Tracing Evolution Backwards

Jupiter’s moon Europa

This meditation is an extension of a series of Facebook posts around the question of what conditions are necessary for the development of life, which itself is an extension of the Drake equation for estimating the probability of finding other life and civilizations in the universe. The proposer, William Maness of my Facebook acquaintance, posted: “Let’s go the other way. Let’s say that Earth’s condition is astonishingly rare. How rare does it have to be to be the only one in the galaxy. How rare to be the only one in the universe?”

And then he proposed conditions for life on Earth as we know it: strong magnetic field, stable sun, Goldilocks zone (meaning both the right part of the galaxy, in terms of density of nearby stars and their radiation, as well the solar system’s “habitable zone,” with planetary temperatures that can support liquid water), a large companion body (to create tides, which set a pattern of inundation and exposure for sea life at the edge of the land, among other things), no gamma emitters nearby, debris-cleared orbit (to minimize life-killing asteroid impacts), abundant liquid water, no conditions that kill carbon life in said ocean, an active lithosphere (with plate tectonics to renew the surface, replenish the atmosphere, and relieve geothermal stresses¹), an active water cycle, and a transparent atmosphere. “These are just a few that come to mind,” he wrote.

My first response was to say that some of these conditions overlap and work to the same purpose. For example, the conditions of having a strong magnetic field and a stable sun are related, as their result is to protect developing and existing life from the solar wind and radiation bursts. Having no nearby gamma emitters is part of that requirement, too. But note that if your definition of life includes—or is excluded to—cockroaches and tardigrades, which seem not to care much about hard radiation, these several requirements may not be absolute.

Having liquid water and an abundance of carbon are nice. But as I’ve noted elsewhere,² you could construct a parallel DNA chemistry from silicon and arsenic. The silicon atom has the same chemical-bonding valence as carbon, while arsenic has the same valence as phosphorus. So silicon might replace the carbon atoms in the ribose rings and the purines and pyrimidines that are the main features of DNA and RNA molecules. And arsenic might replace the phosphorus atoms in the bonds that connect those ribose rings into a long-chain polymer. The resulting molecules would be heavier, of course, having a higher aggregate atomic weight. And they would be somewhat more fragile, because their traded electrons would occupy a higher electron orbit. But these replicant molecules would still function like carbon-based DNA.

And liquid water does have some unique properties. The water molecule is easily dissociated into its component oxygen and hydrogen atoms. The molecule has an asymmetrical arrangement, placing the two hydrogen atoms at sixty degrees apart on one side of the oxygen atom, creating a positive and negative side to each molecule. This arrangement allows other molecules to be either “hydrophilic” and attracted to water or “hydrophobic” and repel water. Water as a fluid is also relatively incompressible—you can’t squeeze it in its liquid phase—so that the water in a deep lake or ocean doesn’t get thicker and sludgier as you descend, becoming paste-like or semi-solid. Instead, the pressure just increases while the density remains the same. These features create an important condition for life forms like Earth’s sea creatures, who are composed of mostly water themselves, metabolize the dissociated oxygen in water, and range freely from the surface to the deeps.

That angular separation on the water molecule forces it to form a hexagonal crystal when frozen, so that the solid phase is actually less dense than the liquid phase, enabling it to float. If solid water sank to the bottom of a pond or ocean, where temperatures are generally cooler, then a temporary drop in ambient temperature might freeze any body of water solid. And there it would stay frozen for who knows how long—not until next summer but more likely until the next extreme in the climate cycle.

But other liquids with a low chemical reactivity and low compressibility could support life almost as well as water does—although it would be chemically and physically different from ours and might prefer different ambient conditions.

Other planetary features like a large companion (for tides) and active lithosphere (for plate tectonics and volcanoes) are only required for the kind of life we recognize. I’m betting that, when we find life out there among the stars, it will surprise us. But that wasn’t the premise of the question as originally posed, which acknowledged that it was working backward (i.e., “going the other way”): What kind of conditions will produce us, the life that we know and recognize? And that may be too limiting a definition.

We can begin as a given that the same laws of physics and chemistry exist elsewhere throughout the universe. Go to any other star with a planet, and you’ll find the same atoms from our Periodic Table—although not necessarily in the same abundance and distribution. They will tend to form similar molecules—although perhaps with different underlying chemical reactions having different, temperature-dependent endo- and exothermic requirements—and so the abundance and distribution of life-creating or life-destroying substances will depend on local conditions. The gravity curve will follow the equations we use to measure it here on Earth—although the resulting values will necessarily be different, based on solar and planetary density and distance. The physics of electromagnetism and radiation will apply—although the quality of the light and its effects on biochemistry and biodiversity will be different, based on the output of the local star.

