The Archean Eon – appearance of life
Geologists refer to the period from about 4000 to 2500 Mya as the Archean Eon. During this time rocks on the surface came together to form cratons, which would become the central cores of continental plates. The oldest still-existing continental rocks date from the beginning of the Archean. Since that time,average surface temperatures have been slowly cooling. There is evidence that the rock cycle (volcanism-sedimentation-metamorphism) has been in action since about then.
The earth’s atmosphere slowly developed. Having originated mostly from gases such as methane or water vapor, the latter spewed out from volcanoes or conveyed by asteroids which crashed onto the earth’s surface, the air was rich in carbon dioxide (CO2) but lacked the free oxygen (O2) essential to animal life as we know it, a situation which changed only early in the Proterozoic Eon. Solar energy received at the surface of the Earth was about 20 to 25 % lower than present, which might have made the planet too cold for life to be established1)“Climate puzzle over origins of life on earth”, http://www.manchester.ac.uk/discover/news/article/?id=10798., but the CO2 retained heat beneath the atmospheric layer, causing a greenhouse effect which slowly raised atmospheric temperatures. Sunlight striking the water vapor caused photochemical dissociation, the breaking up of the water molecules and the bonding of oxygen atoms together to create ozone, or O3, a molecule of three oxygen atoms. In time, the ozone came to protect the surface of the earth from ultraviolet radiation from the sun.
Life first began and developed on earth during the Archean Eon, somewhere about 3.8 or 3.5 Gya, where evidence for the older date is disputed. At that time, surface conditions were still difficult, with high temperatures and an atmosphere lacking in free oxygen but rich in CO2.
Life may be considered as that which can regenerate itself viably. There are different hypotheses about its origin on earth.
- The “primordial soup” hypothesis,
- the hydrothermal vent hypothesis, and
- other hypotheses.
The first type considers life to have been brought about by electricity (from lightning) in a mixture of gases including water and methane. The idea is debated because it depends largely on the composition of the atmosphere at the time.
The third type covers several possibilities, including an origin in clay. For each type, there are variations depending on the details of the chemistry.
The second, hydrothermal model has different flavors depending on what type of vent is concerned and what chemistry takes place there. Here is one version.
In places on the ocean floor, peridotite rock, which is normally found deep in the earth’s mantle, has been pushed up to the surface by faulting. The rock contains olivine, which reacts with sea water to form the minerals serpentine and magnetite, The reactions are exothermic, so a significant quantity of heat is generated. The heat is generated by chemistry and does not come from hot magma, as is the case with so-called “black smokers”. The rocks produced have lower density and so expand and push up. They crack and more sea water moves in to react with remaining olivine. Eventually small cracks and “cells” form within the rock. Rising fluids are very alkaline (basic) and thereby precipitate calcium carbonate and other alkaline substances out when they hit the cold sea water. These then build up on the pile of rock already started and soon “reverse stalactites” are produced by the carbonate left behind by the thermally rising water. Iron in the olivine is oxidized, leading to production of reducing gases hydrogen, methane and hydrogen sulfide.2)“The Lost City 2005 expedition”, NOAA, http://oceanexplorer.noaa.gov/explorations/05lostcity/background/serp/serpentinization.html. These gases in turn are a source of energy. So the rising “chimneys”, which may reach many meters in height, are associated with a source of energy, gases like those in the “primordial soup” and small cell-sized alveoli or compartments. The compartments contain and protect their contents as well as ensuring their concentration, making excellent conditions for the production of inorganic precursors to organic life. From these, prokaryotes and archea would have evolved independently around 3.8 Gya and eukaryotes later, around 2 Gya
Be that as it may, the appearance of cell membranes allowed the formation of simple cells, called prokaryotic cells. As far back as 3.5 Gya. these composed cyanobacteria (“pond scum”). which are the ancestors of the blue-green algae still alive all over the globe today. As such, they are the oldest currently-living beings. But they are extremely important for another reason.
Some of these bacteria formed mats which mixed with sand. As the sandy mixture became muddy, the cyanobacteria migrated upwards and the process repeated, resulting in lumpy layers of colonies called stromatolites, descendants of which survive in isolated parts of the world such as the coastal waters of western Australia.
