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Let us first consider an overview of the basic processes about how life came into being and then evolved on earth, first by means of a verbal summary and then by diagrammatic depiction in clock form. We will then consider in more detail each period and the major events which happened within each. The major steps[1] appear to have been as follows:
Earth formed: 4,550 million years ago
First life formed between 3.6 m years ago. Then:
1. inorganic chemistry
The raw chemical elements of life form – carbon, oxygen, hydrogen, nitrogen – supplied by supernova explosions.
2. organic chemistry
These raw chemicals combined into molecules such as water (H2O), ammonia (NH3) and carbon dioxide (CO2), and then into simple organic molecules like methane (CH4) and others. .. along with warmth
3. biochemistry
These organic molecules find an environment where they can react and become increasingly more complex so as to form biomolecules, the molecules that make up living beings. Water is an essential ingredient for this process. The first steps towards the chain of life in this process are the formation of amino acids.
4. first life
Somewhere around 3.6 million years ago (or earlier), possibly in a drying lagoon, a set of carbon-rich chemicals, including amino acids, reacted with growing complexity as they tried to minimize charge imbalances, creating larger and larger molecular chains. These chains combined with each other, self-organising into increasingly more complex structures. Possibly, simple carbohydrates (foods) appeared as well. Then, somehow, these chains began to develop into imperfect copies of each other
5. prokaryotic cells [2]
From this “simple beginning” to the complex proteins and nucleic acids of the first prokaryotic cells, the steps are also blurry. Perhaps shallow pools (Darwin’s “warm little ponds”) provided a high enough concentration of chemicals; reactions started and molecules grew in complexity until they became a self-sustaining reaction network, extracting energy from the environment to keep on going. At some point a membrane grew around a group of reacting chemicals, creating a protocell - a primitive cell. Isolating these chemicals behind a semipermeable membrane made of fatty molecules, isolating them from the outside environment, allowing reactions to thrive. This was the first prokaryote, the first single-celled organism.
6 eukaryotic cells
We have little understanding of the next step in life’s complexity, the emergence of eukaryotic cells from prokaryotic cells. From 3.6 m years ago, life remained unicellular until about 1.6 m years ago, that is, for roughly 2 billion year, life on Earth consisted only of single-celled organisms, albeit some organized in colonies. Eukaryotes appeared towards the end of this period, when oxygen became more abundant in the atmosphere thanks to collective efforts of the photosynthetic blue-green algae.
7. multicellular life
After about 3 billion years after life’s first known traces, a transition took place from unicellular to multicellular which also possibly also evolved through symbiotic trial-and-error processes.
8. complex multicellular life
Many scientists propose that environmental changes in the Earth played a major role in accelerating the diversity of complex multicellular organisms that climaxed during the so-called Cambrian explosion, about 550 million years ago. Chief among them were the rapid increase in oxygen availability and the advent of plate tectonics and the consequent mixing of surface and ocean chemistry.
9. intelligent life
After about 500 million years of evolving multicellular organisms, including many severe mass extinctions and climate changes, the first member of the genus Homo appeared in Africa some 4 million years ago. Intelligent life as we know it is less than 1 million years old, and has been around for less than approximately 0.02% of the Earth’s history[3].
[1] Source: Marcelo Gleiser, Imperfect Creation, Black, Melbourne, 2010, 182-186; 235-237.
[2] For the ensuing, see graphic at end of page.
[3] Ibid, 237.
Earth formed: 4,550 million years ago
First life formed between 3.6 m years ago. Then:
1. inorganic chemistry
The raw chemical elements of life form – carbon, oxygen, hydrogen, nitrogen – supplied by supernova explosions.
2. organic chemistry
These raw chemicals combined into molecules such as water (H2O), ammonia (NH3) and carbon dioxide (CO2), and then into simple organic molecules like methane (CH4) and others. .. along with warmth
3. biochemistry
These organic molecules find an environment where they can react and become increasingly more complex so as to form biomolecules, the molecules that make up living beings. Water is an essential ingredient for this process. The first steps towards the chain of life in this process are the formation of amino acids.
