Earth scientists, including palaeontologists, glibly talk of millions of years. Some even talk of billions. This boggles the minds of most people, particularly when they are used only to time frames in the order of thousands of years, not millions.
Ancient Rome, ancient Greece, the birth of Christ –all of these events took place in our immediate past – and so to grasp the concept of time that stretches back to the beginnings of the Universe, the formation of Planet Earth, the first appearance of unicellular life four billion years ago and the slow wandering of continents across impossibly distant oceans – distant in terms of time and not physical miles – can be a difficult one. Terms such as “Deep Time” and the “Abyss of Time” have been coined to attempt to convey some sense of the immensity of this time span.
A common analogy, used by deGrasse Tyson and many others is that of a 24 hour day. All of Earth’s history, all 4.5 billion years of it, is represented by 24 hours. Modern humans arrived in the last few seconds before the clock struck midnight. The dinosaurs were around late in the day, between 10h40 and 11h40 pm, it needs to be said, seeing that this is a dinosaur website.
However the dinosaurs got to spend an hour at the party, us humans have barely arrived and we aren’t sure if we are going to survive much longer thanks to our ability to trash the place. Those dinosaurs were much more successful as a species than us humans will ever be – they managed to last for 165 million years.
Staying with the theme, there have been a number of attempts to graphically illustrate the age of the Earth. There is of course the clock analogy already mentioned, and illustrated below by the Northern Arizona University and then there have been several other attempts which are illustrated below. Some are more serious than others, some are downright fun, but all are educational.
24 Hour Clock Analogy Geological Timeline
Humans arrived just before midnight in a 24 hour clock analogy. The party might be just about over but we trash the place anyway. Early life arrives around 3 am, the dinosaurs get there pretty late in the evening, and as for the humans.................
A great illustration from the Northern Arizona University (http://jan.ucc.nau.edu/lrm22/lessons/timeline/24_hours.html)
Spiral Geological Timeline
A great diagram showing the progression of life from the Precambrian through to the present, with more space for detail as we progress towards more modern times. This in a way represents the fossil record and the evolution of life - the further back we go the less species there were and the less well preserved the fossil record.
TheQuest for the Age of the Earth
A huge amount of work went into developing the geological time scale. The age of the Earth was once set at 6000 years or thereabouts, based on the work carried out by the Bishop Ussher. To be more precise he tied it down to 6 pm on Saturday, October 22, 4004 BC. He based his date on the genealogy of the book of Genesis, the biblical account of creation of the world, which runs to 21 generations. Precise dates of the lengths of the lives of the various protagonists are also given, and by totting up the numbers he arrived at a precise date of creation.
We have moved on a bit since then, with various famous scientists of history having a go at a date. Newton wasted his intellect and the latter years of his life trying to tie the history of ancient kingdoms and biblical chronology into the record of astronomical events – eclipses and the arrival of comets – but with extremely limited success. His great friend Edmond Halley, after whom Halley’s Comet is named, came up with the theory that the age of the Earth could be estimated by the amount of salt in the sea. By measuring the rate of increase of the sea’s salinity, and assuming that salt had been washed into the oceans at a constant rate, it would be possible to work backward to a time when the seas contained no salt at all, which would then give the age of the Earth. However observations needed to be made a century apart, so Mr Halley wasn’t around for the second round of tests, so that idea failed too.
The famous French naturalist, Georges Louis Leclerc Buffon was the next to estimate the age of the Earth. He proposed the idea that the Earth’s age could be estimated by the rate at which it cooled and settled on an age of 75 000 to 168 000 years. This was a quantum leap from the 6000 year date which nearly got excommunicated for his troubles.
Charles Darwin came up with a date of 300 million years old, which was poo-poohed by Lord Kelvin, who was the most famous physicist of his day. Kelvin thought the world could only be 24 million years old based on current understanding of physics – no one knew about radioactivity – the power station of our planet – at that time.