The nature of life, however defined, is that is evolves in and adapts to the environment it finds. Otherwise, whatever you find on a new planet is just an artifact or an exception. This presumes, of course, that evolution is present on the planet and is based on either a system of replicating molecules, similar but not necessarily identical to Earth’s DNA-RNA-protein coding system.³ Once the principle of replication-with-modification becomes established and gives rise to “life,” it will already be adapted to the conditions that it finds and then change itself as they change.

This evolution will be able to give rise to organisms that are not like us either physically or chemically. Even on Earth, and working under the DNA-RNA-protein coding system, we can find life that is strange and different. Consider the organisms that our deep-ocean searches have discovered clinging to the sides of undersea volcanic vents: adapted to total darkness and huge surface pressures, tolerating the extreme temperatures of superheated water, and metabolizing sulfur compounds instead of carbohydrates. The life that we recognize from this planet’s surface was able to descend and adapt to that hell. Or rather, our kind of life didn’t adapt itself: any of its great-great-grandchildren who happened to survive because of compounding genetic mutations became able to thrive under those conditions. Remember that the original life on Earth evolved in a carbon dioxide–rich atmosphere. Then plants began metabolizing that carbon in a photosynthetic reaction driven by sunlight and released free oxygen into the atmosphere. Only then did later organisms—“our” kind of life which moves, wiggles, walks, and talks—adapt to breathe and metabolize that oxygen.

As for what conditions might be required to create life, consider the smallest of the Galilean moons, Europa. Jupiter is not in the Sun’s “habitable zone,” with temperatures that generally keep water a liquid. Still, Europa is suspected of having an ocean under its icy shell that is kept warm by tidal flexing in its orbit around the giant planet. The ocean under the ice might contain life, protected not by a thick atmosphere and planetary magnetic field, as on Earth, but by the layers of ice themselves, because water is a good shield against radiation.⁴ Whatever life develops in this ocean would be different from ours—not based on or even seeing the Sun’s light, with no possibility of moving out onto land and developing the things we humans cherish, like fire, metals, and radio and television. But it would still be life under conditions that do not entirely match those on Earth.

When we get out among the stars, we’re going to have to expand our definition of life exponentially. I suspect that will quickly turn our teaching of biology—and so much else—on its head.

1. If you think geothermal stress isn’t important, consider Earth’s sister planet, Venus. By studying the uniformly limited number and apparent recent age of the impact craters on the surface, astronomers have determined that Venus must lack a system of plate tectonics, with its corresponding subduction of surface layers and creation of volcanic hot spots that release core heat, as on Earth. Instead, the planet appears to go through periodic renewals, where the entire surface melts from within and then resolidifies. That would be bad for any life trying to gain a foothold on the rocks there.

2. See The God Molecule from May 28, 2017.

3. For example, a machine-based organism that was able to sample its environment and rewrite its underlying operating code to thrive under those conditions would be a similar but different analog of our biological kind of life. For that matter, you might consider our molecular form of life as simply a kind of nanotechnology.

4. When I was in college, I had a roommate who worked as shift operator at the university’s TRIGA reactor. This was one of those “swimming pool” reactors, used for research, training, and experiments with radiation. When he took me on a tour, we stood at the railing and looked directly down at the reactor core, which when operating glowed with the beautiful blue light of Cherenkov radiation. I pointed at the active core and asked my roommate, “Why am I not dead?” He replied that the twenty feet of water between us and the radiation flux with its fast neutrons was better protection than a foot of lead shielding. I then saw bubbles of gas rising from the reactor and bursting on the surface about eight feet away. I asked what it was, and he said it was a radioactive isotope of oxygen. “Why am I not dead?” Because that isotope possesses a half-life of eight seconds and had mostly decayed to regular oxygen by the time it reached the surface, where any residual isotope dissipated into the room’s atmosphere before decaying further.

Thinking With Our Skin

It’s fascinating to think about how an embryo, a single cell comprising the genetic material of a mother’s egg and father’s sperm, develops into a complex, multi-celled organism with each cell having its own place and function.

