It’s the age-old question: where did we come from? The answer is lost in the mists of time, so far back among the beginnings of our world that any credible answer may not exist. And therefore, given the following thought experiment, we might have to consider some incredible answers.1
Consider that the Earth developed with the rest of our solar system out of the proto-Sun’s accretion disk about 4.6 billion years ago. Consider also that this fresh, new planet appears to have harbored life in the form of single-celled bacteria for approximately 3.8 billion of those years.2 This suggests that in the first 750 million to one billion years, while the planet was still in the late stages of coming together as dust and chunks of tumbling rock, then suffering repeated asteroid impacts and surface volcanism, and finally suffering repeated cometary impacts in order to collect its first surface water, the complex chemical regimes that would later lead to life were able to assemble and sustain themselves.
First would come ribose nucleic acid (RNA), which forms a single strand of sugar rings connected by phosphate bonds, like beads on a string in a relatively straight line. Each of the beads contains a nucleotide base which is compatible with and binds with certain other nucleotide bases. That bonding action allows the RNA string to assemble copies of itself. It supposedly became the first self-replicating molecule, and thus was able to retain and transmit a particular chemical structure.
Next would come deoxyribose nucleic acid (DNA), which forms as paired strands that twist around each other in the double-helix pattern, cross-connecting through those nucleotide bases. RNA is differentiated from DNA by having an OH group attached to the second carbon atom of each ribose ring, where DNA has lost that OH and substituted a hydrogen atom (and thus is “deoxy”). Perhaps the lack of that OH radical allowed DNA to curl around on itself protectively, while the RNA molecule remained straight and wobbly. Another difference between the two molecules is that one of the four bases3 used as attachment points in RNA, uracil, has been replaced by thymine in DNA. The fact that the RNA molecule and its operation are simpler suggests to me that this molecule came first and that DNA is an evolutionary step toward more complexity.
In either case, RNA—and later DNA—would have had no particular reason to exist without its primary cellular function of assembling amino acids into long strands of proteins, which are the workhorse molecules of all life. Simply making copies of the RNA itself from one generation to the next is hardly a viable function—i.e., it ain’t much of a living—for such a complex molecule in a chemically violent world. The mechanism for reading the bases and assembling proteins, the ribosome, itself consists of a complex of both RNA molecules and proteins. So it would seem that both RNA and some proteins, and their reconfiguration into the ribosome, had to be present somewhere near the beginning of life.4
A billion years is a long time. But still, I find it hard to think that the DNA-RNA-protein regime could arise and comprehensively beat out all possible competing chemical processes, and so have established the world’s only complete and functioning prokaryotic cell, in this relatively short amount of time. Or that it could accomplish this amid all the colossal disturbance and heat from volcanism and comet and asteroid impacts.
The mechanism of evolution—trial and testing of an organism to establish its fit with its environment—suggests that other life processes may have existed briefly, been tried but found less appropriate, less functional, less capable than the DNA-RNA-protein regime. But, strangely, we find no evidence on Earth of any other system ever existing. Certainly, fragile molecules that failed to create cells—which were protected, first, with simple phospholipid membranes and then, later, with complex, robust body structures and skins—would tend to disintegrate and disappear, rather than leaving traces in mudstones and other fossil formations. If life ever made a trial start with other chemicals, it certainly left no trace on Earth.
But consider that even the smallest details of the DNA-RNA-protein regime are ubiquitous on this planet, with no surviving competitors to be found. Even the most far-flung and isolated creatures, such as those clustered around deep-sea vents and drawing their life’s energy from synthesizing sulfur compounds through volcanic heat, instead of photosynthesizing carbon compounds from sunlight, all use the same DNA-RNA-protein regime as us humans and our closest ancestors.
