Evolution of the Atmosphere

The first atmosphere on Earth was mostly carbon dioxide and water vapour. It was made of up gases given off from volcanoes and comets colliding with Earth. The carbon dioxide had a warming effect and provided “food” for primitive bacteria and algae the first “life”. They consumed carbon dioxide and released oxygen as waste. Colonies of algae formed stromatolites which produced more oxygen.

 By 1800 million years ago, when the first animals appeared, it was 60 percent of its present level. Solar radiation then turned some into ozone in the stratosphere. Insignificant invertebrates, the protochordates, have a body plan very similar to that of larval fish and amphibians, which were probably their successors in the Cambrian.  By the Devonian period the age of the fist was at its height. The Jawless fish such as Pieraspis which had bony plates and shovel like mouths. Many fish were able to adopt to fresh water. The Devonian also saw first sharks. Since their skeletons are made of cartilage, there are fewer fossils, but their sharp teeth often survive.

 During the Devonian, the first fishes came out of the water. Lobe finned fish developed auxiliary lungs and could haul them out of the water if supplies of oxygen ran low. From them evolved the lung fish and an even closer relative that still lives in the Indian Ocean – the coelacanth. By the Carboniferous, amphibious creatures called labyrinthodonts, with fully formed feet, had established themselves on land.

Geological Timeline

If the evolution of life on Earth seems to go back a long way, it may be put into a geological context by comparing the age of the planet with the life time of a person now 47 years old. Fossils only tell us about life since the Pre Cambrian era began 600 million years ago, but by then our person would have already celebrated their 40^th birthday. Soon after, multi cellular life in the sea diversified into thousands of species. Two years later on the human timescale, planets and insects emerged onto land followed by amphibious animals. Then things began to speed up. It is only a year since the age of the dinosaurs, a week since that last ice age and a mere four hours since our own species, Homo sapiens, first walked on the planet.

During the Earth’s life time, the Solar System has moved around the galactic centre about 25 times. The ocean crust has been recycled 50 times. The continents have accumulated, crashed into each other and broken apart. Landscapes have been eroded and weathered. And the atmosphere has been altered by life forms. Now the globe is being transformed by humans.  Judging from the evolutionary path of the Sun, the Earth has five billion years to go.

 The first living things were microscopic bacteria and protozoa. The first visible sign of life was probably a film of algae. Some algae or filamentous bacteria grew in large mats in shallow after near the tide line, binding sand among them to form layered mounds. Still found growing in warm seas today, fossil stromatolites from the oldest macroscopic fossils in 3,500 million years old deposits in Australia.

Rock Fossils and age of the Rock

Sedimentary rocks contain a record of the changing forms of life on earth. It is not a complete record; most creatures get eaten, rot, or are otherwise destroyed. Many do not have any resistant parts and, of those that do, the remains may later be eroded or never found. Although material from the original shell or bone may survive, it may alternatively be replaced by other minerals or leave just a faint imprint in the rock.

Even so, fossils provide a remarkably detailed picture of life on Earth. It is a picture of rapid diversification and great inventiveness to suit every ecological niche, punctuated by rapid extinctions when times get hard. Thus the changing fossil record provides a powerful means of dating rocks. So called zone or index fossils have been picked as key markers for each time. Ideally they are common, free swimming species that can help correlated rocks of the same age wherever in the world they are found. Sometimes, finding assemblages of different fossils together at one location can narrow the time down further.

Fossils and stratigraphy reveal relative ages of rocks, but not an absolute age. For that, geologists have other techniques. They can simply count the growth rings in trees. Modern mass spectrometers can measure even the slightest trace of an isotope, so tiny crystals can each be dated. Thus the oldest minerals on Earth were found: grains of zircon over four billion years old, eroded and redeposit later. The key to accuracy is purity of the sample. If the sample comes from a lava, its melting will have released any previous gas and reset the clock.

Rock Strata

Reading the stratigraphical notebook is not just a question of opening the pages.  A geological map of the world today is in effect a patch work of different environmental conditions, and the same was true at each stage in the past. The limestone was forming in one place a hundred million years ago does not imply that limestone was forming everywhere at that time. While rocks are deposited in one place, another place may be uplifted into mountains and eroded.

 There are however, gaps in the record, and things are not always what they seem. Although younger rocks are deposited on top of older ones, folding and faulting can be so intense that the younger rocks end up underneath the old. Layers can be folded up to steep angles, or the original layers may be sloping such as current bedding at a river delta or a continental slope. Thick shale beds may have take a few hundred thousand years to form and be tens of meters thick; in the same strata there may be another layer only a few centimeters thick that was five million years in the making. So thickness is no certain clue to age; neither is apparent depth of water. Sea level can change by tens of meters, land level by hundreds. Nor is present latitude much help, when continents have skated across the globe. Britain was once on the equator and there were glaciers in what is now the Sahara desert. But there are plenty of clues for geologists and with their knowledge of processes at work can compute the stratigraphical evidence from around the world to confirm their theories on geological maps.