I’ve written before about the beginning of the development process,¹ as described by the late Eric Davidson at Caltech, who was working with sea urchin embryos. After the original zygote created by the union of egg and sperm had divided again and again to form a hollow sphere of cells called a blastula, he and his team sacrificed these embryos at subsequent fifteen-minute periods to map out the interaction of genes in the nuclei of these undifferentiated cells. The team discovered that, depending on where in the sphere a cell was situated and time after the blastula formed, one gene would produce a bit of microRNA that moved elsewhere inside the nucleus and promoted another gene to make a different miRNA.

This process of promoting different genes continued—all without coding for any proteins—and directed that particular cell toward becoming a different and unique body part, like a backbone spine or a section of gut. These various bits of microRNA and their interactions formed an instruction set and timing mechanism for developing the entire animal. Davidson’s team then compared these miRNA patterns with other animals that had not shared an ancestor with sea urchins for millions of years and found the mechanism to be “highly conserved”—which means that something similar takes place inside the cells of a human embryo.

In mammals and the vertebrate animals closely related to them by evolution, the blastula sphere develops into the gastrula, a hollow, cup-shaped structure composed of three layers of cells. The inside layer, the endoderm, eventually becomes the gut with associated structures like the small and large intestines and organs like the stomach, pancreas, liver, and lungs. The middle layer, the mesoderm, gives rise to the connective tissues including muscles, bones, bone marrow, and blood and lymphatic vessels, and to associated organs like the heart, kidneys, adrenal glands, and sex organs. The outside layer, the ectoderm, becomes the skin with associated structures like hair and nails, the nasal cavity, sense organs including the lens of the eye and the tongue, parts of the mouth including the teeth, and the anus. The ectoderm also forms the body’s nervous tissue including the spine and brain.

So how does that outer layer of this cup-shaped gastrula become something so interior to our bodies as the brain inside its bony skull and the spine inside its chain of bony vertebrae? The answer is that during embryonic development of this outer layer, the ectoderm acquires what’s called a neural fold. A groove forms in the layer that soon folds over to become a hollow tube, the notochord, which eventually becomes the spine inside its sheath of protective bones. The anterior or front end of the spine becomes the brain. In primitive life forms, the brain remains just a cluster of nerve cells, a ganglia. In more developed organisms—from fish on up to humans—this ganglia develops a complex structure with the brain stem, where consciousness originates; the cerebellum or hindbrain, governing autonomic functions like breathing and balance; the neocortex, governing thinking, speech, and fine motor control; and the limbic system, which is associated with emotions, instincts, moods, and the creation of memories.

It’s no accident that the nervous system arises from the skin, because sensation through the skin is the brain’s first major contact with the outside world. Other structures derived from the ectoderm include the eyes, ears, taste buds, and sense organs in the nasal cavity. It might at first seem that the brain, where the cell bodies of the neurons reside, should form in place and then extend long, thread-like nerve fibers, the axons, down along the spine and out into the skin to get such widespread coverage. But instead they all form in place from the same tissue, starting embedded in the skin.

At the same time that the neural fold is forming the spine and brain, the cup-shaped structure of the ectoderm and endoderm curve around and fuse to form the body cavity. That puts the guts on the inside, the skin on the outside, and the skeleton and muscles somewhere in between. All of these tissues are developing together and sometimes—as with mesoderm muscles of the heart and the endoderm structure of the lungs—merge to form integrated systems.

But how does the developing organism know which end of the spine will become the “anterior” as well as what and where the posterior might be? What directs one end of the hollow tube that represents our bodies to become the mouth and nasal cavity, with their sense clusters, and the other end to become the anus? One further set of genes is needed to manage all this, the homeobox genes, or “hox” for short.

This is another highly conserved gene set. Fish have it, as do frogs, lizards, dinosaurs, birds, and all the mammals. So do the insects and arachnids. The hox genes are only active during embryonic development and determine the major body parts that we share with all these other animals: the head with its brain or nerve cluster, the major sensory organs, and mouth parts; the thorax with the heart, lungs, and nexus of the blood vessels; and the abdomen with its digestive and reproductive systems. In human beings, the thorax is enclosed inside the ribcage and separated from the abdomen by the diaphragm. In insects like the fruit fly and arachnids like spiders, the thorax and abdomen are separate body structures. The hox genes also define the limbs and where they are attached: four limbs connected to the spine in the tetrapods, which developed out of lobe-finned fish and first walked on land—that’s us, along with frogs, lizards, dinosaurs, birds, and all the mammals. Other less closely related animals like insects and arachnids have multiple legs attached to the thorax and sometimes wings, too.

