Science: Explaining evolution 3 – Time scales

Here is the third in our series of science blogs exploring evolution.

Explaining evolution 3 – Time scales

One thing we cannot do is turn back the clock to see what has come before on this planet. We use human evidence (e.g. pictures or writing) for recent events, but for changes that happened before humans appeared, we have to rely on geological, geographical and scientific evidence. This combined evidence points to changes over vast time scales – scientists from geology and astronomy confidently estimate the age of the Earth to be approximately 4.5 billion years – the first life on Earth is thought to have started 3.5 billion years.

These time scales are difficult for humans to appreciate. This does not help in the general understanding of evolution which is a very slow process of very small changes over a long time. The white cliffs of Dover are composed of chalk and are thought to be the calcium carbonate remains of skeletons of small one-celled animals that lived millions of years ago. The cliffs are over 100 metres tall and for over 30 million years small pieces of skeleton fell to the sea bed. This sea bed layer was compressed into chalk which became white cliffs when the land rose above the sea. This is an enormous amount of time – 300 years for every millimetre of cliff. Yet the time taken for the chalk layer to form the cliffs is but one hundredth of the time which has elapsed since life first started on planet Earth. I find this awe-inspiring.

Another analogy is to compress the age of the Earth into 24 hours (or one day). If Earth is formed at midnight, early life appears at 4.20am, early plants appear at 9.30pm (yes pm!), dinosaurs appear at 10.40pm and the earliest humans appear at 3.5 seconds before midnight. To understand evolution we need to appreciate the incredible amount of time that passed before life first started on earth and before humans came on the scene. It helps to explain the amazing diversity of life around us – life that has evolved from simple forms 3.5 billion years ago.

John Roach


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