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PRECAMBRIAN
ERA
The Earth's surface was formed some 4600 millions years ago. In
its earliest solid form our planet was one vast inhospitable
desert and looked very different from today. Peaks and hills were
of sharper outline; slopes were more abrupt. Without atmosphere or
water, there was nothing to weather the rocks and wash the debris
down into the valleys; and without water-borne sediment there was
no soil. But in places the Earth displayed very high volcanic
activity. As the crust was constantly broken down and remade, whole
regions rose and sank. New rocks appeared, only to be
remelted, so that their entire structure altered. Some
4000 million years ago dense clouds hung over the bare Earth,
often blocking out the sun's light. The Earth's surface
experienced gigantic storms, while streams of molten lava gushed
forth in many places. Volcanic activity released increasing amounts
of gas and vapor from inside the Earth and these gradually formed
an atmosphere too heavy to escape into space. Water began to
appear in depression in the Earth's surface, first as pools, than
as large sheets of water. In time these united to form the first
oceans, while heat and cold, wind and water, disintegrated the
rocks and produced the first sediments. Life itself did not yet
exist, but the organic compounds needed to build living matter
were already present. Life originated on Earth
about 3500 million years ago. In time, the first plants appeared,
followed by the first invertebrates (animals without backbones).
By about 2000 million years ago the shallower seabed was covered
with different primitive single-celled organisms. These included
blue-green algae, simple prokaryotes that marked an enormous step
forward in the evolution of life, since they contain a complex
green pigment called chlorophyll. This enables plants, using the
energy of sunlight, to manufacture substances from non-living
matter, such as carbon dioxide and water, for their food. In the
process known as photosynthesis, green plants release oxygen,
probably not present in the atmosphere before their time.
TIME
- 4600-570 Million Years Ago |
