For many, the concept of time is difficult to grasp. Yet even in Key Stages 1 and 2, children are expected to understand long periods of time: the activities of the Egyptians 4 000 years ago, the Romans 2 000 years ago or the Vikings 1 000 years ago. If one thousand years is a long time, how can a child comprehend one thousand million years? But not only is it possible, it is intellectually stimulating. Geological time means large numbers and it is necessary to break down these vast periods of time into more manageable pieces. Scaling is a useful tool.
There are a number of 'models' that have been used to scale the passing of the 4 600 million years of the Earth's geological history. Compressing geological time into, for example, a 460-page book, the twenty four hours of the day or even to a single hour have been used. These are misleading, however, as they gives the impression that, with the appearance of humans 'a few seconds before midnight', geological time came to an end and that our species is the ultimate life form at the end of the long evolutionary process. This is far from the truth. If 'survival of the fittest' is the key to evolution, we have a lot to learn from blue-green cyanobacteria!
It has been estimated that the solar system is about half way through its life, so the model presented here scales geological time to that of a middle-aged person. We consider the Earth not as a planet 4 600 million years old, but as a person 46 years old today. Of course to an eight or 12 year old child, it is difficult to imagine that anybody could be as old as 46, but discussion about the age of relatives (parents and grandparents – and teachers?) makes this more understandable. For a child, the same mental agility is necessary to come to terms with 46 years, as for 460 years (the Tudor period), 4 600 years (Egyptians were about to construct the pyramids) and 4 600 million years (the age of the Earth).
The Geological Timeline introduces the question— what of the future? Our middle-aged Earth is part way through its life. It is interesting to debate what the world's environment might be in another million or 4 000 million years. Will humans still exist? What will be the effect of humans on biodiversity? What organisms might evolve? What might the atmosphere be composed of? How will it all end?
A timeline is the ideal way to comprehend the passage of geological time and to demonstrate how life, environment and geography has changed throughout Earth's history. There are several ways that this can be achieved in school.
Whatever method is appropriate, subdivision into 46 units is useful as these give the idea of the 46 'birthdays' (each birthday represents 100 million years of geological time). It is helpful if the last division (representing the last year) is an elongate rectangle divided into 12 'months'— a lot happened during that last 'year'. The children's work can be attached to the timeline—written work, photographs or art work.
The key geological events in the history of life are shown in the Geological Timeline and further details are shown in the table below. It should be noted that some of these events did not fall exactly on the Earth's 'birthdays', but they have been placed next to the nearest one. So the appearance of the dinosaurs 225 million years ago, for example, has been placed at the 44th birthday (so within 25 million years). Of course absolute ages are approximate anyway and the degree of error and uncertainty increases with age. Taking the Carboniferous as an example, its lower boundary been variously dated between 367 and 353 million years ago and the top between 280 and 301, and is, therefore, between 52 and 87 million years in duration.
These will depend on the age and ability of the children:
A number of scientific palaeontology books are available, providing
data that can be used on a timeline. The majority are for advanced
students, but a brief outline of the fossil record, including a short
reference list, is published by the British Geological Survey:
Rigby, S 1997. Fossils, the story of life. 64pp [British Geological Survey, Keyworth]
The Fossil Focus series is also published by the British Geological Survey. These laminated A3 cards colourfully explain the anatomy, distribution and environmental requirements of a number of fossil groups. A list can be found in the BGS catalogue.
