Visions of the Cosmos
Life Among the Stars
Earth - The Genesis of a Living World
Let me lead you on a journey of the mind backwards through the mists of time across all the billions of yesteryears to an age when the Earth and the other planets of the Solar System were born from the proto-solar disc which surrounded our newly created Sun.
The Sun itself was formed out of interstellar dust and gas along with a number of other stars in a stellar nursery. Somewhere a little over four and a half billion years ago in the midst of a gas and dust cloud in an ordinary spiral galaxy we call the Milky Way a huge mass of gas and dust was condensing under gravity. Unlike the cold temperatures of interstellar space the central regions of its inner core were rising towards fifteen million degrees Celsius. Soon in that vast globe of matter thermonuclear reactions were beginning to occur. The massive ball of plasma in its inconceivable metamorphosis gave birth to a star - our Sun The thermonuclear reactions which had now begun would take it on a long journey through time and it would continue its thermonuclear burning for over ten thousand million years. Present theories consider planetary formation to be a natural and common process that takes place during star formation. A proto-planetary disc of gas and dust formed around our new born star. Small aggregates of dust and icy grains condensed within the disc. These aggregates went on to form larger and larger objects - this way small planets were formed. They are called planetisimals. The planetisimals underwent collisions with each other and swept up smaller objects and so the planetisimals increased in mass to finally coagulate to form the Earth and its sister planets.
The processes that formed our Earth began a little over 4.6 billion years ago
In this section of the web-site we are concerned with the long journey through time that our Earth has taken and the wonders that have arisen upon its surface in the four and a half billion years of it existence.
The study of the rocks is called Petrology.
The study of the arrangement of the rocks in the Earth's crustal surface is called Stratigraphy.
The study of the fossils in the rocks is called Palaeontology.
The study of the way in which the continents have moved their positions is called Plate Tectonics.
It is a study of these sciences that has told us most of which we have so far learned about the History of our Planet Earth
The Moon was formed by a different process to the other inner planets. Most authorities now accept the 'Lunar Collision Hypothesis' According to this, about 4.55 billion years ago, quite soon after most of the Earth had formed, a Mars-sized planet collided with the young Earth The collision hypothesis accounts satisfactorily for the geochemical, mineralogical and isotopic compositions of the lunar samples as represented by material returned by Apollo and Luna missions, and by lunar meteorites. Many authorities believe that most of the iron and nickel core material from the smaller planet poured into the core of the Earth. Some of the rocky material was incorporated into the Earth's mantle and some of it formed a ring around the Earth which in a comparatively short period of time formed the Moon. At this stage the Moon was very close to the Earth but from the moment of its formation it began to recede and has been receding ever since. The Earth and Moon would have been extremely hot and volcanic (see illustration below) and were both being subjected to constant bombardment by asteroids, meteorites and comets.
The illustration shows an artists impression of the Earth in that early time referred to at the Hadean Period. Note the apparently huge size of the Moon and the islands of higher melting rocks floating in a magma sea. The ‘sky’ is full of meteors and comets as they bombard the ancient Earth.. Acknowledgements to Watson and Barnet – The World we live in 1975 and http://www.palaeos.com/Hadean/Hadean.html
The scars left by this intense bombardment, which continued for at least 700 to 800 million years have been almost totally erased on the Earth by subsequent reworking of the crust. However the resulting impact craters and lava flows are perfectly preserved on the Moon and on many other bodies in the Solar System such as Mercury whose geological evolution ended long ago. The result is what we see in the imaginative reconstruction on the right. Note that in those early Hadean times the Moon was only 16,000 kilometres from the Earth compared to 384,000 kilometres to-day. For that reason it occupied a large part of the sky. Truly a nightmare landscape!