Then along came Professor Arthur Holmes who pioneered the science of age dating of rocks, pushing the figure out to 3 billion years. This was too much for some, but he had been the closest to the mark yet. It took Clair Patterson, who is a dude strangely enough, to push that date back to 4550 million years. And no one has improved on this figure yet.
I have written a more detailed account of this here.
What I want to focus on now are some of the time periods which make up the geological time scale. But first here is a rhyme to help you remember: "Camels Often Sit Down Carefully Perhaps Their Joints Creak". Which is the first letter of each Period – Cambrian, Ordovician, Silurian, Carboniferous, Permian, Triassic, Jurassic, Creteceous. Our American readers will know the Carboniferous as the Pennsylvanian and Mississippian. Not sure how to revise the rhyme to incorporate those, so look forward to your suggestions. Okay, the rhyme is great, but it leaves out the Precambrian which comes before the Cambrian, and the Tertiary which comes after the Cretaceous.
In the early days of the study of geology no one knew how old the world was. Periods were named based on the rocks that were being mapped in different regions, although mostly in Britain as the pioneers of the subject were mostly based there. However these early workers had now idea that the Precambrian would occupy so much of the geological time scale. Remember that Earth is 4.5 billion years old, and be surprised when I tell you that the beginning of the Cambrian Period is set at around 541 million years ago. So there are nearly 4 billion years of Earth history between the formation of our planet and the beginning of the Cambrian.
Those 4 billion years needed some subdivisions in themselves, so they have been divided up into the Archaean and Proterozoic.
A Brief History of the Geological Time Scale – Precambrian to Quaternary
|Period Name||Named by||Where||Why is it important?|
|Quaternary||Giovanni Arduino in 1759||The young alluvial deposits in the Po Valley in northern Italy.||The Quaternary Period extends from the end of the Pliocene epoch roughly 1.8–1.6 million years ago to the present. It also includes two geologic subdivisions, the Pleistocene epoch (1.6 million years ago to 10,000 years ago) and the Holocene epoch (10,000 years ago to present). Glaciation is what characterises the Quaternary.|
|Tertiary||Giovanni Arduino in Italy.||
Initially geological time had been divided up into Primary, Secondary and Tertiary, with Tertiary being the only one in current use, although this is becoming obsolete. A fourth subdivision called the Quaternary was subsequently added.
In common usage, the Tertiary includes five geologic epochs—the Palaeocene, Eocene, Oligocene, Miocene, and Pliocene. It covers the time from the extinction of the dinosaurs and beginning of the most recent ice age.
First large mammals and primitive primates appear. Then at the start of the Eocene, the first modern mammals began to appear, and with time most modern mammal orders appeared. Mammals became the dominant species during the Tertiary. There also were modern birds, reptiles, fish, and amphibians.
The earliest recognizable hominids arrived Proconsul and Australopithecus. Modern flowering plants evolved, while marine invertebrates and non-mammal marine vertebrates underwent only modest amounts of evolution.
|Cretaceous||Belgian geologist Jean d'Omalius d'Halloy in 1822||Strata in the Paris Basin. Creta, Latin for chalk. He named the period after the widely distributed beds exposed in the Paris strata and those of western Europe.||
It is the last period of the Meosozoic. Warm climate led to high sea levels, which led to flourishing coral reefs throughout the world’s oceans. Marine reptiles and ammonites thrived in these warm oceans, while the dinosaurs dominated the land. Mammals and birds evolved further and flowering plants made their first appearance.
The end Cretaceous extinction put an end to the dinosaurs and marine reptiles, along with a host of other species.
|Jurassic||Alexandre Brongniart, a French geologist this time, in 1829.||Jura Mountains within the French Alps, where limestone strata from the period were first identified.||The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period began with an extinction event, although not of the same order as the end Permian event. Life was dominated by the dinosaurs. Marine reptiles such as ichthyosaurs and plesiosaurs dominated the oceans while the skies were ruled by the pterosaurs. Birds also appeared during this time along with early placental mamals. Crocodilians headed back to a life in the water.|
|Triassic||Friedrich von Alberti in 1834||Germany after three rock strata, namely red beds, overlain by marine limestone, followed by a series of terrestrial mud- and sandstones – the ‘Trias’||
The Triassic began after the end-Permian–extinction, which killed off most of life on Earth. Therapsids and archosaurs were the main terrestrial vertebrates during this time. A specialised subgroup of archosaurs, called dinosaurs, first appeared in the Late Triassic but only became dominant in the Jurassic. Mammals also appeared during this period, along with the pterosaurs.
