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water that formed the oceans. For the next 1,3 billion years (3,8 to 2,5 billion years ago), called the Archean
Period, first life began to appear (at least as far as our fossil records tell us... there may have been life before
this!) and the world's landmasses began to form. Earth's initial life forms were bacteria, which could survive in
the highly toxic atmosphere that existed during this time. In fact, all life was bacteria during the Archean Pe-
riod.
Toward the end of the Archean Period and at the beginning of the Proterozoic Period, about 2,5 billion
years ago, oxygen-forming photosynthesis began to occur. The first fossils, in fact, were a type of blue-green
algae that could photosynthesize.
Some of the most exciting events in Earth's history and life occurred during this time, which spanned about
two billion years until about 550 million years ago. The continents began to form and stabilize, creating the su-
per continent Rodinia about 1.1 billion years ago. (Rodinia is widely accepted as the first super continent, but
there were probably others before it.) Although Rodinia is composed of some of the same land fragments as the
more popular super continent, Pangea, they are two different super continents. Pangea formed some 225 million
years ago and would evolve into the seven continents we know today.
Earth's atmosphere was first supplied by the gasses expelled from the massive volcanic eruptions of the
Hadean Era. These gases were so poisonous, and the world was so hot, that nothing could survive. As the planet
began to cool, its surface solidified as a rocky terrain, much like Mars' surface and the oceans began to form as
the water vapor condensed into rain. First life came from the oceans. Free oxygen began to build up around the
middle of the Proterozoic Period around 1,8 billion years ago – and made way for the emergence of life, as we
know it today. This event, of course, created conditions that would not allow most of the existing life to survive
and thus made way for the more oxygen dependent life forms.
By the end of the Proterozoic Period, Earth was well along in its evolutionary processes leading to our cur-
rent period, the Holocene Period, also known as the Age of Man. Thus, about 550 million years ago, the Cam-
brian Period began. During this period, life "exploded" developing almost all of the major groups of plants and
animals in a relatively short time. It ended with the massive extinction of most of the existing species about 500
million years ago, making room for the future appearance and evolution of new plant and animal species.
And then, about 498 million years later – 2,2 million years ago – the first modern human species emerged.
11. THE OZONE LAYER
Although ozone (O
3
) is present in small concentrations throughout the atmosphere, most ozone (about
90 %) exists in the stratosphere, in a layer between 10 and 50 km above the surface of the earth. This ozone
layer performs the essential task of filtering out most of the sun's biologically harmful ultraviolet (UV-B) radia-
tion. Concentrations of ozone in the atmosphere vary naturally according to temperature, weather, latitude and
altitude. Furthermore, aerosols and other particles ejected by natural events such as volcanic eruptions can have
measurable impacts on ozone levels.
In 1985, scientists identified a thinning of the ozone layer over the Antarctic during the spring months,
which became known as the "ozone hole". The scientific evidence shows that human-made chemicals are re-
sponsible for the creation of the Antarctic ozone hole and are also likely to play a role in global ozone losses.
Ozone Depleting Substances (ODS) have been used in many products which take advantage of their physical
properties (e.g. chlorofluorocarbons (CFCs) have been used as aerosol propellants and refrigerants).
CFCs are broken down by sunlight in the stratosphere, producing halogen (e.g. chlorine) atoms, which sub-
sequently destroy ozone through a complex catalytic cycle. Ozone destruction is greatest at the South Pole
where very low stratospheric temperatures in winter create polar stratospheric clouds (PSCs). Ice crystals
formed in PSCs provide a large surface area for chemical reactions, accelerating catalytic cycles. The destruc-
tion of ozone also involves sunlight, so the process intensifies during springtime, when the levels of solar radia-
tion at the pole are highest, and PSCs are continually present.
Although ozone levels vary seasonally, stratospheric ozone levels have been observed to be decreasing an-
nually since the 1970s. Mid-latitudes have experienced greater losses than equatorial regions. In 1997, the Ant-
arctic ozone hole covered 24 million km
2
in October, with an average of 40 % ozone depletion and ozone levels
in Scandinavia, Greenland and Siberia reached an unprecedented 45 % depletion in 1996.
The amount of UV reaching the earth's surface has been shown to correlate with the extent of ozone deple-
tion. In 1997, UV-B levels continued to rise at a rate of 2 % per annum. Increased UV levels at the earth's sur-
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