We are currently working with your children throughout the coming months on the topic of “time” and we are looking at the past using the Pre-historic timeline.
“By offering the child the story of the universe, we give him something a thousand times more wonderful and mysterious to grasp with his imagination, in a cosmic drama no fable can rival” Maria Montessori
Notes:
Introduction
The history of Earth is divided into two great aeons; the Precambrian, which runs from the formation of the solar system at about 4500 mya until 542 mya, and the Phanerozoic, from 542 mya to the present. The boundary between the two was originally set to be when the earliest fossils were found; though these days fossils of soft-bodied creatures have been found in some Precambrian rocks too.
The aeons are subdivided into eras. There are three eras in the Precambrian aeon; the Hadean, which runs from 4500 to 3800 mya; the Archaean, from 3800 to 2500 mya; and the Proterozoic, from 2500 mya till the end of the Precambrian, 542 mya.
The geological record of Earth only begins at the start of the Archaean era, which started when the Earth’s surface first solidified. The Archaean used to be called the Azoic era, meaning “no life”, but since suspected traces of life are being discovered in earlier and earlier rocks the name isn’t used now. Some scientists claim to have discovered traces of life in rocks dating back to almost 3800 mya, though there’s debate about whether they are fossils or just natural phenomena. The earliest generally agreed fossils are from the beginning of the Proterozoic era.
The Precambrian aeon (and the Proterozoic era) ended with the so-called Cambrian Explosion, when metazoans (complex multicelled creatures) diversified and become plentiful in the fossil record.
The Phanerozoic aeon is also divided into three eras; the Palaeozoic (“early life”), from 542 to 251 mya; the Mesozoic (“middle life”), from 251 to 65 mya; and the Cainozoic (“modern life” – the US spelling is Cenozoic), from 65 mya to the present. The eras are again divided into smaller sections called periods; the periods are further divided into epochs.
PRECAMBRIAN AEON
HADEAN ERA
4500 – 3800 mya
During this era our world was formed from a huge cloud of dust and gases that gradually collected under its own weight to form our Sun, at the centre, and the nine planets and countless asteroids, moons and comets that revolve around it. All these make up the Solar System that includes our planet Earth.
The tighter the dust and gas packed together the hotter it got. So at first the earth was extremely hot, a huge fiery ball of glowing magma. After hundreds of millions of years the earth began to cool and the liquid rock formed a crust. This was the beginning of the Archaean Era.
ARCHAEAN ERA
3800 – 2500 mya
While the Earth now had a solid crust it was still very active volcanically. There was no oxygen in the atmosphere, so it was unbreathable, and it was too hot for liquid water.
Over millions of years the Earth gradually cooled down, and eventually became cool enough to rain; filling the lowlands and forming the seas. But the seas were still hot. The sky wasn’t blue. The air was unbreathable – mostly methane, nitrogen and carbon dioxide with very little oxygen. Only a very few creatures could live on such a poisonous planet. Some of their descendants, called extremophiles, still live on Earth today, in places where nothing else can, such as geysers which shoot boiling hot water high into the air or the “black smokers” deep in the ocean where there is no light and volcanic vents give off burning smoke and poisonous chemicals.
Then the organisms called blue-green algae or cyanobacteria appeared. They were unique, since a byproduct of their metabolism was oxygen gas. They grew so well that they changed the atmosphere of the Earth. The oxygen made it clear and breathable and provided an ozone layer that screened out the harmful UV rays from the Sun. This change marks the end of the Archaean era and the beginning of the Proterozoic.
PROTEROZOIC ERA
2500 – 542 mya
The Proterozoic Era is the age of prokaryotes; simple one-celled animals like the blue-green algae. They may be simple, but they dominated the Earth for almost 2 thousand million years, nearly half of its history.
The first animals on Earth evolved towards the end of the Proterozoic, in the Ediacaran period (600-542 mya) – you can see some fossils here. They included sponges and early corals (not what we call corals today – those didn’t appear till the Triassic period), as well as cnidarians like sea anemones and jellyfish.
PHANEROZOIC AEON
PALAEOZOIC ERA
542 – 251 mya
The Palaeozoic era is made up of six main periods. From earliest to latest they are:
Cambrian life was still confined to the sea. Trilobites were very common. The strange Burgess Shale creatures (see here and here) also date from this period, as do the first vertebrates – creatures with backbones.
