The defining feature of our world is life. For all we know, Earth is the only planet with life on it. Despite our age of environmental destruction, there’s life in every corner of the globe, under its water, nestled in the most extreme environments we can imagine.
But why? How did life start on Earth? What was the series of events that led to birds, bugs, amoebas, you, and me?
That’s the subject of Origins, a three-episode series from Unexplainable — Vox’s podcast that explores big mysteries, unanswered questions, and all the things we learn by diving into the unknown.
The quest of discovering the “how” of life on Earth is bigger than just filling in the missing chapters of the history book of our world. To search for the origins of life on Earth is to ask other big questions: How rare is it for life to form on any planet? How improbable is it for life to form on any planet, anywhere?
We don’t have all the pieces of the story, but what we do know tells an origin story of epic scope that takes us on an adventure to the primordial days of our world.
It all starts with water.
1) Where did Earth’s water come from?
The quest to understand why there is life on Earth must start with water because water is the one thing on Earth all life needs. Different forms of life can survive on extremely different food sources, but nothing lives without water.
So it’s curious that scientists don’t fully understand how water came to cover two-thirds of the surface of our world and create the very first condition necessary for life.
The problem is simple. When the Earth was forming, it was extremely hot. Any water that was around at the beginning would have boiled away.
“So how do you get so much liquid condensing onto the surface of a planet that should be really, really hot?” Lydia Hallis, a planetary scientist at the University of Glasgow, tells Unexplainable’s Noam Hassenfeld.
Scientists can think of a few plausible options. Was it delivered by comets crashing into our world? Or more fantastically, do we only have water due to the extremely circumstantial event of planets like Jupiter wandering toward the sun from the outer solar system? Or was it, somehow, deeply buried within the early Earth?
Hallis has been traveling the world to investigate and try to find some samples of the very oldest water on Earth. Here’s what she’s learned so far:
2) How did life start in that water?
Once there was water, somehow there was life. It’s possible that life didn’t start on Earth at all. But scientists have good reason to suspect it did.
For decades, scientists have been trying to recreate in labs the conditions of that early water-filled Earth. The thinking is, perhaps if they can mimic the conditions of the early Earth, they will eventually be able to create something similar to the first simple cells that formed here billions of years ago. From there, they could piece together a story about how life started on Earth.
This line of research has demonstrated some stunning successes. In the 1950s, scientists Harold Urey and Stanley Miller showed that it’s possible to synthesize the amino acid glycine — i.e. one of life’s most basic building blocks — by mixing together some gasses believed to have filled the atmosphere billions of years ago, with heat and simulated lightning.
Since then, scientists have been able to make lipid blobs that looked a lot like cell membranes. And they’ve gotten RNA molecules to form, which are like simplified DNA. But getting all these components of life to form in a lab and assemble into a simple cell — that hasn’t happened.
So what’s standing in the way? And what would it mean if scientists actually succeed and create life in a bottle? They could uncover not just the story of the origin of life on Earth, but come to a shocking conclusion about how common life must be in the universe.
Unexplainable’s Byrd Pinkerton explores in this episode:
3) What is life anyway?
However life first started, today we know Earth is teeming with it. We know life when we see it. But do we know what life fundamentally is?
“No one has been able to define life, and some people will tell you it’s not possible to,” says Carl Zimmer, science reporter and author of Life’s Edge: The Search for What it Means to be Alive.
That’s not for a lack of trying. “There are hundreds, hundreds of definitions of life that scientists themselves have published in the scientific literature,” Zimmer says.
This question — what is life — feels like it should be easy, something a fifth grader ought to answer. “It does feel like it should be easy because we feel it,” Zimmer says. “Our brains are actually tuned to recognizing things like biological motion. We’re sort of hardwired for recognizing life. But that doesn’t actually mean that we know what it is.”
The problem is, for each definition of life, scientists can think of a confounding exception. Take, for instance, NASA’s definition of life: “Life is a self-sustaining chemical system capable of Darwinian evolution.” But that definition would exclude viruses, which are not “self-sustaining” and can only survive and replicate by infiltrating a host.
On one hand, a definition of life would come in handy as we’re searching for signs of it on other planets and moons. How would we know life when we found it? On the other hand, Zimmer explains, perhaps it’s just not possible to define life — at least not with our current knowledge.
Further reading: What is life? Scientists still can’t agree.