Into the Unknown, page 9
When talking about life in the universe, you’ll also hear a lot about liquid water, and for good reasons. Liquid water is essential for facilitating chemical reactions and as a transport medium in biological systems. The water molecule has some astounding properties that I think are woefully underappreciated. For starters, because of the structure of the water molecule—with the oxygen and hydrogen atoms not being in a straight line, it is slightly negative on one side and slightly positive on the other. This property results in water being a “universal” solvent. To be fair, it isn’t actually “universal” and can’t dissolve everything (thank goodness, otherwise showers would be a very different experience), but it dissolves more things than virtually any other liquid. Dissolving things might sound bad (I’m picturing some gruesome interaction with battery acid), but water’s ability to dissolve things makes it a workhorse in distributing important nutrients.
You may well take the fact that water expands when it freezes for granted, given how common this phenomenon is in our modern lives. But this property is rare. There are a few other materials that will expand when frozen, but generally stuff gets smaller and denser when in solid form. What does this have to do with life? If water got denser when it froze, lakes, rivers, oceans, and so on would freeze from the bottom up until they became frozen solid. However, because ice is less dense than water, it floats on the surface, not only acting as insulation to the underlying water, but fundamentally enabling life to continue below the nice crusty layer of ice. Without this quirky property of water, life on Earth would be very different indeed (and possibly nonexistent).
As far as liquid water goes, the distance of the Earth from the Sun is right smack in what is called the “habitable zone” or the “Goldilocks zone”—meaning we are just at the right distance to have liquid water. A little farther away and water would freeze. A little closer and the liquid water would evaporate. Curiously, Mars is dancing right on the edge of this habitable zone, and there is compelling geological evidence that Mars once had rivers, lakes, and perhaps an ancient ocean. Today there is enough known water ice on Mars to cover the entire surface by 35 meters, and there is likely more hidden away deep beneath the surface. It is entirely possible that life emerged on Mars at some point and is maybe even still there. This possibility has led some to consider attempting to “terraform” Mars to make it suitable for human habitation. However, for the record, terraforming Mars to be “suitable” for humans by nuking the surface (as a well-known billionaire entrepreneur is promoting) is a terrible idea for all sorts of reasons, including: (1) it won’t work, and (2) it is ethically abhorrent.
However, being in a habitable zone is not the only way to have liquid water because (once again for the kids in the back) sunlight isn’t the only source of energy around: if there is frozen water and a sufficient source of energy, liquid water is inevitable. Take Europa, for example, which is one of the plethora of moons of Jupiter (you can be forgiven if you don’t remember them all, neither do I) and way outside the nominal “habitable zone” in the solar system. Europa has a thick shell of ice, underneath which is a mineral-rich subsurface ocean. Kind of like an M&M of planetoids. Europa, for example, gets a whole lot of energy through this whacky phenomenon called “tidal heating.” As Europa orbits Jupiter, its insides get squished around from Jupiter’s gravity (not unlike the tides on Earth, except our tides are on the surface). All this squishing generates friction, and friction means heat. And voilà! Liquid water!
While having some type of liquid seems important for life, and water seems especially well-suited to the task, it is less clear whether that liquid must be water. For example, we know there are lakes of methane elsewhere in the solar system, and we also know there are some extremophiles (e.g., methane ice worms) that don’t mind a lot of methane. So that begs the question of whether some type of life could survive, adapt, and even flourish with an alternate liquid. Or indeed—if we are really pushing the envelope—whether the necessary chemical reactions and transport could happen in some way without any liquid at all.
Finally, another requirement for life to get going is time. Life is going to take a beat to evolve from simple organic compounds, and it needs a nice, comfy, safe place to really start a family. Exactly how long life needs to get started is open for debate, but there is some tantalizing circumstantial evidence to consider. To lay out the case, we need to go way back to the early solar system shortly after it started to assemble. The solar system was a nasty place back then—things weren’t going in nice, orderly, circular-ish orbits yet, and stuff was flying around smashing unsuspecting rocks to bits. The situation got extra intense about 4.1 to 3.8 billion years ago, when evidence suggests that the gas giant planets changed orbits (technically called “planetary migration”), which unleashed dynamic chaos on the whole solar system. This period was called the “Late Heavy Bombardment” because it was “late” in the early evolution of the solar system, and the bombardment was indeed quite heavy. If you can envision a rain of planet-altering asteroid impacts, you’re on the right track.
It’s possible that some form of life emerged on Earth before the Late Heavy Bombardment, but we will probably never know because the entire planet would have been catastrophically resurfaced, almost certainly annihilating any existing life. Perhaps several times. This is where the situation reaches a fresh new level of interesting as far as the “how much time is needed for life” discussion goes, because the earliest evidence for life on Earth via chemical signatures dates from roughly 3.6 to 3.8 billion years ago—right after the late heavy bombardment ended. If you want something more substantial than chemical signatures, the earliest fossil records are from about 3.5 billion years ago, which on astronomical timescales is just a heartbeat later.
