You know we’re going to find life elsewhere in the universe one day. It’s only a matter of time—ever shortening time, too, as we send out more probes and, eventually, begin launching expeditions with human colonists. Either here on the planets and their moons revolving around our own Sol or on other bodies orbiting other stars, the chances are good we will find that “temporary reversal of entropy” we call life.
That is, the chances are excellent that life exists elsewhere. To approach the question in any other way would be to assume that our star, our planet, and our own situation are all somehow unique in the galaxy, if not in the entire universe. But, from everything we’ve been able to see with our light- and radio-based telescopes, our spectrometers, and other more recent detection systems over the past hundred years, our Sol is about as ordinary as a star can get. And from the hints we’ve been able to detect over the past decade or so, based on minute stellar wobbles and variations in their light output, planets seem to abound around our type of star—although most of the bodies detected so far have been massive gas giants like Jupiter and Saturn, and we’ve only recently been able to detect the smaller, more rocky bodies comparable to Earth. Still, whether we’ll be able to find life on any of Sol’s nine planets1 with their dozens of moons, or around a nearby star with a planet in the habitable zone that has been able to maintain stable conditions long enough for life to develop chemically, consolidate and maintain its gains, and then evolve into something interesting … that’s a crapshoot.
Unless, of course, such life reaches out and discovers us first …
But until we do eventually find life, what can we say about it? What can we project? Quite a lot, I believe, based on principles we can observe in the life on Earth.
First, anything we would recognize as alive is probably going to be made up of many repeating units working together under some kind of organization. Single, solitary things—other than stars and planets—don’t seem to last long in our universe. Nor are they able to reproduce, except by growing larger and eventually splitting apart into two new-but-identical subunits, like a bacterium. By “repeating units,” I’m not talking about bees in a hive or soldiers in an army, although that seems to be a pattern, too. I mean cells in an organic body or transistor gates in a complex circuit—out of the many, one.
As we’ve found on Earth, single-celled creatures have a mechanically built-in size limit. The membrane enclosing their interior fluid has limited capabilities for stretch and breaking strength. Sooner or later—really soon, in the case of Earthly microbes—the pressure of water volume inside the membrane overcomes both the cell’s ability to add to the membrane’s surface area and the strength of the covalent bonds between lipid molecules in the membrane itself, and everything just spills out. I suppose you might build a tank—or happen upon, say, a volcanic bubble along the shoreline—that could contain the organism’s undifferentiated cytoplasm in large volumes, with all of the cellular organelles floating free and absorbing food particles, transferring energy molecules, continuously transcribing DNA-analogs, translating RNA-analogs, and replicating protein-analogs. What might be the volume of such a bubble? A gallon? Five gallons? A whole swimming pool’s worth of cytoplasm? But aside from the fact that such a bubble would occur only by chance and its acquisition remain outside the organism’s control, eventually a feeding, replicating, expanding organism would outgrow even that huge volume.2
No, any organic life capable of maintaining its stability and organizing its functions on its own would need to be made up of small units, cells or their alien analogs, which would work together while acquiring different traits and capabilities. In the pattern of life as we know it, cells in any advanced life form would develop into dedicated types. They would specialize to become mechanical structures like bones, skin, exoskeletons, teeth, and muscles. Or they might develop into the life-supporting processes of digestion, circulation, and bodily regulation through the manufacture and secretion of enzyme-analogs. Or they would form a complex information-transmission and -storage system for discerning the organism’s environment, directing the action of its various parts, and capturing and ingesting food … or looking up at the stars and wondering about them.
All these different functions are too complicated and require too much chemical and mechanical specialization for a single-celled creature of whatever size to manage them. Small, self-replicating, autonomous-functioning units, working together as a whole, seem to be necessary for advanced life forms. And after you acquire those differentiated units, what then?