Pangaea began to break apart in the mid-Triassic, forming Laurasia to the north and Gondwana to the south. Hot dry deserts dominated the interior of the continents. The end of the Triassic was marked by another mass extinction which allowed the dinosaurs to become dominant in the Jurassic.
|Permian||Sir Roderick Murchison in 1841||
City of Perm in the Perm Krai region of Russia.
The diversification of the early amniotes into the ancestral groups of the mammals, turtles, lepidosaurs and archosaurs. Earth was joined into two major continents known as Pangaea and Siberia. Collapse of the Carboniferous rainforests led to vast deserts within the continental interiors. Amniotes, better adapted to these desert conditions, rose to dominance.
The end Permian extinction was the grandmother of all extinctions, killing off 90% of marine species and 70% of terrestrial species. It took 30 million years for life to recover – well into the Triassic.
|Carboniferous (Mississippian-Pennsylvanian)||William Conybeare and William Phillips in 1822.||The term Carboniferous means "coal-bearing" and derives from the Latin words carbō ("coal") and ferō ("to carry")||Terrestrial life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates. Arthropods were also very common, and many much larger than those of today. Huge forests covered much of the land which would eventually be preserved as coal beds which are still so important to us today. The atmospheric content of oxygen also reached its highest levels 35% ] compared with 21% today, allowing land dwelling invertebrates to evolve to great size.|
|Devonian||Roderick Murchison and Adam Sedgwick||It is named after Devon, England, where rocks from this period were first studied.||
Free-sporing vascular plants began to spread across dry land, forming huge forests. Evolution of leaves and roots and seed bearing plants. and by the end of the period the first seed-bearing plants appeared. Earth dwelling arthropods also became well-established. Fish diversified to make the Devonian the “Age of Fish". The first ray-finned and lobe-finned bony fish appeared, while the placodermi began dominating almost every known aquatic environment.
The ancestors of all four-limbed vertebrates began to walk on land, as their pectoral and pelvic fins evolving into legs. Sharks became more numerous than in the Silurian and Late Ordovician. Ammonites and species of molluscs appeared. Trilobites, the mollusc-like brachiopods and the great coral reefs, were still common.
|Silurian||Sir Roderick Murchison in early 1830’s||Named after a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales.||A major evolutionary milestone during the Silurian was the diversification of jawed and bony fish. Multi-cellular life also began colonise the land in the form of small plants that grew in wetlands and along rivers and coastlines. Terrestrial arthropods have also been found in the fossil record.|
|Ordovician||Charles Lapworth in 1879||The Ordovician is named after the Celtic tribe of the Ordovices. It as introduced to settle the argument between Sedgewick and Murchsion who were placing the same strata into Cambrian (Sedgewick) and Silurian (Murchison),||
Molluscs and arthropods, dominated the oceans. There was a great increase in the diversity of life, an event known as the Great Ordovician Biodiversification Event. Fish, the world's first true vertebrates, continued to evolve, and the first jaws may have evolved late in the Period.
|Cambrian||Named by Adam Sedgwick||After the Welsh name for Wales, where Britain’s Cambrian rocks are best exposed.||The Cambrian marked a profound change in life on Earth; Complex, multicellular organisms arrived in force with the rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.|
|Precambrian||The time before the Cambrian||Most of Earth’s history, from 4.5 billion years through to 541 million years ago. Life evolved in the Precambrian. It is divided up into the Hadean, Archaean and Proterozoic. The Precambrian accounts for 89 percent of geological time. Erosion and plate tectonics have destroyed much of the Precambrian fossil record and rocks, hence making it difficult to decipher the geological history of this period.|