These timescales indicate that life on Earth emerged basically as soon as it possibly could have. In turn, the apparent speed with which life emerged might suggest that it was relatively easy for life to get going. On the other hand, given that we only have a data sample of one planet with life emerging (so far) we need to be a tad cautious in drawing conclusions. But if we take the existing evidence at face value—because it is the only data we have—it tells us that sometimes (whether “sometimes” is closer to “commonly” or “rarely”) life isn’t so hard to kick off.
There is one more key obstacle to proto-life having enough time in a nice, comfy, safe environment, which is the lifetime of the central star in a given stellar system (i.e., the Sun in our case). As it happens, our Sun is about as “typical” as stars get, which makes it (relatively) boring. Boring is good for us—exciting places in the universe may not be the best for the purposes of life forming and evolving. As a rule, the less massive a star is, the longer it will live. Our Sun, being relatively low mass, will have a total lifetime of about 10 billion years, which leaves a lot of time for life to get going before being destroyed when the Sun dies.
The most massive stars (which do very exciting things) only live for as few as a million years or so, and during their brief lives they are not very nice to be around. Even given the very fast timescale that life got going on Earth after the Late Heavy Bombardment, a million years is perhaps too short for any form of life to evolve, and almost certainly far too short for that life to evolve beyond some microscopic state. On the flip side, the lowest mass stars basically live forever (OK, not technically, but for all practical purposes). But—and this is a big “but”—because they are so low mass, the “habitable zone” around them in which we might expect a planet to have liquid water is exceedingly small and close to the central star, which brings about other issues—first, the low likelihood of a planet being found within that small region, and secondly, the orbiting planet would be so close to the central star that it would probably be “tidally locked”—which means the same side of the planet would face the central star, possibly making one side too toasty for life and the opposite side too cold. The word “possibly” here is key, though—and there are ways to sneak around this (for example, something like a band of life-suitable conditions circumnavigating the planet on the light-side/dark-side interface).
As far as life goes, I think there are things in the category of “need to have” and another category of “nice to have.” I view energy, liquid, and time as fairly hard requirements for life (i.e., needs), but there are bonus features of Earth that are worth keeping in mind that are nice to have. We don’t really know how essential these nice-to-have conditions have been for life, but they probably helped life-as-we-know-it, and they certainly make Earth habitable for humans.
For example, Earth’s atmosphere has been vital for life on the surface (although subterranean life might not care as much). For one thing, having an atmosphere is great for avoiding the rapid vaporization of water. Our atmosphere also does a bang-up job of blocking the high-energy X-ray and gamma ray light, large doses of which are lethal to humans. To be sure, some of the extremophiles we talked about could really care less about radiation, so this is admittedly a human-centric view.
The fact that Earth still has an atmosphere is in part due to Earth still having a decent magnetic field. Without a decent magnetic field, the surface of the Earth would be bombarded with charged particles from the Sun (known as the “solar wind”), and that would cause all kinds of problems. For example, without Earth’s magnetic field, the solar wind would strip away the ozone layer, exposing life on the surface to nasty radiation. We can look to our sister planet Mars for a lesson learned: Mars was formed with a nice thick atmosphere, but after it lost its magnetic field, that atmosphere was swept away, leaving the dry and barren planet we know and love.
Earth also has a nice massive moon. Not only is the Moon lovely to look at, which surely primordial life deeply appreciated, but its gravity has been helpful in at least two ways. First, the mass of the Moon helps to stabilize Earth’s tilt. With a relatively stable tilt, life in different areas of Earth has time to adapt to the local conditions. If the tilt of the Earth were allowed to wobble around on rapid timescales, there would be drastic climate changes, making it hard for life to evolve and adapt fast enough. Second, the gravity of the moon causes tides, and tide pools may have been important for life—in these tide pools the water can evaporate at low tide, leaving highly saturated solutions of organic molecules, making complex combinations more likely to happen.
Then there is the carbon dioxide (CO2) cycle on Earth that is enabled by plate tectonics. This simple little molecule can and does have an outsized impact on Earth’s climate. In the present day, we seem to be doing our best to break this regulation cycle that has been in place for eons. Until the last century or so, CO2 helped to maintain relatively temperate environmental conditions. When this cycle is working, CO2 acts like a rubber band pulling back when the temperature swings too far in either direction. This cycle has done an admirable job keeping the climate relatively stable, which has been key for helping life to evolve and adapt. But of course, after millions of years of working pretty well, this one life-form evolved to become “intelligent” enough to have an industrial revolution and throw the whole system out of whack.
The upshot of all of this is that it’s hard to get around the fact that Earth is pretty darn well suited to life-as-we-know-it. Which kind of makes sense, because if it weren’t well suited, life-as-we-know-it wouldn’t be here or would be very different (i.e., not “as we know it”). But even if the conditions are well suited for life to survive, it doesn’t explain how life emerged to begin with.
How Did Life Emerge to Begin With?