Second, most life is going to be three-dimensional. Human beings have made interesting conjectures about two-dimensional life,3 but unless the creature glides across the core of a heavy planet like Jupiter or the surface of a neutron star, never rising against gravity, two-dimensional life is just too hard to maintain. Such an organism would have to develop and articulate complicated linkages and latches to open a volume that would contain anything representing consumed food or waste products. And without an interior, an exterior would hardly be meaningful. No, two-dimensional life is an interesting intellectual challenge, but not a realistic expectation.
On the other hand, three-dimensional beings capable of ingestions and digestion, orientation and locomotion, and other spatial attributes will have a problem with being required to possess simultaneously both a front and a back, a left and right side, a top and a bottom. And any creature in a competitive environment—one where you are surrounded by similar beings who want to hunt and eat the same food, as well as predators who want to hunt and eat you—will be forced to protect itself and project itself in all directions. If the creature is a filter-feeding bottom dweller like an oyster or a clam, covered by a hard shell, or a soft-bodied worm buried up to its mouth and/or anus in deep sand, issues of protection and projection are not so important. But a mobile organism fighting for its life out in the open needs to worry about seeing, hearing, or otherwise discovering opportunities and detecting threats that may lie in any direction, as well as having a choice of directions in which to chase its prey or flee from its predators.
Such conditions forced the explosive evolution of hard body parts, protective strategies, and sensing apparatus starting in the Cambrian period, half a billion years ago on Earth.
These conditions suggest that the organism will have more than one organ of detection. The creature may be a grazer, like a horse or cow, that needs to watch the surrounding environment for predators and so possess one or more eyes on each side of its head, each eye having a wide-angle lens. Then the eyes working together can take in a nearly complete 360-degree view. Or the creature might be a hunter, like a lion or a hawk, with an agile neck that can rapidly swivel the head to scan the environment, and two or more eyes that are set close together to present slightly divergent views, creating a perception of depth and enabling the organism to determine the range to its target.4
The need for multi-directional mobility suggests that the organism will have more than one leg or fin or other limb for propelling itself against the ground, the surrounding fluid, or some other feature of the environment. A single limb, like a pogo stick, is too hard to control for both propulsion and steering. And a single fin is too hard to use for both purposes—although Venetian gondoliers perform admirably in maneuvering their boats with a single fixed oar, and octopi and squids do well with a single siphon in emulating a soaring jet plane under water.
These are just a few of the requirements—multiple organizing units, multidimensionality, with the need for multiple environmental detectors and multiple propulsive appendages—that an alien life form would need to have. They are the minimal requirements upon which evolution—the preservation of accidental changes through improved adaptation to the environment—can work to the organism’s advantage. And that ability to suffer changes in the first place, and so take part in evolution, would be another requirement.
There may be more requirements,5 but these will begin to shape the rules for alien life. They suggest how future human spacefarers might recognize life on another planet and so distinguish its inhabitants from, say, a rock or a pot of chemical sludge.
1. Nine, if you count Pluto—which I, as a traditionalist, still do. Eight, if not. But again nine, if you keep up with the news that analysis of the orbits of icy planetoids in the outer reaches of the solar system suggests a “massive Earth” somewhere beyond Pluto. The science is not “settled.”
2. And this presumes the cytoplasm in so large a volume could still function homogeneously—that is, discrete operations like acquisition of food, conversion and production of energy molecules, and replication of DNA to produce needed proteins could all be maintained uniformly throughout the mass. If not, then some parts of that gallon of protoplasm might be thriving and expanding while others starved and became dead zones. Such a condition could not possibly be healthy.
3. See, for example, Flatland by Edwin Abbott from 1884.
4. This presumes the organism is passively receiving light or some other electromagnetic wave reflected off the surrounding environment and its objects of interest. Otherwise, the organism might detect its environment by sending out a signal and interpreting the return echo, like a bat. Then the creature’s brain would need to distinguish infinitesimal time lags in determining distance to an object. And it would still need two ears or other detection devices in order to gauge direction.
5. For example, a multi-celled organism will likely have evolved from an active, competitive biome of single-celled organisms, as did higher life forms on Earth. As such, the multi-celled creature will need to maintain an analog to the human immune system in order to protect itself from predation by those competitors in the biome on a cellular level.