by James Morris
When I was a teenager and young adult, I always looked forward to reading Chet Raymo’s column called “Science Musings” in The Boston Globe. Chet Raymo is Professor Emeritus of Physics at Stonehill College in Easton, Massachusetts, and a well-known science writer. His short essays are reflections on science, education, and the natural world.
One of these essays, from the mid-1990s, made such an impression on me that I clipped it out and filed it in my “Science Education” folder, where I keep articles related to science and teaching. The essay is titled “Teaching a Sense of Wonder.” Here, Raymo makes a plea to 6th-grade science teachers, asking them not to emphasize terms and facts, but instead to stand back and think about what every middle school student should learn in a science class.
He boils it down to five important concepts, one of which is the history of life on Earth.
He writes, “Roll out a paper timeline in the longest corridor of the school. Start with Day One, the formation of the Earth. Walk across 3½ billion years of life, most of the way down the corridor, before encountering anything but microbes. Give the dinosaurs their few feet of time. Find our sliver at the end of the line.”
I have taken his message to heart. I teach introductory biology at Brandeis University, and I always take time to teach the history of life on Earth. Many websites convey this information in colorful ways. Some show the timeline linearly, others as a spiral. One particularly compelling website starts with a line representing today, then this month, year, century, millennium, epoch, etc., all the while keeping track of where we are on the growing timeline. Take a look. It’s dramatic and humbling at the same time.
Let’s take a quick tour here. The Earth formed about 4.6 billion years ago. For many of us, that’s a hard number to really grasp. String together 4.6 billion paperclips. They will circle the Earth about four times. That’s the age of the Earth.
For the first half billion years, there were no rocks and no life. But as soon as rocks formed, there is evidence of life. Scientists aren’t entirely clear how life emerged from non-life – it’s possible that early chemical reactions produced the first organic molecules, or that a meteor seeded the Earth with its first organic molecules – or even whether life exists on the many Earth-like planets that have recently been found in our galaxy. But multiple lines of evidence suggest that all living species on Earth can trace their ancestry back to a single living organism. We are all related, just on different branches of one big family tree.
What did the first life look like? It was single-celled, and lacked a nucleus, where the genetic material is housed in our cells. Modern descendants of these early life forms include two of the three great domains of life – Bacteria and Archaea.
The first two billion years of life on Earth (almost half of it) belonged to these single-celled organisms. It was a time of great diversification. Yes, the bacteria and archaea remained unicellular, but they evolved all kinds of different ways to harness energy.
One group of bacteria – cyanobacteria – evolved the ability to carry out photosynthesis, using the energy from the sun to build sugars and producing, as a byproduct, oxygen. So, for the first time, oxygen appeared in our atmosphere. Although we take it for granted today and can’t live without it, at the time, it represented a major crisis for life on Earth, earning the name “oxygen catastrophe.”
No other group evolved this ability – ever – in the history of life on Earth. It evolved exactly once. Organisms alive today, like plants and algae, that are capable of oxygenic photosynthesis can only do so because they incorporated, in one way or another, these bacteria.
The next major stop on our journey is the evolution of the third domain of life – the Eukaryotes – with cells that have a nucleus, like our own. This event occurred about 2 billion year ago, over half of the way along the timeline. These cells are the ancestors of our cells, but the creatures, like the Bacteria and Archaea that preceded them, were all single-celled.
It took another billion years (we’re at about 1 billion years ago, or 80% of the way down the timeline), before cells started getting together to form first simple and then more complex associations.
Unlike the evolution of life and the evolution of eukaryotes, multicellularity didn’t evolve once, but several times independently. For example, it evolved one time in the line of organisms that led to modern-day plants, and it evolved separately in a different line of organisms that led to modern-day animals. Put another way, the common ancestor of plants and animals was unicellular, not multicellular.
The last half billion years, the last 10% of the timeline, starts to look more familiar, and events happen quickly. The first marking point is the Cambrian explosion, a sudden appearance in the fossil record of many marine creatures. Don’t forget, all life is still in the water, where it first began.
If we divide the last half billion, or 500 million, years into five equal parts, we see the first fish around 500 million years ago,
the first amphibians around 400 million years ago,
the first reptiles around 300 million years ago,
and the first dinosaurs and mammals around 200 million years ago.
And what about us? Modern humans evolved just 200,000-300,000 years ago in Africa. In other words, everyone alive today can trace his or her ancestry back to Africa not that long ago. If the history of the Earth were a minute, we wouldn’t even get a second of time. No matter how you look at it, it’s a mere blink of the eye.
The writer Anthony Doerr in his two-minute entreaty imagined the history of Earth as the length of your arm, starting from the shoulder and working toward the fingertips. He asked, “And you? Your grandma’s toffee bars, your CD collection, your treehouse, your best-ever Halloween costume, every regret you’ll ever have, every dream you’ll ever dream, every mouth you’ll ever kiss (or wish you had)—they’ll all ride the microscopic edge of your fingernail, a tattoo so thin you’d need an electron microscope to glimpse it.”
In the course of teaching this material and trying to bring it to life in various ways, I have really come to appreciate how profound it is to understand the rich history of life on this planet and our tiny place on this long and dramatic timeline.
© James Morris and Science Whys, 2017