4. first life
Somewhere around 3.6 million years ago (or earlier), possibly in a drying lagoon, a set of carbon-rich chemicals, including amino acids, reacted with growing complexity as they tried to minimize charge imbalances, creating larger and larger molecular chains. These chains combined with each other, self-organising into increasingly more complex structures. Possibly, simple carbohydrates (foods) appeared as well. Then, somehow, these chains began to develop into imperfect copies of each other
5. prokaryotic cells [2]
From this “simple beginning” to the complex proteins and nucleic acids of the first prokaryotic cells, the steps are also blurry. Perhaps shallow pools (Darwin’s “warm little ponds”) provided a high enough concentration of chemicals; reactions started and molecules grew in complexity until they became a self-sustaining reaction network, extracting energy from the environment to keep on going. At some point a membrane grew around a group of reacting chemicals, creating a protocell - a primitive cell. Isolating these chemicals behind a semipermeable membrane made of fatty molecules, isolating them from the outside environment, allowing reactions to thrive. This was the first prokaryote, the first single-celled organism.
6 eukaryotic cells
We have little understanding of the next step in life’s complexity, the emergence of eukaryotic cells from prokaryotic cells. From 3.6 m years ago, life remained unicellular until about 1.6 m years ago, that is, for roughly 2 billion year, life on Earth consisted only of single-celled organisms, albeit some organized in colonies. Eukaryotes appeared towards the end of this period, when oxygen became more abundant in the atmosphere thanks to collective efforts of the photosynthetic blue-green algae.
7. multicellular life
After about 3 billion years after life’s first known traces, a transition took place from unicellular to multicellular which also possibly also evolved through symbiotic trial-and-error processes.
8. complex multicellular life
Many scientists propose that environmental changes in the Earth played a major role in accelerating the diversity of complex multicellular organisms that climaxed during the so-called Cambrian explosion, about 550 million years ago. Chief among them were the rapid increase in oxygen availability and the advent of plate tectonics and the consequent mixing of surface and ocean chemistry.
9. intelligent life
After about 500 million years of evolving multicellular organisms, including many severe mass extinctions and climate changes, the first member of the genus Homo appeared in Africa some 4 million years ago. Intelligent life as we know it is less than 1 million years old, and has been around for less than approximately 0.02% of the Earth’s history[3].
[1] Source: Marcelo Gleiser, Imperfect Creation, Black, Melbourne, 2010, 182-186; 235-237.
[2] For the ensuing, see graphic at end of page.
[3] Ibid, 237.
The Major Geologic Time Divisions
And now a look at the major geologic time scales depicted in clock form:
This clock representation shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga. Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon. The two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
Evidence from radiometric dating indicates that the Earth is about 4.54 billion years old. The geology or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs and many other groups of life. Older periods which predate the reliable fossil record are defined by absolute age.
Illustration Source: http://en.wikipedia.org/wiki/Geologic_time_scale . Ga or Gigaannum denotes a unit of time equal to 1,000,000,000 (1 billion) years. Ma = millions of years.
Geologists divide the formation of the Earth into categories when different stages of development occurred. Each period and the events which occurred within will now be considered. As we have just seen, the story of biological complexity can be considered as a series of major transitions, including the origin of life itself, the appearance of prokaryotic and eukaryotic cells, sexual reproduction, the construction of multicellular organisms such as ourselves and the appearance of organisms that join together in social groups: Christian, Maps of Time, 108.
The Precambrian Eon, when things were in their developmental stage before the huge explosion of life on Earth, ranges from 4,600 to 542 million years, and the Precambrian Eon is further subdivided into various subcategories called Eras with their main geological events being as follows:
Era Period Principal geological events
Hadean 4,600 – 4,000 Mya* Formation of the Earth, its continents and oceans
Archean 4,000 – 2,500 Mya Earliest fossil record of life; formation of oxygen;
single-celled photosynthesisng organisms emerge
Proterozoic 2,500 – 542 Mya The first multicellular organisms and the earliest animals with shells emerge
*Mya = million years ago
The Archean era is the earliest era of life on earth. The first forms of life on earth evolved in the presence of water. They may have been archaebacteria which evolved in hot volcanic vents in or below the seabed, or they may have been other forms of bacteria[1]. The original building blocks of life were the lighter elements, hydrogen and helium, formed in the aftermath of the big bang. To these were later added the heavier elements, including carbon, oxygen, hydrogen, nitrogen, sodium, iron, the product of supernova explosions successively absorbed by recycled stars. In time, these raw chemicals combined into molecules such as water (H2O), ammonia (NH3) and carbon dioxide (CO2), and then into simple organic molecules like methane (CH4). These organic molecules found an environment where they could react and become increasingly more complex, forming biomolecules, the molecules that make up living beings. Water (in its liquid phase due to warmth) was an essential ingredient of this process, the first steps of which were the formation of amino acids.
