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I. Early Atmosphere
The atmosphere we enjoy today is radically different from the atmosphere that formed with the Earth billions of
years ago. And yet, the Earth’s early atmosphere somehow transformed into the life giving atmosphere we enjoy
today.
The Earth formed with the Sun 4.6 billion years ago. At this point, it was nothing more than a molten ball
of rock surrounded by an atmosphere of hydrogen and helium. Because the Earth didn’t have a magnetic
field to protect it yet, the intense solar wind from the young Sun blew this early atmosphere away.
As the Earth cooled enough to form a solid crust (4.4 billion years ago), it was covered with active
volcanoes. These volcanoes spewed out gasses, like water vapor, carbon dioxide and ammonia. This early
toxic atmosphere was nothing like the atmosphere we have today.
Light from the Sun broke down the ammonia molecules released by volcanoes, releasing nitrogen into the
atmosphere. Over billions of years, the quantity of nitrogen built up to the levels we see today.
Although life formed just a few hundred million years later, it wasn’t until the evolution of bacteria 3.3
billion years ago that really changed the early Earth atmosphere into the one we know today.
1. The timeline of life on Earth:
a. Anaerobic bacteria: Scientists have fossil evidence of bacterial life on Earth ~3.8 billion years ago. At
this time, the atmosphere of the Earth did not contain oxygen, and all life (bacterial cells) was anaerobic.
b. Photosynthetic bacteria: About ~3.2 billion years ago, fossil evidence of photosynthetic bacteria, or
cyanobacteria, appears. These bacteria use the sun's energy to make sugar. Oxygen, released as a
byproduct, began to accumulate in the atmosphere. However, oxygen is actually pretty toxic to cells, even
our cells! As a result, anaerobic cells were now a disadvantage in an oxygen-containing atmosphere, and
started to die out as oxygen levels increased.
c. Aerobic cells appear in the fossil record shortly after that (~2.5 Billion years ago). There cells were
were able to use that 'toxic' oxygen and convert it into energy (ATP) and water. Organisms that could
thrive in an oxygen-containing atmosphere were now 'best suited to the environment'.
Origin of Life: The Heterotroph Hypothesis
Life on Earth began about 3.5 billion years ago. At that point in the development of the Earth, the atmosphere
was very different from what it is today. As opposed to the current atmosphere, which is mostly nitrogen and
oxygen, the early Earth atmosphere contained mostly hydrogen, water, ammonia, and methane.
In experiments, scientists have showed that the electrical discharges of lightning, radioactivity, and ultraviolet
light caused the elements in the early Earth atmosphere to form the basic molecules of biological chemistry,
such as nucleotides, simple proteins, and ATP. It seems likely, then, that the Earth was covered in a hot, thin
soup of water and organic materials. Over time, the molecules became more complex and began to collaborate
to run metabolic processes. Eventually, the first cells came into being. These cells were heterotrophs, which
could not produce their own food and instead fed on the organic material from the primordial soup. (These
heterotrophs give this theory its name.)
The anaerobic metabolic processes of the heterotrophs released carbon dioxide into the atmosphere, which
allowed for the evolution of photosynthetic autotrophs, which could use light and CO2 to produce their own
food. The autotrophs released oxygen into the atmosphere. For most of the original anaerobic heterotrophs,
oxygen proved poisonous. The few heterotrophs that survived the change in environment generally evolved
the capacity to carry out aerobic respiration. Over the subsequent billions of years, the aerobic autotrophs and
heterotrophs became the dominant life-forms on the planet and evolved into all of the diversity of life now
visible on Earth.
Miller/Urey Experiment
By the 1950s, scientists were in hot pursuit of the origin of life. Around the world, the scientific
community was examining what kind of environment would be needed to allow life to begin. In 1953,
Stanley L. Miller and Harold C. Urey, working at the University of Chicago, conducted an experiment
which would change the approach of scientific investigation into the origin of life.
Miller took molecules which were believed to represent the major components of the early Earth's
atmosphere and put them into a closed system
The gases they used were methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O). Next, he ran
a continuous electric current through the system, to simulate lightning storms believed to be common on
the early earth. Analysis of the experiment was done by chromatography. At the end of one week, Miller
observed that as much as 10-15% of the carbon was now in the form of organic compounds. Two percent
of the carbon had formed some of the amino acids which are used to make proteins. Perhaps most
importantly, Miller's experiment showed that organic compounds such as amino acids, which are essential
to cellular life, could be made easily under the conditions that scientists believed to be present on the early
earth. This enormous finding inspired a multitude of further experiments.