All life on Earth not only uses the same carbon-based DNA as its recording and transcribing medium, and RNA and its ribosomes as its translating medium, but all life uses them in exactly the same way. DNA bases taken in groups of three as a reading frame call for exactly 20 out of the 500 possible amino acids in exactly the same way to produce the myriads of possible proteins.5 No living creature or fragmentary ancestor has ever been found that uses a two-base or a four- or five-base reading frame, or whose genetic code calls for just 18 or 19, or 21 or 22, amino acids. Other such systems might have been more robust, more or less susceptible to mutation and the beneficial effects of evolution, but we wouldn’t know. All life on Earth came down from the winner of the evolutionary lottery that’s encoded in our genes.
And isn’t that odd? We find no variations in the basic system—except for minor discrepancies, like the fact that thymine replaces uracil in the transition from RNA to DNA coding, or that two extra amino acids are added to some proteins in rare instances by non-genetic means. But even these are ubiquitous cases. Nowhere on Earth do we find coding systems using different amino acids, or a wider or narrower reading frame, or a different translation scheme. Surely, in the competitive arena of evolution, chemical systems must have arisen and might have remained functional that were almost as good as the current version of the DNA-RNA-protein regime, that offered parallel strategies for success, and that sometimes made a better fit to a slightly different environment—like those volcanic vents—which would reward a different mutation rate or provide a richer lode of different chemicals.
It might be possible, for example, to produce an analog to the DNA-RNA-protein regime using other elements with similar electron configurations that form similar covalent bonds.6 For example, silicon can form the same types of molecules as carbon, because it has four valence electrons—available for sharing in molecular bonding—the same as carbon. Since the available electrons in a silicon atom are in the third electron shell, while those of carbon are in the second shell, the silicon bonds will be farther away from the atom’s nucleus and so relatively weaker. Similarly, arsenic has the same electron configuration as phosphorus. Indeed, for a while, biologists studying bacteria from Mono Lake in California—where large amounts of arsenic are naturally present—believed they had found arsenate groups substituted for phosphate groups in the backbone of a certain bacteria’s DNA. That finding has since been disproved.7
On a planet rich in silicon and arsenic, and relatively poor in carbon and phosphorus, an alternate set of life molecules similar to the DNA-RNA-protein regime might evolve. Creatures made with these molecules would be heavier and denser than Earth life, because silicon has an average atomic weight of 28 compared to carbon’s 12, while arsenic has an average weight of 75 compared to phosphorus’s 31. But such life would also be more fragile—more prone to mutation, molecular breakage, and even disintegration—because the electron bonds holding the molecules together would be weaker.
It’s possible, of course, that the lottery-winning, carbon-based DNA-RNA-protein regime originated on Earth, just as different chemicals might form different life molecules on other planets with other environments. A billion years is, after all a long time. And the right combination of chemicals only has to come together once—if it’s robust and yet flexible enough to survive and thrive. It’s possible the Earth produced one clear winner in the evolutionary lottery, one system to dominate the planet, coming out of the time when the liveliest thing on Earth was a string of sugar rings held together with phosphate bonds—and that all competing systems suddenly became eternal losers, washed away in the sea of life.
But still, until we learn different, the possibility exists that the DNA-RNA-protein regime did not originate here. “Learning different” might involve someday finding an alternative chemical regime that evolved here on Earth, or finding evidence of the DNA-RNA-protein regime in the sands and clays of Mars, on the moons of the outer planets, or on the planets of other stars. And if it did not originate here, then the DNA-RNA-protein regime might have been seeded here when the right conditions existed on Earth: not too hot, not too cold, offering liquid water that isn’t frozen over or boiling away, offering adequate amounts of the right free gases like oxygen and nitrogen, and not too much ultraviolet sunlight coming through the atmosphere.
The seeding mechanism need not have involved silvery spaceships landing among the rubble heaps of the early Earth and gloved hands pouring out beakers of chain-linked chemicals, or even simple cell structures, into the early seas. The seeding could have been accomplished by shooting snowballs laden with chemicals and spores into the universe, hoping that one of them would add its secret to the Oort cloud of icy particles left over from the accretion disk surrounding a Sun-like star. Then all it would take is an orbital jumble and a fumble to send a laden comet down to crash into the primeval sea of a possible planet in the star’s habitable zone. One snowball might be all it took. And all the rest might as well drift forever through the galaxy or perish in impacts on a burning Mercury or a hostile Io, where compatible life is not possible.