It’s not just a coincidence that we share the same basic body structure with fish and frogs. It’s written into our genes. I always marveled at the movie Avatar, where on the planet Pandora the humanoid natives, the Nav’i, are four-limbed like the Earthly humans, but every other species in close evolutionary proximity to them has six limbs. Given that the hox gene set is relatively stable, creatures so closely related that they can attain near-telepathic communication by mixing the tail ends of their neurons really ought to have a parallel body structure.

The hox gene set is also the reason that we classify mythological creatures like Pegasus, the flying horse; gryphons, which are half lion–half eagle; and dragons, which have four legs and a pair of wings, as “chimera,” or impossible animals. The hox gene set simply doesn’t allow for mashups of six-limbed creatures that closely parallel the known tetrapods. It also forbids angels with two arms, two legs, and a pair of wings. All of them are violations of basic body structure.

We still have a lot to learn about fetal development. And certainly the hox gene set deserves more study. But I find it fascinating that the process of going from a single cell to a complex organism passes through a multi-layered sphere that then folds inward and outward like a piece of origami. And it’s a bit chilling to understand that we all think and feel with cells that originate in our skin.

1. See Learning as a Form of Evolution from December 10, 2017.

Keeping Busy

Storyteller in a Turkish coffee house

We human beings are endlessly concerned with finding our “purpose” in life. It’s a question that faces a child from the first time he or she is asked “What do you want to be when you grow up?” Answering “I just want to be” is not considered sufficient, although it’s the answer that every other life form, every bacterium, plant, and animal on this planet has for the question.

Biologists define life with a number of different characteristics. First is cellular organization—any organism, even a one-celled prokaryote, has an arrangement of pieces and parts, systems and subsystems, that enable it to function. Second is reproduction—it survives for a time and then divides into or buds off daughter cells, or joins with a complementary partner to form a new organism sharing the traits of each. Third is metabolism—it ingests nutrients such as proteins and carbohydrates, or in the case of plants, minerals and sunlight, and excretes waste products. Fourth is homeostasis—it tends to maintain a stable internal environment and seeks to maintain a stable external environment. Fifth is heredity—it can trace an ancestry based on changes through mutation from its parent cell or organism. Sixth is response to stimuli—it senses and reacts to its environment, moving toward light or nutrients or prey, avoiding predators or unfavorable conditions. Seventh is growth and development—the result of that heredity and metabolism is successful accumulation of resources and changes in structure. Eighth is adaptation through evolution—while the individual may not always change in response to its environment, the hereditary line changes through natural mutations that enable some future individuals, but not necessarily all of them, to survive.

These characteristics are not immutable like the laws of physics. Bacteria don’t react to their environment as readily as a gazelle being chased by a leopard. And not every individual successfully reproduces. Some of the characteristics listed above, also, are concatenated on other biologists’ lists, such as heredity being an element of evolution. But the principle is the same: life reacts to its environment in a way that, say, a stone weathering on a mountainside does not.

For every other species on Earth, this is enough. My dog does not question her life. She does not attempt to be something other than a part of the situation in which she finds herself. This is a shame, really, because in an earlier age of the world she would have been hunting small mammals, finding and mating with a male dog, digging a den and giving birth to litters of puppies, and only occasionally getting to lie in the sun in contentment. It would have been an active life full of interesting activities with occasional moments of terror. As it is, she is an adjunct to my household and has the primary function of nuzzling my hand when she wants something and having her coat stroked and hearing soothing words when I choose to give her attention—or feeling the tugs of the brush and the terror of the toenail cutter when I groom her. She won’t mate or reproduce because that potential was surgically removed at the shelter where I found her. So her life is reduced to eating the food that I put down for her, exercising her excretory functions only when I take her for a walk, and otherwise lying in the sun or on a cushion under my desk, waiting for something to happen. But it’s a life.

Human beings would go mad in this situation. We cannot be kept as pets—or not most of us, and not the best of us. And therein lies one of the basic problems of our modern world.

