“The important point is that since the origin of life belongs in the category of at least once phenomena, time is on its side. However improbable we regard this event, or any of the steps which it involves, given enough time it will almost certainly happen at-least-once. And for life as we know it, with its capacity for growth and reproduction, once may be enough.” –George Wald
That there’s an amazing story of life’s evolution on Earth is a scientific certainty. The evidence is encoded in the nucleic acid sequences of every living organism ever discovered, and the history of life on this world is traceable back for literally billions of years.
But the origin of the very first organism that could be called “alive” is very much still an open scientific question. Was the first living creature on Earth created on Earth? Was the material that it came from present on Earth since our planet’s formation, or was it carried here during our planet’s early bombardment by asteroids, comets and protoplanets? Or, did it perhaps originate elsewhere, on one of those spaceborne projectiles, and then seeded life on Earth?
Based on what we know about planetary and star formation, all of these are possible.
Here’s why we think so.
Above is a multi-wavelength, composite view of the Milky Way’s center. It’s a region rife with the births and deaths of stars, very rich in the heavy elements that wind up forming planets and solar systems like our own.
But we don’t just find the elements by themselves, in isolation in these parts. They come — in interstellar space — assembled into what we traditionally think of as complex, organic molecules.
Known as polycyclic aromatic hydrocarbons, they give the image above its eerie, greenish, misty glow, and are revealed in great detail spectroscopically. (And it’s not really green; this is a false coloring of infrared wavelengths.) But these and other complex organic molecules, including amino acids, sugars, and ethyl formate (the compound that makes raspberries smell the way they do) are found in great abundance in interstellar space.
When the Earth formed out of our Solar System’s protoplanetary disk, it’s very likely that these elements, molecules and compounds existed in all of these locations: on Earth, on asteroids and comets, and on the proto-planetesimals that assembled into all of our Solar System’s planets.
Now, the way things have gone in our Solar System, we know they have, in fact, given rise to complex life in our particular instance. There’s evidence of that all around us, including in you, yourself.
But the unanswered question is what started it all, and where did that come from?
We’ve now got a huge, observed collection of stars with worlds around them, some of which have multiple planets, many of which have been around for long enough that — if they have worlds within what we consider their “habitable zone” — could have complex life as well.
An open question is whether asteroids can help or harm worlds in their efforts to evolve complex life. On the one hand, getting struck by asteroids can introduce new organics and materials into the ecosystem, and can knock off the apex animals of the time, paving the way for new species to mutate and fill niches. On the other hand, getting struck by too many asteroids can wipe out life too frequently, and could perhaps create a “disaster-world” where life never evolves beyond the simplest of organisms, or gets wiped out entirely.
An excellent piece of science was recently done by Rebecca Martin and Mario Livio. What they did was to analyze the Solar Systems of stars containing the 520 giant planets discovered outside of our own, the 90 known stars with warm dust (consistent with having asteroid belts), and compared that data with simulations they ran of Solar System formation. What they wanted to find out was an answer to the following question:
How frequently do we get an asteroid belt — far enough away from the parent star for snow/ice to form — that’s thin, and yet close enough to a gas giant that the belt will get thrown in towards the inner solar system?
When they compared their simulation with the data, this is what they found.
They found that a belt like the one found in our Solar System happens only 4% of the time, which is pretty rare! It’s not incredibly rare, but it’s not something that happens the majority of the time. Most times, either a gas giant moves inwards too quickly, disrupting or destroying a potential asteroid belt, or is formed too far away from the “snow line,” the line where it’s finally cool enough / far enough away from the Sun to form snow and ice.
But that does not mean that what we have is necessary — or even necessarily preferred — for complex life!
Life may form — complex life may form — even more easily in either of the other two cases. It’s a huge and not necessarily correct assumption to say that just because you can envision a catastrophic effect if we swing too far in either direction, that we’re in the “sweet spot.”
Life happened here on Earth and we have the history that we have; that’s a given. But until we discover complex life in other places or gain a significantly better understanding of how life originated on Earth in the first place, there is not enough evidence to state that the conditions found in the Solar System are preferred for creating complex life in the Universe.
Yes, it’s a very neat find that 4% of the solar systems out there — based on what we’ve observed — have similar asteroid-belt structures to our own, and that’s worth sharing with the world. But going beyond that and stating that those 4% of cases have a better shot at promoting complex life is a reach that goes well-beyond what the science tells us.