It's that time of year again: Over the next week, or so, most of us will celebrate events that occurred many, many years before we were born.
For Christians, it's the birth of Jesus (2016 years ago). For Jews, it's Hanukkah's story of the Second Temple's rededication 100-plus years earlier. These events mean a lot of things to different people.
For me, as an astrophysicist, one of the things it means is time.
The time between now and the events that these holidays celebrate means lots of trips around the sun for lots of generations of human beings. How should we think about all that time and all those people?
Well, one way to get the whole story of humanity's remarkable journey on this planet is to watch this video from the American Museum of Natural History. In it, you can see the growth of the human population from our beginnings as a separate species — about 200,000 years ago — right up to the modern era. If you really pay attention, you can catch some details that put our own moment into perspective.
Here are some of the highlights you can pick up from the video:
- For the first 190,000 years of our existence, we were pretty sparse on the ground. While we had expanded across a lot of the world's land masses, there were fewer than a few million human beings in total.
- The development of agriculture 10,000 or so years ago drove an explosion in our numbers. No matter how you slice it, farming and city building were revolutionary for us (and the planet).
- By A.D. 1 there were 170 million human beings on Earth. The video does a wonderful job of showing where all those people were living (mostly Han Dynasty China, northern India and the Roman Empire). At first, 170 million might seem like a lot of people. But then you realize that's the entire human population of the entire planet. That means metropolitan Tokyo (to take just one modern city) now houses one quarter of the entire human race as it was when Jesus was born.
- Between A.D. 1 and the beginning of the Renaissance (almost 1,300 years) the human population barely doubled.
- You can actually see the world population decline — a rare event — during the Black Death.
- We reach 1 billion souls around 1800, at the beginning of the Industrial Revolution. Then — boom! — the population skyrockets. In just two centuries, we add 6 billion more new human beings.
Taken as a whole, this video gives us an example of exponential growth. That's the world we're experiencing now, in everything from population to fossil fuel consumption. Unfortunately, our evolutionary experience makes us ill-equipped to wrap our minds around exponential growth. But after watching the population bloom, it's not hard to understand why we're reaching all kinds of critical thresholds now. Still, we can leave the lessons of climate change, etc., for another time. For now, it's enough to think about something simpler — but equally challenging to grasp.
All those people.
All those hopes, all that suffering, all that joy, all those birthdays and holidays lived year after year. All those lives and all their stories.
For me, this video shows how important it is to try — even if just for a moment — to put myself in my proper place at the head of that line and see my commonality with all who have come before me. Each person represented in the population plot, as it marches across millennia, was just like me in his basic humanity. And someday I, too, will be nothing more than a point on that line marking out the whole of humanity's story.
Finding our place on that line gives us a great reason for reverence and humility, two emotions that are perfect for the holiday season.
Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4