I try not to spend too much time looking back on my life, but there are times when it is good to do so and I have been reminded of a blog post I sent out early last year. A few years ago now a good friend sent me an article about a daughter learning about Darwin’s Theory of Evolution and then her mother telling her about the Sanskrit Avatars, which tell their version on the beginning of life here on Earth. I appreciated that, but to me there are other people, for example the Aborigines, also the American Indians who all have their traditions. Whatever way is right, however things occurred, I really do believe that this world, along with the rest of the Universe, didn’t just happen by accident. With looming discrepancies about the true age of the universe, scientists have taken a fresh look at the observable, expanding universe and have now estimated that it is almost 14 billion years old, plus or minus 40 million years. Considering that as well as our sun, our star, there are around 100,000 million stars in just the Milky Way alone, our Earth is a bit small! As well as that, there are an estimated 500 billion galaxies. With all the fighting and killing that we humans have done in the (extremely relatively) short time that we have been around, it is perhaps a good thing that the nearest star system to our sun is Alpha Centauri, which is 4.3 light-years away. That’s about 25 trillion miles (40 trillion kilometres) away from Earth – nearly 300,000 times the distance from the Earth to the Sun. In time, about 5 billion years from now, our sun will run out of hydrogen. Our star is currently in the most stable phase of its life cycle and has been since the birth of our solar system, about 4.5 billion years ago and once all the hydrogen gets used up, the sun will grow out of this stable phase. But what about our Earth. In my blog post last week I said about what might happen if we were to take an imaginary quick jaunt through our solar system in the potential ‘last days’ of the sun. There has been speculation, there have been films, tv series, all giving a view on how things were or might be. The film ‘2001 A Space Odyssey’ is just one example. Others films like Star Trek have imagined where beings from other worlds colonised Earth, some have considered life if another race were to change life completely here. A favourite of mine, Stargate, started out as a film and then became a series where the Egyptian ruler, Ra, travelled via a star-gate to a far-distant world where earth-like creatures lived. We can speculate and wonder, it is a thing that we humans can do. Though if you know of the late, great Douglas Adams and his tales, we should not panic. Just remember that in his writings, at one point the Earth was destroyed to make way for a hyperspace bypass and that just before that happened, the dolphins left Earth and as they did so, they sent a message saying “So long, and thanks for all the fish”. But I digress. I cannot possibly detail the full history of our Earth here, but I can perhaps highlight a few areas and I shall do my best.
Many attempts have been made over the years to comprehend the main events of Earth’s past, characterised by constant change and biological evolution. There is now a geological time scale, as defined by international convention, which depicts the large spans of time from the beginning of the Earth to the present and its divisions chronicle some definitive events of Earth history. Earth formed around 4.54 billion years ago, approximately one-third the age of the Universe, by accretion – this being the growth or increase by the gradual accumulation of additional layers or matter. It came from the solar nebula. Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. Whilst the Earth was in its earliest stage, a giant impact collision with a planet-sized body named Theiais is thought to have formed the Moon. Over time, the Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface. The Hadean eon represents the time before a reliable fossil record of life, it began with the formation of the planet and ended 4 billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution. The succeeding eon was divided into three eras, which brought forth arthropods, fishes, and the first life on land, the next which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs and the following one which saw the rise of mammals. Recognisable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale. The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago after which a geological crust started to solidify. There are microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia and other early physical evidence of a biogenic substance is graphite found in 3.7 billion-year-old rocks discovered in southwestern Greenland. Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose. This developed over time and culminated in the Cambrian Explosion about 541 million years ago. This sudden diversification of life forms produced most of the major algae, fungi, and plants known today, and divided the Proterozoic eon from the Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived on Earth, over five billion have become extinct and estimates on the number of Earth’s current species range from 10 million to 14 million, of which about 1.2 million are documented, but over 86 percent have not been described. However, it was recently claimed that 1 trillion species currently live on Earth, with only one-thousandth of one percent described. The Earth’s crust has constantly changed since its formation, as has life since its first appearance. Species continue to evolve, taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments. The process of plate tectonics continues to shape the Earth’s continents and oceans and the life which they harbour.