For a million years or more, our hominid ancestors lived as hunter-gatherers. Life was a struggle. We lived from one animal kill to the next, from one berry bush to the next. And when the seasons changed and the streams dried up, we suffered. We mated according to our hormones and our opportunities. We carried our feeble young along on the trail by instinct alone, not dreaming of a different or better life for them. We had an existence prescribed for us by circumstance, full of interesting if repetitive activities with occasional moments of terror. No one among this primitive species—or almost no one, surely—looked up into the sky at night and wondered about the Moon and the stars and what they might be or mean. Almost no one asked if there might be any other purpose to life. Everyone was just too busy surviving to ask such stupid questions.¹

All of that started to change when human beings settled down in the fertile river valleys, planted crops and tended domesticated animals, invented city life with its artificial hierarchies and its wonder at the Moon and stars and what supernatural beings might lurk behind them. We suddenly had more food—most of the time—than one person could hunt or gather and eat by him- or herself. We had an unfamiliar condition called abundance. And we could indulge the pastimes of people who did not directly produce food, shelter, or clothing and yet still wanted to eat, sleep indoors, and cover their own nakedness. We had room for priests, shamans, storytellers, tax collectors, and other government officials. We began having a civilization and all of its questions.

Things have only gotten better—or worse, depending on your point of view—with the advent of science, technology, and modern methods of agriculture, production, and distribution. Where the labor of one person on the soil might once, in that fertile river valley, have supported two or three more people in the nearby town, now the labor of one or two people plus a cohort of robotic machines and systems support a hundred more. Working to stay alive and wondering where your next meal is coming from are no longer the primary concerns of most people in the Western and developed countries.

Physical needs have been replaced in our modern society by existential needs. A person who eats, lives in, and wears the products of other people’s labor has to question his or her own existence, no matter how the value of those goods and services in terms of dollars, credits, or other forms of exchange was acquired. More importantly, without the requirement of spending every waking moment concerned with the fulfillment of those physical needs, what is the person going to do just to keep busy? The question “What are you going to be when you grow up?” becomes “What are you doing here in the first place?”

Some people have a specific answer to that question. They are usually the humans lucky enough to be born into a family with a tradition of productivity: the family farm, the family business, or a profession followed by parents and grandparents such as medicine, law, or engineering. These family situations set a child’s mind in a pattern of work, responsibility, and obligation.

Many people transfer the question of personal purpose to a higher authority. They know they are valuable and worth the food they eat, the shelter they inhabit, and the clothes they wear because their deity sets apart all human life as having such value. What they do in their day-to-day occupation or their role as homemaker and caregiver is secondary to this important and holy purpose.²

My own role, which I think came about from my maternal grandfather’s love of books and my own father’s lifelong interest in reading, is that of perpetual student, then as an interpreter and explainer of life and the world, and finally as a storyteller. The family thought that, with my facility for languages, I would become a lawyer, like that same grandfather, but I lacked the aggressive instinct for courtroom battle. Instead, I became fascinated with stories themselves, with fictions that make more sense of the world than the daily lives we all encounter, with their power to sum up and explain the human condition. I spent my high school and college years learning the literature of my culture as an English major. This was not just the language but its use in the business of transmitting personal and cultural experience. I worked my entire professional life as a communicator. First, I was a book editor and technical editor, helping authors and engineers tell their stories in a coherent and pleasing manner. Then I was a technical writer, a speechwriter, and an internal communicator, telling about and explaining the business—whatever business I found myself in: engineering company, public utility, pharmaceutical company, or maker of genetic analysis equipment—to its operators and its other employees.

And all the while I knew that I was peripheral to that corporation and to society as a whole. The publishing business, in which I was a direct contributor to the end product, is a nice-to-have in a civilized society but not need-to-have in the way that farmers, carpenters and masons, weavers and tailors, and the truck drivers that move their products to market are necessary to life. As a technical writer and internal communicator, I was not even central to the business function but a convenience to the employees who do the actual work and the managers who want to see it continue. As a novelist, I might directly bring my readers moments of interest and even joy—or at least a release from tedium while waiting for a bus—but I am not central to their lives.

I don’t regret any of this, and performing these peripheral functions has paid me well over the years. I’m one of the living examples that an English major does not necessarily have to teach or ask “Do you want fries with that?” But I also know that my function in society has not been critical to its operation. If I had disappeared years ago, no one would have starved, been made homeless, or gone naked to the elements. And when the end finally does come, I will know that my life has been an elaborate and complicated form of keeping busy.

But that’s more than some people have. And it may be better than chasing rabbits with a sharpened stick or pulling berries off a bush for a living. At least I never had to run from a leopard, either.