Stromatolites existed from about 3.5 Gya to 0.5 Gya, but are still found in a few places such as Shark Bay, Australia, or the Pacific Coast of Baja California. They survive only in especially salty water (twice the sea’s normal saltiness) or in places with especially strong currents, as both conditions limit predators such as snails which otherwise would devour them.
Cyanobacteria have been called the “working-class heroes of the Precambrian earth”3)Knoll (2003), 42 and were fundamental to the development of life. The importance of these organisms cannot be stressed too much, as they were the first organisms to carry out photosynthesis, the use of energy from the sun to convert carbon dioxide into nutrients and free oxygen, which is returned to the atmosphere. Over hundreds of millions of years during the Archean and Proterozoic Eons, as cyanobacteria used photosynthesis to get the energy necessary for their own metabolism, they brought about the gradual transformation of atmospheric CO2 into oxygen necessary for other forms of life4)The capability of stromatolites to accomplish this task alone has been questioned and other mechanisms suggested., such as ourselves. At the same time, the greenhouse effect was reduced and, thereby, global temperatures. Much CO2 was also dissolved in the seas, where it combined with calcium to form calcium carbonate, which in turn solidified to form limestone. Limestone, ocean water and corals are huge stores of carbon dioxide (carbon sequestration).
Photosynthesis took place in the top layer of stromatolites and each layer lived off the layer above. As such, they represented an early symbiosis or way of living together – an example of what we now call ecology.
The Proterozoic Eon — the dance of the continents
The Proterozoic Eon ran from about 2500 to 542 Mya. It has been so designated because of the appearance of more complex organisms during this period, in spite of widespread glaciation early on. At this time, plate tectonics came into its own, with cratons moving about on the surface of the earth in what has been called a “stately dance”, i.e., a slow one. It was like some kind of round, with one continent dancing for a while with another, then separately, then with a third. At least three times, they all came together to form a single supercontinent. As they smashed into each other, they brought about the rise of mountains, a process geologists call orogeny. As they rifted and came apart, seas formed between them.
As shown by fossil evidence, stromatolites thrived in the Proterozoic and continued their conversion of atmospheric CO2 into O2. The first oxygen produced had been gobbled up by chemical reactions with elements like iron. Several types of indirect evidence, based on the presence of certain types of molecules in rocks, indicate that around 2000 Mya, the content of free oxygen in the atmosphere increased significantly. In spite of evidence for important fluctuations in oxygen levels over the millenia since then, the oxygen content of the atmosphere has been generally increasing for the last 2000 million years.
With the atmosphere richer in oxygen, other forms of life evolved. More complex cells called eukaryotes appeared about 1400 Mya. Such cells incorporate smaller components called organelles. Examples are the cell nucleus and the mitochondria essential to the generation of energy for the cell. It is widely accepted that some of the organelles within eukaryotes were originally bacteria which entered the original cell, be it prokaryote or some sort of proto-eukaryote, and stayed. This is another example of ecological coexistence.
Prokaryotes divide by a process of mitosis, after which each “child” organism is essentially a clone of the “parent”. Eukaryotes also duplicate themselves by mitosis, but they reproduce by meiosis, a process in which a selection of genes from each parent is combined with a selection from the other.5)This subject will be discussed in more detail in the chapter on biochemistry and cellular biology. This method of reproduction leads more rapidly to greater diversity of genes and, so, to the formation of new species. Only eukaryotes form multicellular organisms, a necessity for more advanced forms of life.
Taking into account biochemistry and evolutionary history, biologists now divide life into three domains: bacteria and archaea, (both prokaryotes), and eukarya, the last two being descended from the first in a yet-to-be-determined order. Current eukarya include plants and animals – such as us.
Fossils usually only show the harder body parts of the fossilized organisms. But from the end of the Proterozoic, around 575-542 Mya, fossils were discovered which also showed the softer body parts of strange and complex organisms. Named after the Ediacaran Valley in Australia where they were first discovered, they have since been found around the world in places such as Charnwood Forest, England, or Mistaken Point, Newfoundland.