So the history of Earth is divided into four great eons, starting with the formation of the planet. Each eon saw the most significant changes in Earth’s composition, climate and life. Each eon is subsequently divided into eras, which in turn are divided into periods and which are further divided into epochs. In the Hadean eon, the Earth was formed out of debris around the solar protoplanetary disk. There was no life, temperatures were extremely hot, with frequent volcanic activity and hellish-looking environments, hence the eon’s name which comes from Hades. Possible early oceans or bodies of liquid water appeared and the Moon was formed around this time, probably due to a collision into Earth by a protoplanet. In the next eon came the first form of life, with some continents existing and an atmosphere is composing of volcanic and greenhouse gases. Following this came early life of a more complex form, including some forms of multicellular organisms. Bacteria began producing oxygen, shaping the third and current of Earth’s atmospheres. Plants, later animals and possibly earlier forms of fungi formed around that time. The early and late phases of this eon may have undergone a few ’Snowball Earth’, periods, in which all of the planet suffered below-zero temperatures. A few early continents may have existed in this eon. Finally complex life, including vertebrates, begin to dominate the Earth’s ocean in a process known as the Cambrian Explosion. Supercontinents formed but later dissolved into the current continents. Gradually life expanded to land and more familiar forms of plants, animals and fungi began to appear, including insects and reptiles. Several mass extinctions occurred though, amongst which birds, the descendants of non-avian dinosaurs, and more recently mammals emerged. Modern animals, including humans, evolved at the most recent phases of this eon.
The standard model for the formation of our Solar System, including the Earth, is the Solar Nebula hypothesis. In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas, composed of hydrogen and helium created shortly after the Big Bang some 13.8 billion years ago and heavier elements ejected by supernovae. At about 4.5 billion years the nebula began a contraction that may have been triggered by a shock wave from a nearby supernova, which would have also made the nebula rotate. As the cloud began to accelerate, its angular momentum, gravity and inertia flattened it into a protoplanetary disk that was perpendicular to its axis of rotation. Small perturbations due to the collisions and the angular momentum of other large debris created the means by which kilometre-sized protoplanets began to form, orbiting the nebular centre. Not having much angular momentum it collapsed rapidly, the compression heating it until the nuclear fusion of hydrogen into helium began. After more contraction, a ’T Tauri’ star ignited and evolved into the Sun. Meanwhile, in the outer part of the nebula gravity caused matter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runaway accretion, successively larger fragments of dust and debris clumped together to form planets. Earth formed in this manner about 4.54 billion years ago and was largely completed within 10 to 20 million years. The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produce other accretion disks around virtually all newly forming stars in the universe, some of which yield planets. Then the proto-Earth grew until its interior was hot enough to melt the heavy metals and having higher densities than silicates, these metals sank. This so-called ‘iron catastrophe’ resulted in the separation of a primitive mantle and a metallic core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of its magnetic field.
This Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of lighter elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth’s heat would have driven off this atmosphere, as a result of which the atmosphere was depleted of these elements compared to cosmic abundances. After the impact which created the Moon, the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen. Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen. The new atmosphere probably contained water vapour, carbon dioxide, nitrogen, and smaller amounts of other gases. Water must have been supplied by meteorites from the outer asteroid belt also some large planetary embryos and comets may have contributed. Though most comets are today in orbits farther away from the Sun than Neptune, some computer simulations show that they were originally far more common in the inner parts of the Solar System. As the Earth cooled, clouds formed, rain created the oceans and recent evidence suggests the oceans may have begun forming quite early. At the start of the Archean eon, they already covered much of the Earth. This early formation has been difficult to explain because of a problem known as the ‘faint young sun’ paradox. Stars are known to get brighter as they age, and at the time of its formation the Sun would have been emitting only 70% of its current power. Thus, the Sun has become 30% brighter in the last 4.5 billion years. Many models indicate that the Earth would have been covered in ice and a likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes whilst another greenhouse gas, ammonia, would have been ejected by volcanos but quickly destroyed by ultraviolet radiation. One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in the laboratory fall well short of the minimum complexity for a living organism. The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nuclei and amino acids that are the building blocks of life. An experiment in 1953 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning. Although atmospheric composition was probably different from that used by Miller and Urey, later experiments with more realistic compositions also managed to synthesise organic molecules. Additional complexity could have been reached from at least three possible starting points, these being self-replication, an organism’s ability to produce offspring that are similar