|The timeline in 'birthdays'||Approximate age of the Earth (millions of years)||Approximate time before present (millions of years)||Notes on key events along the timeline|
|0||0||4600||Earth formed from a dust cloud with the sun in centre. When Earth was about 80% of its present size, it crashed with another planetoid. Debris around the Earth fused together to form the moon.|
|1||100||4 500||Earth's core formed when dense metals sank to the centre. Eventually the stony crust cooled and solidified. Little is known about Earth (no crustal rocks survive).|
|2||200||4 400||Oldest known minerals to form on Earth are zircon crystals in Australia. Inclusions in the crystals said to indicate oceans had formed by this time, although this is controversial.|
|5||500||4 100||End of the Hadean (the name of the essentially unknown phase of Earth's history, not represented by crustal rocks).|
|6||600||4 000||The Ancaster Gneiss (Greenland) are Earth's oldest known crustal rocks (c. 4 000 Ma).|
|8||800||3 800||Akilia Gneiss (3 850 Ma) said to have carbon traces of life, but this is controversial.|
|9||900||3 700||Banded Ironstone Formations (BIFs) are considered to have been created by bacteria. Oxygen created by bacteria caused ferrous iron in the ocean water to oxidise and precipitate as a red layer of iron on the sea floor. At times when oxygen was not being created, grey cherts were precipitated instead. These layers built up alternately to form BIFs. BIFs provide the earliest signs of photosynthesis. They began to form about 3 700 Ma, but no fossils are known. However, photosynthesising bacteria must have evolved from non-photosynthesising ancestors, which in turn evolved via non-biological evolution.|
|11||1 100||3 500||The Apex Chert (western Australia), 3 465 Ma old, was once believed to contain the earliest fossils, but this is now considered unlikely. However, in the Pilbara region, NW Australia, silica-rich rocks dating to about 3 500 Ma contain tubes about 40 microns long and thinner than a human hair. Although some may have formed inorganically, there are some geologists who believe that they were formed by rock-eating bacteria. This is still controversial.|
|16||1 600||3 000||Stromatolites formed by blue-green cyanobacteria, microscopic, single celled, photosynthesising organisms. The organic mats precipitated calcite and trapped sediment particles into the layer. Blue-green bacteria are still making stromatolitic domes in Shark Bay (Australia) 3 000 million years later.|
|25||2 500||2 100||Oxygen was a waste product produced by bacteria and would have been poisonous to these early life forms. Initially the oxygen was chemically trapped in the rocks, e.g. BIFs and limestones, but eventually there was too much to store in this way and it escaped into the atmosphere. Terrestrial 'Red Beds' were created 2 100 Ma, by the oxidisation of iron. Eventually free oxygen began to accumulate in the atmosphere.|
|31||3 100||1 500||Small amounts of oxygen in the atmosphere. The first eukaryotes appear, the basic cell type that almost every living thing on Earth is made of—protista, fungus, plant, animal kingdoms (only bacteria, Kingdom Monera, have the simpler prokaryotic cell). Eukaryotes require oxygen for their metabolism. Sexual reproduction is said to have evolved at about this time. Rocks 1 000 Ma old show an increase in diversity of these early eukaryotes, protista.|
|39||3 900||700||Geneticists have suggested animal life began c. 1 000 Ma ago, but there is no evidence for this in the geological record. Choanoflagellates are protistids with genetic material also found in animals and it has been suggested that the animal kingdom evolved from something similar. The earliest trace fossils in Australia (made by the activities of presumably soft-bodied animals) and Africa are about 700 Ma old.|
|40||4 000||600||The first multicelled animal fossils including the 'sea-pen' Charnia, worms, sea urchin-like creatures and jelly fish, are a little over 600 Ma old. Fecal pellets discovered in 600 million year old rocks in Scotland must have been left by an animal with a gut.|
|41||4 100||500||Animals with hard parts (shells and skeletons) e.g. trilobites and molluscs evolved 545 million years ago (at the beginning of the Cambrian). Soon afterwards, all kinds of organisms with hard parts began to evolve, including corals, crinoids, brachiopods, nautiloids, graptolites and microscopic species too (e.g. foraminifera). The earliest fish evolved in the early Cambrian. The first fish with calcareous back bones (rather than cartilaginous notocords) evolved a little later. Comparison can be made to other vertebrates including ourselves.|
|42||4 200||400||Sufficient ozone in the atmosphere allowed plants to evolve from algae and colonise the land. Invasion of land by plants began with the evolution of non-vascular bryophytes in the Mid Ordovician, about 450 Ma ago. Cooksonia, the first vascular plant, evolved in the late Silurian, c. 420 Ma ago. Soon afterwards animals followed the plants.|
|43||4 300||300||Animal life has been found on the marshy land associated with early plant fossils. Worms, snails and, by the late Devonian (about 350 Ma), the first amphibians (tetrapods) left the aquatic realm. Amphibians rapidly evolved into lizards. Lizards had developed a water proof egg that did not have to be laid in water. They did not have a need to stay close to bodies of water and keep wet. The first tropical rain forests evolved (the coal forests) and began to spread about 320 Ma ago.|