The evidence of the early bombardment is clearly seen on the Moon and Mercury. All evidence of the early cratering has been erased on the Earth. Until about 3.9 billion years ago the planets were sweeping out debris from their orbits and the bombardments were slowing down. Then evidence, mainly from the Moon, indicates that there was a period in which the planets of the inner solar system were subjected to very heavy bombardment by comets and/or meteorites. This period is referred to as the late heavy bombardment and is discussed in a separate page of the web in the second chapter on Planetary Science and Space Science under the Solar System. The renewed bombardment diminished until a point in time about 3.85 billion years ago. The term Hadean is well named for during those countless millennia the environment of our planet was indeed hellish. The earliest terrestrial environments were harsh: High levels of atmospheric carbon dioxide, methane and water vapour must have created a strong greenhouse effect, with high global temperatures. Oceans formed early, between 4,400 and 3,900, from condensation of atmospheric water vapour. Estimates suggest that the earliest oceans were hot (between 80 and 100°C) and acidic. Radioactivity was many times greater than it is at the present time and there was heavy volcanism. The first fragments of crust must have been very unstable in the early times and easily reabsorbed by the liquid mass of magma and sucked into the depths. Only, with the further cooling of the planet, might those fragments become numerous and large enough to form a first, thin, solid cover -- that is to say, a true primitive crust. This primordial crust might have developed as a warm expanse of rocks (some hundreds of degrees Celsius), interrupted by numerous large breaks, from which enormous quantities of magma continued to erupt. In Hadean times a massive moon hung in the sky raising enormous tides. The day was very short since the planet was rotating much quicker than it does to-day - huge nimbus clouds would have hidden the light of the Sun, the Moon and the stars for long periods and gargantuan storms and high winds would have screamed across the surface of the world. Every so often the air would have been rent by huge explosions as volcanoes exploded and comets and massive meteorites smashed into the planet. Huge lava fields encompassed the planet and rivers of molten rock ran across the surface. And so it was for many millions of years.
Recent discoveries in the Jack Hills area of Western Australia have cast doubts on the length of the Hadean Period. It was widely believed that the fierce conditions of the Hadean lasted until about 3.8 billion years ago. However groups of geologists led by John Valley of Wisconsin State University and Mark Harrison of the University of California in Los Angeles have discovered small crystals of a mineral called Zircon in some of the oldest rocks in Western Australia. Studies of oxygen isotope ratios and titanium concentrations within the zircons points to a very early age for the zircons of about 4.4 billion years. This indicates that the Earth's crust cooled much quicker than had previously been thought and that conditions may have been possible for life only two or three hundred million years after the creation of the Earth rather than seven to eight hundred million years as most people had previously believed. However, none of this negates the fact that the early Earth was a very inhospitable place.
The Coming of Life - The Archaen
Opinions differ as to when life began. A rough estimate is usually about 3.8 billion years ago. However the recent discovery of the zircons, which indicate that relatively hospitable conditions may have existed even 4.3 billion years ago, has led some authorities to suggest that the dawn of life may have been much earlier than previously believed, even if it had to endure the late heavy bombardment The vast period of time that was occupied by the earliest forms of life is referred to as the Archaen.
All we can say at present is that in the remote past a strange and marvellous chemistry began to evolve. The exact details of how atoms of carbon, nitrogen, oxygen, hydrogen, phosphorous and sulphur were selected to form large self-replicating molecules in a watery solution of iron, sodium, potassium, calcium , magnesium and chloride ions is a mystery that is hidden in the mists of time. This question was surveyed at a discussion meeting entitled 'Conditions for the Emergence of Life on the Early Earth' at the Royal Society in February 2006. The introduction is available on line - the hyperlink is given in the section at the end of the website under external hyperlinks. The full proceedings are for sale but are also available in scientific libraries.
Current theories suggest that the first primitive organisms evolved deep in the ocean floor at volcanic thermal vents. They belonged to groups of single celled anaerobic bacteria and archae. Unlike present day organisms they relied on chemical reactions involving molecules of hydrogen sulphide and other sulphur containing molecules and metallic ions. They lived in an environment free of molecular oxygen - in fact oxygen would have been highly poisonous to them. Although to-day the vast majority of organisms live in oxygen rich environments, there are still bacteria and archae which thrive in oxygen free environments. They are known as anaerobes.
The Proterozoic Era
The period of Earth's history that began 2.5 billion years ago and ended with the Cambrian Explosion 544 million years ago is known as the Proterozoic. Near the beginning of the Proterozoic, stable continents first appeared and began to accrete, a long process taking about a billion years.
It is divided up into:
Paleoproterozoic (2.5 to 1.6 billion years ago)
Mesoproterozoic (1.6 billion to 900 million years ago)
Neoproterozoic (900 to 544 million years ago).