to itself, metabolism, its ability to feed and repair itself and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances. The earliest cells absorbed energy and food from the surrounding environment. They used fermentation, the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an oxygen-free) environment. The evolution of photosynthesis made it possible for cells to derive energy from the Sun. Most of the life that covers the surface of the Earth depends directly or indirectly on photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water, and sunlight into food. It captures the energy of sunlight in energy-rich molecules, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product. Some organisms, including purple bacteria and green sulphur bacteria, use an an oxygenic form of photosynthesis that uses alternatives to hydrogen stripped from water as electron donors, such as hydrogen sulphide, sulphur and iron. Such organisms are restricted to otherwise inhospitable environments like hot springs and hydrothermal vents. The simpler form arose not long after the appearance of life. At first, the released oxygen was bound up with limestone, iron and other minerals. The oxidised iron appears as red layers in geological strata which are called banded iron formations. When most of the exposed readily reacting minerals were oxidised, oxygen finally began to accumulate in the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth’s atmosphere to its current state. This was Earth’s third atmosphere. Some oxygen was stimulated by solar ultraviolet radiation to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonise the surface of the ocean and eventually the land. Without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells. Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the oxygen catastrophe. Resistant forms survived and thrived, and some developed the ability to use oxygen to increase their metabolism and obtain more energy from the same food. The Sun’s natural evolution has made it progressively more luminous during the Archean and Proterozoic eons and the Sun’s luminosity increases 6% every billion years. As a result, the Earth began to receive more heat from the Sun in the Proterozoic eon. However, the Earth did not get warmer. Instead, geological records suggest that it cooled dramatically during the early Proterozoic. Glacial deposits found in South Africa based on paleo-magnetic evidence suggest they must have been located near the equator. Thus, this glaciation, known as the Huronian glaciation, may have been global. Some scientists suggest this was so severe that the Earth was frozen over from the poles to the equator, a hypothesis called Snowball Earth. The Huronian ice age might have been caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane (CH4) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO2, a less effective greenhouse gas. When free oxygen became available in the atmosphere, the concentration of methane could have then decreased dramatically, enough to counter the effect of the increasing heat flow from the Sun. However, the term Snowball Earth is more commonly used to describe later extreme ice ages during the Cryogenian period. There were four periods, each lasting about 10 million years, between 750 and 580 million years ago, when the earth is thought to have been covered with ice apart from the highest mountains, and average temperatures were about −50°C (−58°F). The snowball may have been partly due to the location of the supercontinent straddling the Equator. Carbon dioxide combines with rain to weather rocks to form carbonic acid, which is then washed out to sea, thus extracting the greenhouse gas from the atmosphere. When the continents are near the poles, the advance of ice covers the rocks, slowing the reduction in carbon dioxide, but in the Cryogenian the weathering of that supercontinent was able to continue unchecked until the ice advanced to the tropics. The process may have finally been reversed by the emission of carbon dioxide from volcanoes or the destabilisation of methane gas.
I think that sets the basic scene for the Earth itself, but there is still much to write about in terms of colonisation of land, extinctions and human evolution. But I think this is more than enough for now. Change has continued at a rapid pace and along with technological developments such as nuclear weapons, computers, genetic engineering and nanotechnology there has been economic globalisation, spurred by advances in communication and transportation technology which has influenced everyday life in many parts of the world. Cultural and institutional forms such as democracy, capitalism and environmentalism have increased influence. Major concerns and problems such as disease, war, poverty and violent radicalism along with more recent, human-caused climate-change have risen as the world population increases. In 1957, the Soviet Union launched the first artificial satellite into orbit and, soon afterwards Yuri Gagarin became the first human in space. The American, Neil Armstrong, was the first to set foot on another astronomical object, the Moon. Unmanned probes have been sent to all the known planets in the Solar System, with some, such as the two Voyager spacecraft having left the Solar System. Five space agencies, representing over fifteen countries, have worked together to build the International Space Station. Aboard it, there has been a continuous human presence in space since 2000. The World Wide Web became a part of everyday life in the 1990s, and since then has become an indispensable source of information in the developed world. I have no doubt that there will be much more to find, learn, discover and develop.
This week, as we begin a new year…
When attempting to remember the order of planets in our Solar System, I have found they can be remembered by:
‘My Very Educated Mother Just Served Us Nachos’
(Pluto was first discovered in 1930 and described as a planet located beyond Neptune, but following improvements in technology, in 2006 it was then reclassified as a ‘dwarf planet’ in 2006.)