|44||4 400||200||Lizards evolved into dinosaurs 225 Ma ago. There are a number of differences, but the most obvious is in the construction of the hip so that dinosaurs were able to stand with straight legs beneath their body. Some were bipedal. 'Mammal-like reptiles' evolved into the first mammals—shrew-like insectivores about 210 Ma ago.|
|45||4 500||100||Archaeopteryx, the first bird, evolved from feathered theropod dinosaurs about 140 Ma ago. Soon afterwards (c.130 Ma ago) flowering plants evolved—Archaefructus was the earliest angiosperm (it had carpels but no flower), but soon afterwards species related to magnolia appeared (oldest fossil flower).|
|8 months ago||4 535||65||Mass extinction of 65 to 70% of all species, including all the ammonites, belemnites, flying reptiles and dinosaurs (although birds, the last of the evolutionary line of the dinosaurs continued to thrive).|
|7 months ago||4 550||50–60||After extinction of the dinosaurs, very rapid mammalian evolutionary radiation, especially in the Eocene (about 40–55 million years Ma), occupying land, sea and air.|
|4 months ago||4 565||35Ma||The first primates, related to lemurs, evolved late in the Cretaceous, just before the mass extinction and the disappearance of dinosaurs, ammonites etc. Monkeys evolved about 35 Ma ago and started to evolve rapidly: the dryopithecines appeared about 25 Ma and the first apes 17 Ma. .|
|2 months ago||4 585||15||Grass evolved. This is, perhaps, the most important flowering plant so far as humans are concerned as it provides us with wheat, barley, maize, rice, etc. The first grasslands evolved during a prolonged phase of climatic cooling. A number of animals took advantage of the expanding grasslands, including some of the primates that live on the ground rather than the trees.|
|About 3 weeks ago||Almost 4 600||c. 5-6||The hominid Australopithecus evolved ('hominid' and 'human' should not be confused; Australopithecus was not human). There have been several species. Recently a skull c. 7 Ma old was discovered that has been suggested to be the earliest hominid but some believe it to be the skull of an ape. Australopithecus afarensis ('Lucy') evolved about 5 Ma ago and the last species of Australopithecus boisei ('Nutcracker Man'), lived from 2.3 to 1.4 Ma ago.|
|About 7–8 days ago||2–2.5||The first species of human evolve in Africa—Homo habilis. Fossils of their brain case shows that the speech centre is only just beginning to develop.|
|c. 6–7 days ago||2–1.6||Homo erectus is considered by some to be two species-H. ergaster and H. erectus. They evolved in Africa about 1.6 Ma ago. Homo erectus was the first human species to migrate across Europe and Asia.|
|c. 5 days ago until 'last night'||1.3||Ice ages start, but this was a period of very variable climate–Britain was sometimes buried beneath about 1 km thick ice caps, sometimes tundra developed, sometimes it was warmer than today. During warm periods, lions, hippo and rhino lived in Britain, but when the tundra developed, mammoth, wolves and giant elk lived here. The last glaciation in Britain ended about 10 000 years ago.|
|c. 2 days ago||0.5||Homo heidelbergensis evolved. The earliest fossils are c. 500 000 years old. 'Heidelberg Man' is also known as 'Swanscombe Man' and 'Boxgrove Man' in Britain.|
|The timeline in 'birthdays'||Approximate age of the Earth (millions of years)||Approximate time before present||Notes on key events along the timeline|
|46||c.13 hours ago||4 600||150 000 years ago||Homo neanderthalensis evolved and spread throughout Europe during the 'Ice Ages' - fossils are found particularly in Mediterranean countries, but also Germany (the type area) and they also reached Britain. It possibly evolved from Homo heidelbergensis.|
|c. 12 hours ago||130 000 years ago||Modern humans (Homo sapiens) evolved in Africa about 130 000 years ago, perhaps from 'Rhodesia Man', Homo rhodesiensis, but the evolution of Homo is controversial and there are a number of different evolutionary theories. The idea that we evolved from Homo neanderthalensis is flawed.|
|c.3.5 hours ago||c. 35 000 years ago||Humans left Africa c.35 000 years ago to spread over Europe and Asia. Neanderthals became extinct about 30 000years ago.|
|c.1 hour ago||c.11 000 years ago||Man became a farmer.|
|c.1 minute ago||250 years ago||The Industrial Revolution. During the last 60 seconds we have pollution, radioactive waste, hole in ozone later, acid rain, mass extinction, etc.|
|The future||4 600-9 000||The next 4 600 million years|| What next? The world has changed so much in the last 4 600
million years who knows what will happen in the next.
In the short term- More pollution? More extinctions? Changes in the atmosphere? Global warming? Rising sea levels and flooding of the continental margins?
In the medium term—the extinction of human beings. What will evolve to dominate the world then? The mammals had to wait for the extinction of the dinosaurs before their sudden evolutionary radiation. What is waiting for our extinction? Could it be the turn of the insects? Or something totally unknown?
In the longer term—reorganisation of the continents. Greenhouse and Icehouse Earth? And finally the sun becomes a red giant, destroys the planets and dies.