Achieving this kind of scattershot seeding would not be a random act. It’s not likely that an asteroid impact on a green planet in some distant star system would throw up a cupful of water bearing microbes, which would then freeze in space, find its way out of the planet’s gravity well, then out of the star’s gravity well. We know from human experience how hard it is to eject a probe from the neighborhood of the Sun: developing and launching Voyagers I and II took concentrated effort. As our own Oort cloud and its comets prove, things in the neighborhood of a star tend to fall inward, not outward. So it’s unlikely to happen by chance that a cupful of sea water left its native planet even once, let alone the thousands or millions of times needed to satisfy the long odds of its microbes reaching Earth.
No, if our genetic stuff was made elsewhere, its delivery—either by hand or by the scatter shot of dirty snowballs—was a conscious act. Some intelligence had to want the universe in general or the Earth in particular to share the gift of life. And how those star-seeding intelligences got their own life in the first place … well, as the old woman said about a cosmology that rode on the back of a giant turtle, “It’s turtles all the way down, young man”—for all we know, an infinite regression.
This does not mean that any aliens we are likely to meet will be humanoid. Such a conclusion does not follow any more than the notion that the evolved primates called humans, kin to chimpanzees and gorillas, are the only form of life that can reach intelligence. Evidence of intelligence among dolphins, whales, and elephants aside, we have no way of knowing that dinosaurs were not philosophers of exquisite insight, and only the lack of long arms with prehensile thumbs kept them from expressing that intelligence through art, architecture, and written lines that might have survived the Chicxulub disaster to be found among the mudstones today.
No, if we were seeded here, it was as a mere possibility, a potential written into the primitive code of a one-celled animal, or even into the humble mechanism of an RNA strand, perhaps as a virus with a lipid coating. What we became, what all life on Earth became, was prefigured more by the planet’s environment and the adaptations it forced, than by the code string that enabled those adaptations.
Whether that first glimmer of life developed here on Earth or elsewhere, its flowering here into all the different forms of life—some of them bearing self-awareness and enough intelligence to look up at the stars and wonder—is the true miracle.
2. From G. M. Cooper, The Cell: A Molecular Approach, 2nd edition (Sunderland, MA: Sinauer Associates, 2000). Available at the National Center for Biotechnology Information. The page gives a pretty good view of the origin and evolution of cells.
3. The base rings of both the DNA and RNA molecules serve as attachment points for their complementary strands. These rings are built mostly out of nitrogen atoms, and their complementary bonding consist of pairing one of the purines—adenosine (A) and guanine (G)—with its matching pyrimidine—either thymine (T) or uracil (U) with adenosine and cytosine (C) with guanine.
4. The other part of the process—the twenty amino acids from which all of the earthly proteins are constructed, or at least their precursors—have been detected on dusty grains of ice floating in interstellar space. See “DNA and amino-acid precursor molecules discovered in interstellar space” from the Kurzweil Accelerating Intelligence news site for March 2, 2013.
5. Actually, 22 amino acids are sometimes used in protein synthesis, but two of them—selenocysteine and pyrrolysine—are added to the protein string by other biological mechanisms, rather than encoded by the genetic material.
6. Alternative chemistries have been proposed and artificially created in the search for life’s origins. For example, some scientists have substituted threose, a molecule similar to ribose, to create a long-strand molecular chain capable of self-replication. See “Strange cousins: Molecular alternatives to DNA, RNA offer new insight into life’s origins” from Science Daily of April 19, 2013. To my knowledge, none of these alternatives have been found in nature. Another page at the National Center for Biotechnology Information offers an overview of alternative structures for DNA, such as different spiral patterns, crosses, and loops.
7. See “Study Confirms Bacterium Proteins Bond To Phosphate, Not Arsenate” at the RedOrbit news site from October 4, 2012.