If millions and billions of years leave you unmoved, if names like Ordovician (485 to 444 million years ago), Triassic (-252 to -201 million years ago) or Palaeoproterozoic (-2.5 to -1.6 billion years ago) mean nothing to you, that's perfectly normal.
It's very difficult for a human being to imagine such long periods of time. Even for geologists, who deal with these concepts on a daily basis, it is often difficult to get an idea of the chronology of events that have marked the history of our planet.
Yet there are wonderful stories to be discovered in the Earth's past. It is also an opportunity to become aware of the past diversity of the planet's environments, in which living organisms have played a key role. Moreover, geological subdivisions of time are based on the different fossil life forms found in the corresponding geological layers.
The chequered history of life on Earth
The course of events over the last 4.5 billion years has not always been a smooth one. Initially, there was a virtual absence of oxygen (current levels were made possible by the emergence of cyanobacteria), massive volcanic eruptions, the formation of gigantic mountains, mass extinctions, impressive asteroid falls, climate change and variations in sea level that are nothing like what we are observing now.
All this has affected the forms of life that have been able to survive on Earth. Over time many species of living organisms have emerged, but there have also been major biological crises. There have been five major ones in fossil time alone – that is, the last 540 million years – even though this represents only 12% of our planet's history since its birth.
It is precisely the different forms of life that scientists use to characterise the different geological periods. To determine the sequence and duration of these events, geologists have studied the fossilised traces of organic life in the sediments. Subsequently, other techniques, in particular those based on isotopes, have been used to quantify and refine the geological time scale.
Considering our world on a geological time scale enables a calm view of the future
Tracing the history of the Earth is a humbling experience. The 20th century saw spectacular advances in knowledge of the Earth sciences, with the conceptualisation of plate tectonics, isotope dating and the discovery of life forms at the bottom of the oceans and in the Earth's mantle.
The 21st century is one in which societal issues are directly contingent on the Earth sciences. Whether we're talking about mineral or groundwater resources, natural hazards, spatial planning or adapting to climate change, Earth science specialists play an essential role.
Will we be able to find the necessary resources within an acceptable development framework? Will we be able to anticipate the adaptations needed to cope with climate change? Will we be able to predict and mitigate the effects of natural disasters? To answer these questions, it is essential to have a good knowledge of geological time, a precious tool that is constantly being improved and basic to any study.