes in a rock sample,
and knowing the isotope’s decay rate,
we can closely determine when the
rock developed into its present form.
Where do we find these rock layers, particularly ocean rocks, to analyze? Some rocks used to lay as sediments on the bottom of the ocean but
have been pushed into mountains on
land primarily through long-term tectonic processes [Fig. 4]. Other such
outcrops exist in ancient ocean areas
such as parts of the Midwestern
United States. Studying these types
of sedimentary deposits can give us a
window into the history of ocean life.
Figure 5. Glomar Challenger deep-drilling rig and core
sample. Specially designed ship was able to drill the ocean
floor while floating over three miles above. It was succeeded
in 1985 by the larger JOIDES Resolution.
Of course, a lot of oceanic crust is still under the ocean. Core samples of ocean sediment and
rocks are obtained by various means, including drilling ships [Fig. 5]. Deployed from the ship, a
hollow drill penetrates the ocean floor, drawing layers of sediment into a cylindrical tube. The
most recent deposits of sediment form the uppermost layers. The deeper the drill penetrates, the
further back in geologic history we sample.
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THE ORIGIN OF LIFE IN THE OCEAN: WHERE IT ALL STARTED
The fossil record has been the basis for our understanding of the origin of life. Evidence
supports that life began in the ocean about 3.5 billion years ago. Stromatolites, which were layered mats of photosynthetic bacteria, are one of the oldest known fossils. However, there is also
evidence that photosynthesis occurred in sediments of even greater age. Between the time of
stromatolites and the much later appearance of large multi-celled organisms, life was likely dominated by single cells. However, the size and simplicity of one-celled organisms makes recognizing their fossil remains very difficult.
About 600 million (or 0.6 billion) years ago, most of the presently known groups of invertebrates appeared for the first time. Common to all of the oldest fossil organisms is that they were
adapted to life in the water. In fact, the similarity between the salt content of most organisms’
internal liquids and of seawater, together with the fossil evidence, support the belief that life
developed in oceans or pools.
The atmosphere of early Earth did not have today’s protective layer of oxygen and ozone, so
Earth’s land surfaces must have been bombarded by ultraviolet radiation; a type of radiation
which destroys most organic molecules. Water provided protection from this hazardous short
wavelength radiation. It was not until about 425 million years ago that the atmospheric oxygen
level was sufficiently high so that living organisms could leave the protective water to live on
land. The oxygen build-up was a joint effect of oxygen production from photosynthetic activity
and the disassociation of water.
To better understand the timing of fossil record, let’s suppose that life on Earth began at
midnight and lasted only 24 hours. The oldest multi-celled organisms did not appear until 7:20
in the evening, plants started living on land at around 9 P.M., and
fish flourished in the period 20
minutes afterwards. Dinosaurs
and early mammals roamed the
earth at 9:40 P.M., and man appeared only 2.4 seconds before the
end of the day. Life on Earth consisted of single-celled organisms
for the first 20 hours and during
much of it, bacteria reigned supreme!
TODAY’S DIVERSITY OF LIFE
Today’s oceans are populated
with a great diversity of life [Fig.
6], ranging from the tiny bacteria
to the largest animal that ever existed, the great blue whale. Life
Figure 6. Diversity of ocean life.
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in the ocean adapts to water motion and the patterns of physical and chemical properties. On a
long time scale, decades to hundreds of years or more, the distribution of species is driven by
natural selection. This is the process by which organisms that are best able to adapt to changing
environmental conditions prevail. For example, species that are adapted to cold water will likely
survive glacial periods while the tropical ones will not. In some unusual cases, humans can help
to preferentially “select” certain characteristics within a species. For many generations, superstitious Japanese crab fishermen would examine their catch and throw back a rare variety of crab
whose shell or carapace resembles a human face. After many decades of this practice, this formerly rare variety of crab began to dominate some Japanese fishing bays.
The panorama of species that we see today is but a small portion of all species that have
existed since the beginning of life. In fact 99.9% of all species for which we have a record are now
extinct. Perhaps it is the fate of all organisms to become extinct, the ultimate consequence of
their changing environment. When the environmental change is too rapid to be compensated for
by mutation or migration, the species cannot survive.
EXTINCTIONS
In general, extinction occurs gradually with time and occurs at the level of individual species.
However, there are examples in the fossil record of mass extinctions that wiped out many species
in a relatively short period of time. Of course a short period of time in the geological record is
about half a million years!
A well-known example of
mass extinction occurred at the
transition between Cretaceous
(K) and Tertiary (T) periods,
about 66 million years ago. Most
of the extremely diverse marine
life, such as a whole group of onecelled organisms and invertebrates, were lost during this “KT extinction.” The K-T extinction
also coincided with the demise of
the dinosaurs. Although this
topic has received considerable
attention by scholars, they have
yet to completely agree on exactly
what happened and why.
Near the K-T extinction
boundary, global stratigraphic
records show concentrations of
the element iridium that are
about twenty times normal. Because the iridium concentration
Movie. Earth’s orbital parameters. Three important variables
are (1) the precession of Earth’s rotational axis, (2) the tilt of Earth’s
rotational axis, and (3) the elliptical path of Earth around the Sun,
or orbit eccentricity.
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in meteors is higher than that on Earth, this sharp worldwide increase suggests an extraterrestial
origin. One proposed theory is that an asteroid, approximately 10 km in diameter, crashed onto
Earth’s surface. Such an impact would have created clouds of dust and particles capable of
blocking out light for months to years. This condition would cool Earth’s atmosphere and inhibit photosynthesis.
Opponents of the asteroid impact theory suggest that there is no evidence of a sudden extinction of land plants, as would be expected from such a long-term lack of light. Moreover, unlike
the sudden extinctions in the marine realm, the disappearance of the dinosaurs appears to have
been relatively gradual. Therefore, details of the K-T extinction are still under discussion. Interestingly, mass extinctions are observed in the fossil record approximately every 26 million years.
Some theorize that this may be related to cyclic shifts in the distances between Earth, its neighboring planets, and the Sun [Movie].
CONCLUSION
The various life forms that populate Earth’s oceans form a complex and dynamic system.
Interactions among different organisms, as well as interactions between marine life and the physical--or non-living--ocean environment, are responsible for the patterns of change we have observed in marine life. Some of the changes in ocean life are relatively short-term, such as those
changes caused by El Niño events. Other ocean changes, such as the evolution of species, occur
over periods of millions or billions of years, and must be inferred from the fossil record and other
indirect evidence.
VOCABULARY
algae
chemosynthesis
dynamic
extinct
inorganic
magma
mutation
nutrients
radiometric dating
species
superposition, principle of
ultraviolet
asteroid
crustacean
evolution
fossil
invertebrate
mid-ocean ridge
natural selection
ozone
seafloor spreading
strata
tectonic
upwelling
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carapace
disassociation
El Niño
geologic time scale
isotopes
migration
nuclear fission
photosynthesis
sediment
stratigraphy
timescale