The Ediacaran fossils are difficult to interpret. They seem to be generally flat, multi-sectioned organisms, often described as “quilted”, without any internal structure. Charnia, for instance, seems to be a flat, fractal construction without any central stalk. They do not resemble any modern organisms and are generally considered to represent an evolutionary dead end in spite of their being complex, multi-celled organisms. In any case, since they date from as much as 575 Mya, they do show that multi-cellular life existed before the Cambrian. After the Ediacarans had lived alone for up to 90 million years, they disappeared forever as small shelled organisms and trilobites took over.
The Paleozoic Era – rise of complex organisms
The Phanerozoic Eon is divided into three eras:
- the Paleozoic (542-251 Mya),
- the Mesozoic (251-65.5 Mya) and
- the Cenozoic (65.5 Mya to today… about).
During the Paleozoic, the buildup of cratons and mountains continued; glaciers and shallow seas were formed. Life spread from the sea to occupy the land; and fishes, reptiles and primitive mammals evolved.
Geologists have found a huge increase in the number, variety and, especially, the complexity of fossils dating from around 542 Mya in western Canada and in China. This date has therefore been adopted as the beginning of the Paleozoic Era, which is considered to run from 542 to 250 Mya. It is itself broken down into six subdivisions called periods:
- the Cambrian (542-500 Mya),
- Ordovician (500-440 Mya),
- Silurian (440-410 Mya),
- Devonian (410-360 Mya),
- Carboniferous (360-290 Mya) and
- Permian (290-250 Mya).
Tectonic plates continued moving during the Paleozoic, causing continents to grow and to smash together. Sea levels rose and fell. The continents came to have shapes resembling those of today, but they were associated in different ways. By the end of the Paleozoic, all the major continents were fused into one mighty supercontinent, Pangea. A huge mountain chain stretching diagonally across the great continent left its remains after Pangea disintegrated: The Appalachians of North America and the mountains of Ireland, northern Scotland and western Scandinavia were originally part of the same chain. During the following era, the Mesozoic, Pangea rifted and broke apart into the two huge continents, Laurasia and Gondwana (or Gondwanaland). Later, when Gondwana split up, the rift between Africa and North America became the Atlantic Ocean.
Life in the sea
The extraordinary increase in the number of multi-cellular animal phyla which took place at the beginning of the Paleozoic is referred to as the Cambrian Explosion. It is seen today as an explosion of fossils, the most famous of which are those of the Burgess Shale field of about 540 Mya, now in Canada. The first beings of the Cambrian were tiny and shell-like and lived in or next to the sea; land away from the sea was still lifeless. The Cambrian Explosion started in vigor about 540 Mya with the appearance of arthropods (the family of spiders and insects) with articulated members and lasted for some 10 million years. Evolution of the period has been likened to a game of chance, in which many species appeared only to die out. It is speculated that if one could go back to that period in time and start the evolutionary process over, the results would be quite different.
Although there do exist fossil tracks of mostly worm-like creatures from 555 Mya, the organisms represented by the Cambrian-period fossils were of a new kind. Cambrian organisms were more complex and bigger than before due to their support structure. During the early Paleozoic, continents were under shallow seas for periods of several million years at a time, so life was dominated by creatures of the seas, including reef builders. These organisms had no internal skeletons, making them invertebrates, but they did have a hard exoskeleton or carapace. The support this gave was advantageous in several ways: It shielded them from the sun, allowed them to retain moisture, gave support for a muscle system and protected them to some extent from predators. Later, skeletons would provide a mineral store, since bones store minerals like calcium and phosphorus from the blood and are able to pass them back to body cells when they are lacking. Many types of these creatures existed in the Paleozoic seas. From tiny creatures, larger ones evolved.
Trilobites, a type of arthropod, were a dominant form of marine life during the period and existed in thousands of different species on every continent for some 270 million years. They lasted so long that they have been referred to as the “mascots” of the Paleozoic. They ranged in size from several millimeters to over 50 centimeters. Some had eyes with many crystalline lenses, like fly eyes. Thousands of species of trilobites existed on every continent and over time. Near the end of the Cambrian, there were three trilobite mass extinctions due to climate change and other factors (continental movements, evolution of predators). But trilobites survived.
Diversification took off. Radially symmetry echinoderms were the ancestors of today’s starfish and sea urchins. Brachiopods, shellfish with hard upper and lower valves (as opposed to the left and right valves of modern oysters and scallops, to mention the most edible of them), grew wild on the sea floors next to many other creatures.