1. So you can imagine that the subjunctive mood was not a part of their speech patterns. There’s not a lot of need for expressing potential or counterfactual conditions—shoulds, woulds, coulds, oughts—when you’re chasing a rabbit with a sharpened stick or tasting a new and unfamiliar kind of berry for the first time. You do or you die.

2. Not having a personal god—nor even an abstract idea of any god—I cannot rely on this definition of personal value. Unless the thoughts of my brain are made real by writing them down and preserving them in my function as a student, explainer, interpreter, and ultimately a storyteller, I have no more personal dignity or right to life than a bacterium or a dung beetle.

New Kids on the Block

The question of finding intelligent life in the universe was most concretely addressed by two scientists years ago. One was Enrico Fermi and his paradox, which to paraphrase asks: If life is so common among the billions of stars in the trillions of galaxies, and if space travel is even relatively achievable, then where is everybody? The other was Frank Drake and his equation that attempts to quantify—or at least set the framework for quantifying—the likelihood of intelligent life appearing elsewhere in the universe.

Both of them miss a point that is, I think, obvious to anyone who thinks about it deeply enough: the universe is really big and really old.¹ We ourselves have only been around for comparatively the last two seconds of all that time. So far, we’ve managed to explore the skin of just one local planet and populate only a fraction of it in any numbers. And we’ve placed a couple of dozen robot probes on the surface of, or in orbit around, the other planets in our own system. We have accomplished the latter in a remarkably short time, considering the age of our own species. In terms of the age of the universe, we’ve gone outside the Earth’s atmosphere in just the last microsecond.

Okay then, time scales. The universe, according to our best guesses and measurements, is about thirteen billion years old. Considering its vast size, that’s a bit younger than it should be if the whole works has been expanding at even roughly the speed of light for all that time since a putative Big Bang. To account for this, astronomers posit an “inflationary” period right after the dense monoparticle containing everything we can see, know about, or infer exploded, so that it all expanded faster than light speed just to catch up with modern observations. But I digress …

Our own solar system is approximately four billion years old. That is, the Sun and the planets started condensing out of a cloud of dust and gas some nine billion years after the whole shebang exploded or started expanding exponentially. And that dust and gas was the residue of earlier stars that had lived and died, vomiting up a rich mixture of hydrogen, helium, and every other type of atom on our periodic table. We live in a second- or third-generation—possibly a fourth-generation—star system and are richer for it. But I digress …

That curious reversal of entropy that we call “life” actually appeared on the surface of this third planet—one of only three in the habitable zone where water can appear as a liquid rather than a vapor or a solid—soon after the bombardment of planetoids and meteorites stopped and the planet’s surface had cooled enough to be solid and entertain pools of liquid. The first life was nothing remarkable and not really visible to the naked eye: single-celled bacteria and blue-green algae that processed the chemicals available in their environment and the sunlight streaming down by using a relatively sophisticated DNA-RNA-protein coding system—or perhaps just RNA-protein coding to begin with, because the DNA form may have developed a bit later. These simple creatures worked to—or I should say “evolved to”—exploit the existing chemistry of the planet and replace its original atmosphere of carbon dioxide, water vapor, ammonia, and methane with one that was mostly nitrogen and oxygen.

These bacteria and blue-green algae, which would only be really visible as mats of colored slime on the edges of the seashore, persisted for two or three billion years. Over that time, they developed environmental niches and separate species, but they still remained simple, mindless cells that processed chemicals and sunlight, grew and divided, and reworked the planet’s surface and atmosphere. It wasn’t until about five hundred million years ago, or three and a half billion years since the solar system formed, that the first multi-celled organisms appeared. This required variable expression of that DNA-RNA-protein coding system and the development of cell types that were different from each other but still originated with a single cell and worked together as a single system. This generative period was called the Cambrian Explosion because suddenly the Earth—or at least its seas—had plants and animals that a visitor from beyond the stars could recognize with the naked eye and avoid stepping on, if he-she-it were wading in a tide pool or shallow lake. The animals and plants didn’t move up onto the land masses until about a hundred and fifty million years later, during the Devonian period.

Here we’re still talking about relatively mindless beasts: fishlike vertebrates and scorpionlike arthropods who spent their entire lives grazing or hunting and reproducing in kind. Even when the land animals developed study legs and grew to the size of houses, such as the dinosaurs, they were still just predators and prey, fighting for survival, reproducing, and not much else. It wasn’t until sixty-five million years ago, when the Chicxulub asteroid wiped the slate of life nearly clean and allowed little mouselike nocturnal creatures, the earliest mammals, to survive and develop, that we got the sort of brains and intelligences that we recognize in primates, whales, dolphins, elephants, and ourselves. And even then, any visitor to this planet would not have found much in the way of interest, nothing to report home.