By a process which is not well understood, a set of self-sustaining chemical reactions capable of absorbing energy from the environment and of replication occurred. Maybe shallow pools provided a high enough concentration of chemicals, reactions started and molecules grew in complexity until they became a self-sustaining reaction network, extracting energy from the environment to keep on going.
Bill Bryson traces these early beginnings of life from the time when a “little bag of chemicals fidgeted itself to life” about 3.85 billion years ago, absorbed some nutrients, and then in due course cleaved itself, “passing a tiny bundle of genetic material from one living entity to another” producing an heir[2]. Then at some point in the first billion years of life, cyanobacteria, or blue-green algae, tapped into the hydrogen in water molecules, releasing the oxygen as waste, thereby inventing photosynthesis, causing the world to fill with O2. Then about 3.5 billion years ago, the cyanobacteria combined with tiny structures of dust and sand, forming a kind of bacterial rock called stromatolites,
The evidence is that life came early to the planet. The earth’s oldest rocks are about 4 billion years old, and the oldest biological signature found in rocks anywhere on the planet is in rocks 3.8 bullion years old at Isua in Greenland. The evidence consists of the presence of the C12 isotope and its biological altered isotopic ratio from C12 to C 13, meaning that some cells lived, respired, segregated carbon, died and had been buried in these particular rocks[3].
Some of the world’s best preserved early Archean rocks have been located in the Pilbara region of Western Australia. Initially, stromatolite fossil remains 3.46 million years old were found at a remote place called North Pole near Marble Bar in the same region. Then in 2011 microscopic fossils about 3.4 million years old were found in Pilbara’s Strelley Pool formation and the following year stromatolite fossils were found going back some 3.49 million years, this time in the Dresser Formation of the Pilbara[4]. These are the oldest known organisms on Earth, dating to a mere billion years after the Earth formed. By 3.46 million years ago the North Pole stromatolites had already evolved to the stage where they were producing oxygen by photosynthesis using prokaryotic chlorophyll. They must therefore have evolved some time beforehand.
Modern stromatolites, the kind which were first discovered in 1961 in the shallow waters of Shark Bay on the remote northwest coast of Australia, are formed by simple ecosystems of several types of bacteria that build domical and columnar rocky masses. They are rare and are generally only found in bodies of highly saline seawater such as that which occurs adjacent to the open waters of Shark Bay. They would perhaps have died out by now, but they still exist at Shark Bay because the waters are too saline for the predators that would normally feast on them[5]. Yet there they can still be seen, doing their job respiring oxygen in the form of tiny bubbles into the atmosphere day by day. When the North Pole stromatolites were alive the days were shorter and the tides were higher. One of the requirements for most, if not all stromatolites, is that they need to be submerged for at least part of every day. Hence they mostly grow in the intertidal zone, growing to lower heights as the high tide mark is approached. So their maximum height is limited by the depth of water at high tide. At that time the Earth was spinning faster than now and the moon was closer. Since its formation, the Earth has been gradually slowing and the Moon has been slowly moving further away, hence the gravitational pull it exerts on the Earth is less now than it was when the North Pole stromatolites were forming, as a result the present day tides are lower. This is probably why the stromatolite fossils at North Pole, and those also on the banks of the Nullagine River WA are much taller than their present-day relatives, meriting description in fact as “huge cliffs”[6].
