One of the most challenging things I’ve tried to explain in my science communication career is the concept of ‘deep time’, or the geological timeline of the earth. While it is crucial to understanding the history of the Earth and life as we know it, the vast span of time involved can be an incredibly difficult concept for young people to grasp – and rightly so. When an 8-week summer term can feel like an idyllic lifetime, 4 billion plus years is mind-blowingly huge. However, having a strong conceptualization of scale and the relationships between scales is essential in being a scientifically literate consumer of information (Tretter, et al., 2006), so how do you explain geological time in a way that young people can grasp?
An important engagement tool is helping people to relate to the facts you’re telling them, making them invested in learning more rather than counting on them to want to gain knowledge just for knowledge’s sake. Many educators use the hook of human existence when explaining geological time, which not only interests the audience but gives an excellent reference to the vastness of scale with which you are dealing.
You could try to calculate it out in numbers: The history of life on Earth began about 3.8 billion years ago, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it’s only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma. Mammals didn’t evolve until 200Ma and our own species, Homo sapiens, only 200,000 years ago. So humans have been around for a mere 0.004% of the Earth’s history. (http://www.bbc.co.uk/nature/history_of_the_earth)
Many people turn to analogies, those things which are like… other useful things. For example:
The 24-hour clock: Did you know…that if we compress the Earth’s 3.7 billion years existence to a 24-hour time scale, the first human species appeared about 47-94 seconds before midnight, and our species (Homo sapiens) appeared roughly 2 seconds before midnight? (http://www.rigb.org/contentControl?action=displayContent&id=00000001056)
Clocks are actually a very common visualisation tool: http://deeptime.info/
The book of the world: If you were to write a history of the Earth’s past, allowing just one page per year, your book would be 4,600,000,000 pages long. That’s a very thick book — 145 miles to be exact. An average reader, reading about 1 page every 2 minutes would need more than 17,503 years to finish it. And that’s with no time out for anything else — no time to eat, sleep, ride a bike, or go to school. Even if you were an amazing speed reader and could read 2 pages every second, it would still take you nearly 73 years to read the entire book. (http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/)
You can even get creative with a roll of toilet paper:
One of my favourite ways to explain just how long the earth’s timeline stretches is to hold out my arm and ask my audience to imagine the entirety of the earth’s history is represented along its length. It’s billions upon billions of years old, and things changed verrrrry slowly. The first life – bacteria – started forming here, around my bicep, and oxygen didn’t even start building up in the atmosphere until around here, my elbow. After that things start developing a bit more quickly, with blue-green algae appearing here, eukaryotic cells here, and down by my wrist the first multicellular organisms finally enter the story. In just the short span of my hand most of the life we know evolves and flourishes, and eventually – at the very end – human beings appear. How long have they been around, in the grand scheme of things? No more than the sliver of my fingernail at the end of my middle finger. So while we like to think that humans are the dominant species, superior to all others, in the grand scheme of things we’re newcomers on this planet – ones who could be erased with the flick of a nail file!
And just in case you were wondering how things stacked up in a galaxy far, far away: http://io9.com/5890342/a-chart-that-explains-how-long-ago-star-wars-actually-took-place