If we want to know if there might be other life in the universe, we need to understand the processes by which it could originate. The general scientific hypothesis is that complex molecules come in contact with each other over time amid different physical conditions, and they are able to form increasingly complex configurations through chemical reactions. Even if the likelihood of any particular chemical reaction is low, given enough time and opportunities for interactions, even rare reactions are bound to happen. Eventually, we end up with things like proteins and amino acids.
Given that we conjecture only a few ingredients are necessary to get life of some form going, you might be wondering whether we’ve ever tried to just do it ourselves and see what happens. The short answer is yes, we have tried, and no, we haven’t succeeded. Yet. But the short answer doesn’t quite do justice to the complexity of the topic. There is, of course, a fancy-sounding name for this type of experiment—“abiogenesis,” which refers to the processes from which life could have arisen.
One of the earliest experiments to try to identify a chemical origin of life on Earth was started back in the 1950s by Stanley Miller and Harold Urey (and not coincidentally referred to as the “Miller-Urey experiment”). In a nutshell, they took a bunch of chemical compounds believed to be in the atmosphere of the prebiotic Earth, put them in a sealed flask, and shocked them with fake lightning. They expected to create perhaps a dozen or so amino acids, which are the building blocks of peptides, which are the building blocks of proteins, which are the building blocks of life-as-we-know-it. In the 2000s a group of scientists opened a set of sealed vials that remained from the original experiment and found twenty-two different amino acids.
Since the 1950s we’ve learned a lot more about potential chemistry and the prebiotic Earth. More recent work suggests that clay surfaces could have an important role in helping these lonely amino acids join hands and make nice happy peptides. Now there is a whole new much more sophisticated version of the Miller-Urey experiment in Canada, called a “planet simulator” (which looks amazingly like a very high-tech pizza oven). Maybe by the time this book is published, they will have some early results and this section will already be outdated.
One of the primary hurdles for these experiments, and the researchers working on them, is simple statistics. The prebiotic Earth had a lot more real estate to work with than a few dozen small vials or a pizza-oven-sized experiment. It also had a lot more time to sit around and let things happen than, say, a human lifetime—like thousands of times more time. It may be overly optimistic to expect it to happen on timescales that keep people’s attention.
There are also ongoing experiments that start with proteins and lipids to see what happens, and whether they can create synthetic cell-like structures. These experiments skip over the part that Miller and Urey were trying to mimic, jumping straight to working with more advanced molecular building blocks. If one experiment can generate the peptides and proteins, and another experiment can take peptides and proteins and generate living cells, then it is only a matter of time before we string them together. Then what?
There is, however, a big fat chasm between the “can we” and “should we” questions. It is easy to get caught up in how cool it would be to see living cells form out of basic materials and everything that might teach us about our origins. I would argue that we should consider taking a step back from time to time and genuinely question whether we should—and consider the conditions under which we should not. The ethics here are complex, but whenever we talk about doing anything with life, we should really give deep thought to the issues involved. I would rather see our ethical understanding ahead of our science than the other way around. The same goes for terraforming Mars.
One interesting hypothesis in play is that life didn’t originate on Earth at all, but rather hitched a ride here from somewhere else. This idea is known as “panspermia.” The idea is that somehow (whether by accident or intention) microscopic life traveled through interplanetary space by hitching a ride on a comet, asteroid, meteor, alien spacecraft, and so forth, landed on Earth, and flourished. In this scenario, our very distant ancestors were aliens.
Given what we know about extremophiles in our own biosphere, the idea that tiny life-forms could survive interplanetary travel intact is not as crazy as it might initially seem. If life finds a way to withstand the harsh radiation of space (check!), survive ultracold temperatures (check!), and can go into some sort of cryptobiotic state for long periods of time (check!), it has a fighting chance to make an interplanetary journey. Given enough time, this almost seems inevitable, like migratory birds dispersing seeds to distant lands after crossing oceans.
Planets in the solar system have been exchanging meteorites pretty much since the beginning of the solar system. For example, we have identified over two hundred Martian meteorites on Earth—these rocks were ejected from Mars, perhaps as the result of an asteroid impact or volcano, meandered through interplanetary space for maybe millions of years, and through pure chance ended up landing on Earth (a few have likely landed on other planets, too).
Given all of this, I would argue that it is possible that the panspermia conjecture is correct. I’m not going to go remotely so far as to argue it is likely, but we certainly can’t rule it out. At least not yet.
Before we move on from the mystery of the origin of life, I want to mention a disconcerting idea that stretches our notion of life and how it might have arisen. This particular idea also does a great job of reminding us how provincially we tend to think about life and intelligence. In a nutshell, the idea goes as follows: given enough time, it is statistically more likely for particles in the universe to randomly come together and spontaneously form a “brain” (not a literal brain, but an arrangement of particles with similar functionality) than it is for the universe to go through all the steps of making galaxies, stellar systems, planets, life, and finally… actual physical brains. In other words, the spontaneous “brains” are a smaller deviation from equilibrium than the requirements to form actual physical brains, which would make them more abundant. The statistical arguments for (and against) these brain-like structures get deep into cosmological predictions about the nature of the universe, and you can be sure that they are hotly debated.