Above, left and right: Modern day stromatolites in the waters of Shark Bay, Western Australia, 26 Aug 2014. Photographer: Elwyn Elms
Modern-day stromatolites consist of three distinct layers that form an ecosystem of interdependent producers and consumers. The uppermost layer of a modern stromatolite colony is mostly composed of photosynthetic cyanobacteria that use chlorophyll to power the conversion of water and CO2 into carbohydrates and oxygen. The middle layer is formed by anaerobic bacteria, sulphate reducing bacteria and methanogenic archea which use the energy present in the waste products excreted by the upper two photosynthetic bacterial layers. Because the upper two photosynthetic layers need light to survive, the upper surface of the stromatolite grows upwards towards the light. The slime produced by the surface bacteria traps calcite and clay mud. Over time the calcite layer becomes thick enough to block out the light, or sometimes mud that builds up in the slime can also block out the light. When this happens, the photosynthetic layer re-establishes itself above the opaque material, “over its congealed ancestors” as David Christian describes it[7] and the whole cycle repeats over and over again producing layer after thin layer of carbonate cemented rock, and so the stromatolite grows[8].
Because the textures that indicate the presence of bacteria in the rocks can also result from other causes, the team from Old Dominion University (Norfolk, Virginia) led by Nora Noffke, responsible for the Dresser Formation discovery corroborated its findings by measuring the carbon making up the textures in the rocks. About 99% of carbon in the non-living stuff was carbon-12, a lighter version of the element than the carbon-13 that accounts for most of the remaining 1%. Microbes that use photosynthesis to make their food contain even more carbon-12 and less carbon-13. That bias, a signature of “organic” carbon that comes from a living being showed up in the rock.
The presence of bacteria similar to modern day cyanobacteria (blue-green algae) suggests that the early earth was already full of life. At first these primitive forms of life ate or absorbed chemicals and then those living near the surface learned to use sunlight for their energy. The cells of cyanobacteria contained molecules of chlorophyll, which enabled them to process sunlight in the fundamental chemical reaction known as photosynthesis[9].
Remember that photosynthesis, the process by which algae and other plants capture sunlight to make food, is the result of the presence of oxygen in the atmosphere. It began more than 2.5 billion years ago, during the Great Oxidation Event, but it took hundreds of millions of years for enough oxygen to build up in the atmosphere and ocean to support complex life. Early levels of oxygen were simply not sufficient to support life, and it took about 2 billion years, or roughly 40% of the Earth's history, for oxygen levels to reach more or less modern levels of concentration in the atmosphere[10]. During this period, Earth’s atmosphere and ocean had no free oxygen. However, as photosynthesising bacteria and plants evolved and diversified in the ocean, and eventually on land, oxygen levels increased while carbon dioxide levels dropped. First of all, large amounts of oxygen were pumped in to the ocean until, after about 700 million years, the ocean itself (in reality, the ocean’s rocky floor) began to rust[11]. This waste product from bacterial slime is the rust coloured iron ore which is being mined today. After about 700 million years the oceans simply ran out of iron, and this new and more powerful metabolic technology began to transform the early atmosphere by pumping into it huge amounts of free oxygen, a gas that was poison to most early life forms[12]. The oxygen released by these simple organisms gradually accumulated in the atmosphere and ocean, and this made possible the blossoming of complex life.
So far as the fossil record is concerned, fossils can tell us a great deal about the history of the earth over the last 700 million years, but this is less than one-fifth of the period life has existed here. However, paleontologists have learned how to find and analyse tiny “microfossils” of bacteria, the oldest of which dates back some 3.5 million years, close to the earliest signs of life on earth. In recent years, biologists have also made increasing use of techniques for studying and comparing the genetic material of different modern species, which can reveal evolutionary links between modern species that cannot be detected from the fossil record alone[13].
[1] Christian, 109.
[2] Bill Bryson, A Short History of Nearly Everything, Broadway Books, 2003, 292-299.
[3] Tom Hubble, Big Bang to Life Sydney Uni CCE course, 2011. See also the section on carbon dating herein.
[4] See http://www.odu.edu/about/odu-publications/insideodu/2013/11/11/topstory1
[5] Bill Bryson, op cit, 299.
[6] “Australia: The Land Where Time Began: A biography of the Australian continent”, http://austhrutime.com/north_pole.htm See also ABC TV programme “The history of Australia”. op cit.
[7] At op cit, 111.
[8] Hubble, op cit.
[9] Elaborated upon by Christian, op cit, 110.
[10] Bill Bryson, op cit, 300.
[11] Ibid, 298. This process is said to have taken place in the late Achaean era (2.8 to 2.5 mya).
[12] For the foregoing, see Australia, The Time Traveller’s Guide, Episode 1, ABC TV, 25 March 2012.
[13] Christian, op cit,108-9.