At the end of the Ordovician and the beginning of the Silurian, two mass extinctions took place, separated by around 4 million years. They are referred to as the Ordovician-Silurian extinction events. Since most life was in the sea, it was this sea life which suffered, It is estimated that 60% of marine invertebrates were destroyed. The extinctions were probably largely caused by climate change due to movement of the continents.
In the Silurian period, eurypterids (looking like scorpions or crayfish) developed which were capable of living in salt or fresh water, an important step in animal evolution. The ammonoids and nautiloids whose fossils we find so beautiful appeared toward the end of the Paleozoic.
Evidence for the second stage of the Cambrian Explosion, 525-520 Mya, comes from Chengjiang, China, a site where some soft parts of the organisms have been conserved in the fossil record. Among the Chengjiang finds is the oldest fish, which is also the oldest vertebrate.
The first fossil evidence of fishes show species which had spinal cords (making them chordates) but no internal skeletons or jaws. The latter evolved from gills only later. Fish became numerous in the Devonian Period, which is often referred to as the “Age of fishes”. Although many types later became extinct, some of their ancestors survive even today: cartilaginous fish, like sharks or rays; fish with bones, like today’s trout or bass; and lobe-finned fish, like today’s lungfish.
At the end of the Devonian, another series of extinctions referred to collectively as the Late Devonian mass extinction took place. Individual events may have been separated by over millions of years. Mostly marine life was affected and trilobites were almost finished off.
Life on land
Plants first developed in water. The date of their migration onto land is still debated but seems to have taken place at least by around 480 Mya and perhaps as early as 600 Mya, in the late Precambrian. Low mossy plants appeared on land during the Ordovician. The migration of plants to land was facilitated by the development of a cellulose-based support structure and the ability to transport water in their stems. The oldest known such vascular plant dates from the mid-Silurian, about 430 Mya, and represents an important advance, as such plants had internal tubes by which water and nutrients could mount from the soil to replace moisture that was eliminated from the plant’s upper parts.
With the advent of woody stems, plants developed to the point where the Carboniferous Period was one of dense areas of vegetation, tree-like plants and swamps. This plant material decayed and was eventually transformed by heat and pressure into the fossil fuels we are busily burning up in a tiny fraction of the time it took to make them. Carboniferous plants were all seedless and so had no flowers.
The Carboniferous was a period when the oxygen content of the atmosphere was 50-100% greater than now (see the figure) and this had an effect on evolution. Giant insects evolved, including a dragonfly with a 65cm wingspan. Later, when oxygen levels came back down, the giant insects disappeared.6)Fortunately…
During the Silurian, tiny arthropods appeared on land. They did not have a digestive system capable of making them herbivores, but lived off decayed matter. During the Devonian, skeletal changes which permitted animals to support themselves on land facilitated the transition from fishes to tetrapods (four-footed animals, including birds). The first land-based tetrapods were still aquatic or amphibious animals and probably lived mainly in ponds, but were capable of breathing air, so they could move to another pond in times of drought. They also laid their eggs in water, which furnished nutrients for the young, which were essentially fish (like tadpoles).
So first plants moved onto the land. They were followed by small arthropods, which ate decayed matter from the plants. And then tetrapods followed and ate plants and arthropods. It is all about getting enough to eat.
A very important evolutionary step was the development of the amniotic egg. This protected the young inside a protective cover and provided the nutrients that young amphibians were forced to get from water. This development contributed greatly to the evolution of amniotes (the first of which resembled small lizards), which now could leave the water completely. These animals split into two groups, synapsids (early mammals) and sauropsids (early reptiles).7)There is some disagreement here. Some authors refer to the first amniotes as reptiles and later speak of “mammal-like” reptiles. or “stem mammals”. It seems easier to speak of amniotes which were the ancestors of both mammals and reptiles. The first reptiles date from the mid-Carboniferous, during which life on land and sea reached a new peak of development and diversity.
