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Life on Earth Topic 2 : Evidence for the origin of life. Part of the Evolution of Australian Biota Module Biology in Focus, Preliminary Course Glenda Childrawi and Stephanie Hollis. DOT POINT.
Part of the Evolution of Australian Biota Module
Biology in Focus, Preliminary Course
Glenda Childrawi and Stephanie Hollis
The approximate age of the Earth, based on geologic, magnetic, radiographic and palaeontological studies, is 4.5 billion years. During the Hadean eon (4.5-3.8bya) was the formation of the Earth.
4 billion years ago the Earth was a very different place than it is today. The sun was only 70% of its present strength, Earth was spinning much faster and day was only 18 hours long and there was no free oxygen in the atmosphere How was anything to survive on such an inhospitable planet?
We know Earth transformed from a gaseous cloud into a solid body. The heavier molten elements, like iron, sank down to become the core. The lighter elements, like silica, rose to the surface and formed the crust.
But how did we get our atmosphere?
Intense heat and pressure, due to the extreme forces of gravity, below the surface cause the material to remain in a molten state. We know that hot material rises (think of boiling water), and so does this molten material in the centre of the Earth.
Once this molten material reaches the surface we get volcanoes! These amazing geologic features not only bring to the surface fresh minerals and rock, they also bring gases trapped within the material.
It was these gases that formed the original atmosphere of our planet.
The process of volcanoes releasing gases into our atmosphere is called outgassing and included:
The atmosphere was anoxic meaning it had no free Oxygen (O₂) as all the oxygen was bound within other compounds such as water and carbon dioxide. This meant that there was no ozone (O₃) layer exposing the Earth’s surface to ultraviolet radiation.
Most of the hydrogen gas escaped into space, as happens today, whereas the other gases accumulated and remained in the atmosphere.
Early Earth provided an environment in which the production of organic carbon-containing molecules could have happened fairly easy. The energy for driving these reactions could have come from a number of sources which include:
In Early stages of Earth’s formation, organic molecules would have most likely formed in the lower atmosphere or at the Earth’s surface. We’ll look at the stages of change thought to occur in the early Earth.
Dense clouds formed in the steamy atmosphere. These clouds were formed of water from meteorites and hydrated minerals. These clouds then formed a reflective shield from the suns penetrating heat
About 500 million years later, meteorite impacts declined due to this protective layer and the friction caused on entry. The Earth cooled and the temperature fell below 1000C, forming a stable rocky crust.
The release of gases from volcanoes increased air pressure in the atmosphere. When the temperatures cooled, this assisted in causing the immense clouds of water vapour to condense into liquid and fall as rain.
Rain would have washed organic molecules into lakes and ponds rich in dissolved minerals. This created an environment for reactions to occur which produced new organic molecules and compounds.
Carbon Dioxide dissolves readily in water to form carbonic acid (H₂CO₃). It would have been flushed out of the atmosphere by rain and into the oceans where it reacted with calcium to form calcium carbonate (CaCO₃)
At first, the rain evaporated as it fell on the hot rock surface but the evaporation gradually cooled the crust until water could accumulate in the lower regions of the Earth’s surface forming oceans. The first rives were created where the water, dissolving and transporting minerals along the way, eventually ran back into the oceans.
The dissipation of heat into space cooled the Earth, causing crust fragments to become numerous enough to form a first thin, solid cover.
Over the next 3.5 billion years, the amount of carbon dioxide in the atmosphere was reduced as it became incorporated in rocks (limestone). The main gas remaining in the Earth’s atmosphere was nitrogen.
The composition of Earth’s atmosphere today is somewhat different to that proposed on early Earth. Today it consists of 78.1% nitrogen, 20.9% oxygen and 1% trace elements such as carbon dioxide, water vapour, methane, hydrogen and ozone.
There is much known about the composition and conditions of Early Earth, however there are a lot of unanswered questions. Scientists continue to search for more evidence reflecting the conditions of early Earth that may have existed when life began.
If these conditions are known, then we may perhaps discover more about the building blocks from which life began….