We humanoids of the genus Homo didn’t come down from the trees and out onto the grasslands until about one or two million years ago. We didn’t develop the sapiens, or “wise” species, until about sixty-five thousand years ago. And even then we were in competition with the neanderthalensis species, named for a valley in Germany where their bones were first discovered. These other Homo species may or may not have been as developed intellectually as our direct ancestors. But any visitor from another star system would have found both the neanderthalensis species and our sapiens ancestors picking berries and killing slower-moving animals with sharp sticks and edged rocks, then sucking the marrow from their bones. These collective ancestors probably used a decipherable language, and some of them may have carved a whistle or rudimentary flute from a shinbone in an attempt at making music. But for the most part, you had to look and listen closely to distinguish them from a troop of chimpanzees or baboons. And still, the number of Homo sapiens—the species with such future promise—was pitifully thin on the ground.

It wasn’t until about ten thousand years ago that our ancestors began gathering in groups, usually along fertile river valleys, to practice farming, domesticate animals, and build shelters larger than a tent made of saplings and animal skins. It wasn’t until five thousand years ago that they begin to think about baking clay forms into useful pots, smelting metals for better tools and weapons, and writing down their grunts and squeaks as an encoded language that would last longer than this morning’s conversation.

It wasn’t until a hundred and eighteen years ago that Guglielmo Marconi sent the first long-distance radio transmission—across the Atlantic Ocean—and then that message was in coded a series of dots and dashes. Even in its heyday, radio was a broadcast system rather than beamcast, which meant that its signal strength dissipated exponentially under the inverse square law. The signal from a 50,000-watt radio station in Kansas, belting out Patsy Cline in the 1950s, would be something less than a whisper by the time the wave front passed Saturn, and hardly louder than a bug fart by the time it got out of the Oort Cloud on its way to the stars. That famous first television broadcast of Adolph Hitler opening the Berlin Olympic Games in 1936 would not have fared any better. And these signals would then have had to compete with the blasts of radio noise coming from our own Sun. And now that so much of our communications is carried by coax and fiber-optic cables, and beamed down from satellites in near-Earth orbit, our planet will have gone virtually dark within our local spiral arm of the Milky Way.

So, even if the universe is crawling with life at the stage of bacteria and blue-green algae, or shambling along with creatures that resemble the dinosaurs or our own Homo habilis, it’s not listening for us and not able to visit us. Even if an advanced species has developed radio sets and antenna with which to search the skies—and remember, we didn’t develop radio telescopes until the engineers at Bell Labs tried to establish the source of bothersome static on long-distance radio-telephone calls in the 1930s, about eighty years ago—they wouldn’t be likely to hear anything that sends them cruising toward Earth.

And did I mention that space is really, really big? The nearest star system is four light years away, which means that even if we could travel at light speed—and our mathematics says we can’t—it would take four years to make the one-way voyage, even if we had the proper technologies for propulsion and life support. Forget about science-fiction tropes like warp drives, wormholes, matter-antimatter energy sources, and other forms of magic. Going to the stars will still be a civilizational undertaking—for us and for any other species out there. Stellar empires might grow in the minds of speculative writers and nuclear physicists like Fermi, but establishing one and holding it together as an enterprise of cultural and economic exchange, under the conditions of generational time lag presented by the distances involved, would be a daunting and perhaps fruitless task.

Exploring the cosmos just to see if some other planetary system has developed something more than slime molds and dinosaurs, or even humanoids knocking over pigs and butchering them with sharp rocks, would be a remarkably altruistic or academic pursuit costing a huge percentage of a planet’s and a culture’s resources. Even beaming signals out in all directions in the hope of one day getting a response that made any kind of sense would be a significant undertaking.

pWhere are all the other intelligent species? I think they’re out there, but they’ve got better things to do than visit us.

1. I know, fellow writers, “really” is an adverb and we should eschew the use of adverbs. When you want to write “really” or “very,” just write “damn,” because it means the same thing. Still, I could use “really” about four times in succession to describe the universe’s magnitude and about twice to describe its age—but that would just sound silly. Hence, the rest of this essay.