Lying on my back on a mountain top in Maine, I find myself jerked past Alpha Centauri, cruising back in time as I move away from Earth. As the stars rush towards me, my breath catches in my chest. The trip through universal history is awesome, and I can experience that time warp without ever leaving my comfortable seat on Earth. The astronomical events we witness on a nightly basis do not occur contemporaneously. Light emitted from celestial bodies must make an arduous journey across light-years to reach our eyes. As a result, each glance into the night sky is a look into the past. But once light from our own sun makes its eight minute morning commute, we are cut off from the voyage into history and must return to life here and now.
It makes me wonder, though, are we truly so isolated on our tiny third rock from the sun, so seemingly ideal for life and growth? Does life not commune with outer space in other ways? As I write this, a gray sky insulates, making me feel alone and distinct, a passenger in a controlled capsule cruising through space. But the journey from Baltimore to New York I am currently making is nothing compared to the possible distances my components may have crossed. Could the building blocks of life, or even life itself, exist in what we understand to be little more than conglomerations of rock and ice?
Panspermia proponents postulate just that, and quite plausibly. Consider the North East Regional train that is currently carrying me up the east coast on a set trajectory. External fluctuations in environment have small and superficial effects on its composition and performance. Layers of insulation leave the inside a controlled climate, allowing its passengers to travel in comfort. It leaves a trail of dust and gases in its wake. Similarly, a comet could be seen as a pan-solar system transporter. Formed on the outskirts of the solar system, comet bodies contain ice and other frozen volatiles, and in their early existence, radiation from the explosion of a nearby supernova melted their primitive interiors.
The warm and wet environment inside a comet could provide ideal breeding ground for anaerobes, or, more simply, a primordial soup for the synthesis of organic compounds. Explosions or collisions on life-harboring planets may propel living matter into space, which then becomes encapsulated in comet bodies. Because some extremophiles, not only among archaea and eubacteria, but even within the kingdom animalia, can survive the vacuum and elements of space, some living matter could hover in the emptiness until they are picked up by a nearby piece of matter. Perhaps, by this mechanism, life as we define it has persisted since long before the dawn of our planet or solar system.
Comets carrying programs for life may inseminate a body, maybe a world like Earth, upon impact. If the comet delivers its cargo to a body mature enough to nurture and support development, then that initial deposition could cascade into the evolution of complex ecosystems. Growing, dividing, and branching, vast arrays of life adapted to local environments would arise from alien ancestors. All native species, then, would be alien. Even humans, so used to thinking that the Earth is their home and their God-given habitat, may truly be extraterrestrial in origin if life on Earth originated via a panspermic event.
Perhaps this is why we have such a fascination with outer space. We constantly try to look deeper and further, we search for clues to the existence of other inhabitable planets, we speculate about the existence of life elsewhere. Perhaps something soft-coded in our epigenomes, handed down from generation to generation, drives us to find our true homes or seek out long lost relatives. I personally feed my insatiable need to explore our world and others through imaginary space and time trips by consuming mass quantities of science fiction.
Science fiction books, movies, and television shows, at least those that depict more peaceful and relatable intelligent life forms, tend to portray aliens as humanoid. Sometimes these species, which, in the imaginary universe, originated in entirely different worlds, interbreed with humans to form hybrid offspring. I always find that strange; why should life from different and unique worlds even operate using the same molecular mechanisms, let alone have evolved to reproduce by the same means? Is there something fundamental about DNA and proteins that implies that all life forms must utilize the same nucleotides as the carriers of developmental and regulatory programs, and implement them with the same set of amino acids, translated in the same way? This is certainly not true for amino acids; we can re-engineer cells to utilize amino acids not found in the standard 20 piece toolbox. But had life originated on a single planet, moon, comet, or asteroid, which then launched its living material into space to disperse and fertilize multiple bodies, common mechanisms like those depicted in science fiction would sit better with me viscerally. Because we would share a common ancestor with other-worldly beings, the rules set out on our shared ancestral world would likely persist in the new environments. Biology, once created, does not generate anything new; it simply modifies and builds on what already exists.
So does it then make sense that humanoids with compatible reproductive systems evolve? Even more basically, should we expect life forms from independent evolutionary histories to share any body patterns at all? Paleontologist and evolutionary biologist Stephen Jay Gould would argue no, because natural selection and evolution depend on a high level of chance. He said that if you were to rewind the evolutionary clock to the beginning then allow it to run again, a completely different set of organisms would arise than the set we see today. By similar arguments, we might anticipate that the founding members of ecosystems on different planets, even if they are from the same species, would diverge and evolve independently. But thermodynamics would suggest otherwise.
Systems, whether designed or evolved, strive for maximum output for a minimum energy input. As a result, when evolutionary forces select for certain designs, they would move to more efficient constructs. While differing environmental conditions may have different optimal constructs to execute certain functions, certain constructs may be universally optimal under comparable environments. Therefore, paradigms like quadrupedial locomotion with feet immediately under hips (like a dog), which is more efficient than quadrupedial locomotion with feet splayed outwards (like an alligator), may have evolutionary advantage, and therefore selected for, in earth-like worlds. Also, gene regulation mechanisms, which could bound the capabilities of developmental programs, would have been determined by the founding organisms in each world; therefore, worlds with common alien ancestors would share limitations on how species can develop and regulate themselves.
It would not be so far-fetched, then, to find similar patterns among Earth creatures and creatures found on other earth-like bodies that share a common ancestor and have been subject to evolutionary forces on a similar time scale.
Evidence of panspermia is weak: a meteorite carrying nucleic acids, a sterilized space shuttle ferrying bacteria on its exterior, but nothing has of yet refuted the theory. I am certainly not sold on the idea, but I find it an exciting playground for the imagination. Consider a corollary: if life or life-bits exist on extraterrestrial bodies, we may, in fact, be constantly communing with the alien. Microscopic information transfer could be occurring unwittingly as life programs from earth are carried to space on the backs of space shuttles, which then bring back responses from extraplanetary debris. Depositions of organisms from outer space could grow and evolve in our upper atmosphere. These organisms, dragged down to Earth by objects passing through the atmosphere, could constitute invasive species of a whole new from. Gallivanting across Earth in the form of pandemics, alien invaders may be waging biological warfare with us even now. Some of us may be harboring extraterrestrial terrorists in our own bodies.
But even if alien invaders, whether employing biological warfare on a microscopic level or engaging native species with advanced weapons and planned attacks, caused a mass extinction event, life will still persist. Once life has begun, it is nearly impossible to end. Populations will rapidly evolve to fill the niches of those killed off, and others will rapidly evolve to resist the foreign entities and perhaps even fight back. Maybe the alien invaders will take over the Earth, and evolve whole new ecosystems. Even if the Earth explodes, expelling its constituents into the vacuum of space, life will still persist.
Extremophiles that can tolerate the vacuum of space already exist. Some may cling to the ruins of their shattered world, while others, free floating in space, may score passage on comets or other celestial bodies. Conceivably, even one cell could make its way to a body that can support it, give it the nutrients to grow, and seed life again.