Sun Comes Up from Behind Earth in Space.
The mainstream planetary science community has become so obsessed with the hunt for life offworld, that it often forgets how little we know about how life formed on our own planet.
Astrobiology, after all, is not just about finding life on planets circling other stars, or on the moons of Jupiter and Saturn, but also about understanding how microbial life first started on our own planet. Why did it happen here? Did it need help from meteorites that seeded the planet with the necessary chemistry for life to first begin?
And did life here originate more than once? And did it happen coevally around our planet at about the same time?
At least one prominent astrobiologist thinks it’s possible that the answers to the last two questions may both be yes.
We still don’t know how life on Earth originated, Zita Martins, an astrobiologist at Portugal’s Instituto Superior Tecnico, told me in her office in Lisbon. But we’ve discovered little pieces of a puzzle that build this big picture about prebiotic chemistry, meaning what happened just before life originated. We also know a lot more about the chemistry, geology and physics when our solar system first began, and that’s crucial information that we didn’t have when I started 25 years ago, she says.
Life may have originated within two hundred million years after Earth’s formation. But the oldest evidence for life dates back to at least 3.5 billion years ago.
As for how it happened?
Lots of the organic molecules that make up cellular life are present in meteorites, asteroids and comets, says Martins. And that organic matter was brought to Earth early in Earth’s history, she says. But these same impacts may have also aided the onset of life by creating local niches in which life could develop.
Nearly a decade ago, Martins and colleagues from the University of Kent in the U.K. simulated comet impacts onto a planetary surface. Such impacts generate so much energy, that that energy needs to go somewhere, says Martins. And obviously, the impact energy is going to be enough to break molecular bonds and create more complex molecules, she says.
Martins says once Earth cooled enough to harbor liquid water on its surface, our planet have hit a critical stage where life could begin to form. She say that it’s even possible that microbial life may have evolved in more than one spot on the globe at about the same time.
But Martins also notes that getting a full picture of the early solar nebula’s chemistry is also important in understanding the feedstock of molecular components that went into the evolution of the first life here. Over and over, she emphasizes the surprising rich molecular chemistry that she and colleagues continually find in meteorites, on comets and on asteroids.
Glimpses into the makeup of the gas and dust from which our solar system formed is also important in giving us clues to what type of interstellar chemistry contributed to our early solar system’s first chemical building blocks.
Martins wonders why life choose certain specific molecules to use and not others that were available. That’s one of the things that are not so sure about if we are ever going to answer, says Martins. We can have a glimpse of what happened, but I’m not sure if we’re ever going to answer that question, she says.
Then there’s our lack of understanding of how life enabled the big jump from just having prebiotic chemistry to actually having biology, says Martins. We’re still not there, but the important thing is tried to understand and know which type of molecules were available from which life could first start, she says.
What personally puzzles Martins most about the origin of life on Earth?
When it happened, and why and whether there was some event that actually kickstarted the origin of life here, says Martins. I’m particularly interested to see if similar conditions that happened at the time that life originated here on Earth, also happened at other locations of our solar system, she says.
Martins also wonders if life evolved here more than once? Did it repeat several cycles of origination and destruction before finally taking hold? And is there any way to scientifically prove that life did evolve here more than once?
It’s still a very difficult question to answer right now, says Martins. We have to keep analyzing samples, doing field work and studying different fossils to determine if they have a common genesis or not, she says.
Will we ever understand exactly how life started here?
I’m not so sure if we will ever know exactly how life started on Earth and when, says Martins. But certainly, we will understand more about the conditions in our solar system just before life started here, she says.
Ironically, we spend all this time looking for life, but we still have a very difficult time coming up with a definition of exactly what life is. If you ask two different molecular biologists, you are likely to get two different answers.
It should be self-sustained system capable of replication is a very simple definition.
And it should have metabolism, obviously, she emphasizes.
That leaves out viruses, which are parasitic in their nature and are metabolically inert.
As for Martins?
Even though viruses like Covid regularly wreak havoc here on our own planet, Martins still doesn’t consider them to be a life form, simply because they are not self-sustaining.