Tectonically, what today would be Europe and North America were then situated in tropical climates near the equator. Indeed, because no land mass was over either pole, polar ice caps were limited and the earth’s temperature gradient was less pronounced. On land, huge tree-like plants grew in swamps and life reached all the continents. Insects and tetrapods swarmed through the undergrowth. But no bird sang and no flower lent color to the scene.
Near the end of the Carboniferous, as Gondwana (the southern continent comprising today’s South Africa, South America, Antarctica, Australia and India) approached the poles, as seen in the following figure, there was a period of glaciation which lasted into the Permian. Remaining glacial features on these continents provide evidence for plate tectonics, as some of these continents now occupy much warmer latitudes8)Benton, 90.
The Permian Period was dominated by the existence of the supercontinent Pangea. Around the equator, the Carboniferous swamps had given way to deserts and these arid conditions were well suited to the development of reptiles.
To the east, projecting into the continental land mass, was the Tethys Sea, which was swarming with life. This was also true of the Zechstein Sea in the north, the area of current northern Europe. Parts of the Zechstein evaporated, leaving behind minerals (evaporites) which helped furnish raw materials for the Industrial Revolution – plaster of Paris, gypsum and substances used for the production of acids and ammonia.
The distribution and variety of organisms today is a result of the existence and subsequent breakup of Pangea. During its existence, no waterways blocked migration roots, so animals, at least those who could support the aridity of the interior, were free to move about to new habitats. The later breakup of Pangea was an equal boon to evolution as organisms isolated from one another tend to evolve in different ways from similar beginnings. Simply put, “isolation begets diversity.”
The time of Pangea was one of much development in the forms of life. By its end, dinosaurs and early mammals had developed. Many insects existed, including cockroaches, which are still with us, alas.
The end-Permian extinction
The Paleozoic Era ended with the greatest of all the mass extinctions, the end-Permian extinction. It is estimated that 96% of sea and 70% of land species disappeared9)McDougall, 321.. The date of the extinction marks the end of the Paleozoic and the beginning of the Mesozoic Era, largely accepted as 251 Mya. The causes of the extinction are still uncertain. It may have been initiated by volcanic eruptions in Siberia which increased the amount of methane and CO2 in the atmosphere, bringing about a “runaway greenhouse phenomenon”.10)Benton, 118. This in turn could have caused ocean currents to change in such a way as to lower oceanic oxygen content, bringing about extinction of the bottom end of the aquatic food chain. It could have caused acid rain which killed land plants vital to survival of animals. The CO2 would have been absorbed by the oceans and led to their acidification, wiping out many marine organisms. What is clear is that it took around 20 million years for life to recover, far longer than after the other known mass extinctions. When life did attain its previous diversity, its forms had changed. The few remaining trilobites had been completely eliminated.
A recent study11)“Ancient whodunit may be solved: The microbes did it!|” March 2014: MIT News, newsoffice.mit.edu/2014/ancient-whodunit-may-be-solved-microbes-did-it. indicates that eruptions in the Siberian Traps increased the amount of nickel in the earth’s crust and this was a nutrient for a microbe, a methane-producing archaea called Methanosarcina, which had undergone a genetic change at about that time. It is suggested that the microbe emitted vast amounts of methane into the atmosphere and that changed the climate. Well, maybe.
Don’t stop now. Continue global history with the ages of reptiles and mammals.
Notes [ + ]
|1.||↑||“Climate puzzle over origins of life on earth”, http://www.manchester.ac.uk/discover/news/article/?id=10798.|
|2.||↑||“The Lost City 2005 expedition”, NOAA, http://oceanexplorer.noaa.gov/explorations/05lostcity/background/serp/serpentinization.html.|
|3.||↑||Knoll (2003), 42|
|4.||↑||The capability of stromatolites to accomplish this task alone has been questioned and other mechanisms suggested.|
|5.||↑||This subject will be discussed in more detail in the chapter on biochemistry and cellular biology.|
|7.||↑||There is some disagreement here. Some authors refer to the first amniotes as reptiles and later speak of “mammal-like” reptiles. or “stem mammals”. It seems easier to speak of amniotes which were the ancestors of both mammals and reptiles.|
|11.||↑||“Ancient whodunit may be solved: The microbes did it!|” March 2014: MIT News, newsoffice.mit.edu/2014/ancient-whodunit-may-be-solved-microbes-did-it.|