During a large part of the history of the Earth it is known that the continents appeared to drift over the face of the planet by a process which used to be called 'continental drift' whose main discoverer was Alfred Wegener. Now that the mechanism is better understood it is referred to as plate tectonics. Plate Tectonics and its importance in understanding continental drift, earthquakes, tsunamis, volcanic activity, sea floor spreading, mountain building and the creation of many island archipelagos will be frequently referred to in the web-site. A separate page of the web-site is devoted to the study of Plate Tectonics
Gas Warfare - The Oxygen Revolution and the Coming of the Cyanobacteria
Cyanobacteria are aquatic and photosynthesize. Because they are bacteria, they are quite small and usually unicellular, though they often grow in colonies large enough to see. They have the distinction of being the oldest known fossils, and have been found in some of the oldest rocks in the Archaen Era around 3.5 billion years old. Until the emergence of this new kind of organism there was virtually no free O2 (oxygen molecules) in the atmosphere and for the types of life that had so far appeared on the planet oxygen was a deadly poison just as chlorine gas is for us. The emergence of the Cyanobacteria began the very slow process of changing the atmosphere of the Earth forever. During the day they took in CO2 and water and converted them to sugars and molecular oxygen. At night they reversed the process but the overall effect was an increase of O2 in the atmosphere. This resulted in the death of anaerobes to whom oxygen was a deadly poison. The coming of free oxygen resulted in the evolution of a whole plethora of bacteria, fungi, plants and animals to whom oxygen was the very breath of life. The aerobes as we call them acquired one enormous advantage - oxygen metabolism enabled them to extract sixteen times as much energy from 'food' as the process used by the anaerobes.
It was not until the Proterozoic that the cyanobacteria reached their pre-eminent importance and that the amount of free oxygen in the atmosphere became abundant. Cyanobacteria used to be called blue green algae but are now clearly known to be bacteria and not plants. Many of them grew in colonies on rocks together with other organisms-the whole colony is called a stromatolite.
Stromatolites still exist to-day in a few small isolated communities. One of the best known sites is at Sharks Bay in Western Australia
|Stromatolites at Hamelin Pool at low tide||Stromatolites at low tide Photo Stanley
|Hamelin Pool is one of only two places in the world with living marine stromatolites, or "living fossils".|
Hamelin Pool is one of only two places in the world with living marine stromatolites, or "living fossils". It also has the distinction of being Western Australia's only marine nature reserve. The stromatolites are able to survive in the area because Hamelin Pool's water is twice as saline as normal sea water. They look like rocky lumps strewn around the beach but are actually built by living organisms. Within the structures are communities of diverse inhabitants such as species of cyanobacteria with population densities of 3000 million individuals per square metre! The organisms use sediment and organic material to build stromatolites up to 1.5 metres high - up to 10 million times their size. Because they grow very slowly, a metre-high stromatolite could be about 2000 years old. When the Shark Bay stromatolites were discovered by scientists in 1956, they were the first growing examples ever recorded of structures, found fossilised in very old rocks, that had puzzled geologists for more than a century. The living microbes that built the stromatolites are similar to those found in 3500 million year old rocks, which are the earliest record of life on Earth.
Plate tectonics was active 1,300 million years ago reflecting the evolution of the earth’s lithosphere. Around this time there was one main supercontinent which is called Rodinia
As the supercontinent broke up, shallow marine environments formed at plate boundaries.
stromatolites became predominantly widespread in these areas.
Credit Hooper Virtual Museum Carleton University Ottawa Canada http://hoopermuseum.earthsci.carleton.ca/stromatolites/PROT1.htm
Although there were stromatolites as long ago as 3.5 billion years ago there were very few of them. The diagram shows that they increased in number until they became very numerous around 1 billion years ago. Oxygen levels in the Archaean had been less that 1% of present levels in the atmosphere, but by about 1.8 billion years ago, oxygen levels were greater than 15% of present levels and rising. (Holland, 1994). It is when 'gas warfare really began - most of the organisms that we are familiar with not only tolerate but require oxygen to live. However, oxygen is a powerful degrader of organic compounds. Even today, many bacteria and archae are killed by oxygen. Organisms had to evolve biochemical methods for rendering oxygen harmless. one of these methods, The coming of free oxygen resulted in the evolution of a whole plethora of bacteria, fungi, plants and animals to whom oxygen was the very breath of life. The aerobes as we call them acquired one enormous advantage - It was oxidative respiration and enabled them to extract sixteen times as much energy from 'food' as the processes used by the anaerobes.
The Snowball Earth Hypothesis
This popular theory suggests that on an at least two periods -some estimate three or four times the whole planet was completely covered by ice. These glaciations occurred between 580 million and a thousand million years ago. The two favoured periods are around 750 million and 580 million years ago. It is suggested that during the periods of intense glaciation there were clear blue skies and that the Sun's heat and light would have been radiated back into space by the intensely white ice-fields and snowfields. This situation would have seemed irreversible but the action of volcanoes would have produced massive quantities of carbon dioxide which would have caused a sudden and rapid thaw which would have produced a dangerously high temperatures. Some scientists hypothesize that the sudden shock at the end of the 580 million year ago ice age triggered of the Cambrian Explosion.
The history of life in the Cambrian Explosion and the preceding Vendian Period is dealt with in the next page of the web-site.