Primitive fish appeared in the Ordovician period.
Life first appeared on land in the Silurian – early plants, and relatives of spiders and centipedes. In the sea were coral reefs and the first fish with jaws. The first freshwater fish also evolved in this time.
Plants grew larger – ferns & horsetails developed. The first amphibians appeared – able to survive both on land and in water. Ammonites developed in the oceans.
Amphibians need water to lay their eggs in. In the Carboniferous period some began to lay shelled eggs, which meant they could spend their entire lives on land. They became the first reptiles.
The first true trees – conifers – appeared. Meganeura, the largest dragonfly ever discovered, is from this period. It had a wingspan of 70cm. A main hunter at this time was Dimetrodon, which had a sail of skin on its back. It’s one of a group of creatures called pelycosaurs or ‘mammal-like reptiles’ which are the ancestors of the mammals.
MESOZOIC ERA:
251 – 65 mya
The Mesozoic era, the Age of Dinosaurs, is made up of 3 main periods. From earliest to latest they are:
The first dinosaurs appeared in the Triassic period, as did primitive pterosaurs – reptiles that flew like bats or birds.
The earliest mammals – tiny shrew-like creatures – date from this period, as does the Archaeopteryx, a small dinosaur that developed feathers and wings and became the ancestor of modern birds. The famous Tyrannosaurus and Stegosaurus date from the Jurassic period, as do the seagoing Plesiosaurus and Ichthyosaurus.
Many familiar dinosaurs such as brontosaurs, Allosaurus, Velociraptor, Iguanadon and Pteranodon come from this period; but it’s also when the first flowering plants appeared. Magnolia and ginkgo trees still grow today.
No one knows why the dinosaurs became extinct 65 million years ago. These days most scientists think that they died out because a gigantic meteorite struck the Earth and made the weather so bad that the dinosaurs couldn’t survive. They’ve even found the crater it left at a place called Chicxulub.
If you want to find out more about dinosaurs, this BBC website is a good place to start; while this Dinosaur Dictionary is useful for looking names up.
CAINOZOIC ERA:
65 mya – present day
The Cainozoic is divided into the Tertiary and Quaternary Periods, which are subdivided into epochs. The BBC’s Walking with Beasts site has a nice animation where you can drag a slider to see how the shapes of the continents changed through the Cainozoic, and click on buttons to see the different kinds of creatures at different times and places.
Placental mammals first evolved.
The ancestors of whales returned to the sea. Dog and cat-like predators hunted on land.
Grasses appeared for the first time. Some creatures developed the ability to eat grass and lived on the new plains of grass.
The first monkeys and apes appeared during the Miocene.
Many giant mammals lived in the Pliocene, including glyptodonts (giant armadillos), megatheria (giant ground sloths) and Gigantocamelus, a 15-foot tall camel. Australopithecus – an early ancestor of humans – first appeared in this epoch.
Note: start of Age of Man (see below)
The Pleistocene was an epoch of ice ages. Creatures like mammoths and woolly rhinoceroses grew thick coats to survive the cold. Sabretooths, big cats with enormous teeth, also date from the Pleistocene, as does the human ancestor Homo erectus.
The last 10,000 years. Includes all recorded human history.
THE AGE OF MAN:
from about 3 million years ago – present day
When we’re teaching the children the timeline we use this as a convenient term for the time that human beings have existed on Earth; it’s coloured red on the chart so is sometimes known as the Red Era in Montessori terminology. The earliest Australopithecus fossils date from about 4 mya. Australopithecines walked upright and had humanlike hands but very small brains. By about 2.2 mya Homo habilis had appeared. They had much larger brains than australopithecines, though only about half the size of modern humans. They may have been able to speak, and they made simple stone tools. Homo erectus appeared at about the beginning of the Pleistocene. We’ve found some of their camp sites, so we know that they’d learned to use fire. Neanderthals, who were very close to modern humans, lived during the Ice Ages from about 200,000 years ago. We’ve found some of the caves they lived in. It was only 30,000 years ago that modern humans like us, Homo sapiens, first appeared.
You can find more information on human evolution on the internet.