We’ve made the mistake many times before, of thinking that Earth was unique in various ways. And we’ve made the opposite mistake: assuming it to be completely typical. When we first realized Earth was one of a handful of worlds, many seventeenth-century astronomers and natural philosophers imagined the surface environments of other planets as essentially identical to Earth, populated by similar creatures and peopled with minor variations on European societies. When we don’t have any details, our minds tend to go with what we know, filling in gaps with the familiar. Today still our thoughts and calculations about alien life and civilizations are unavoidably rife with assumptions based on terrestrial evolution and history.
When we talk about extraterrestrial species and civilizations being at “our level” or “more advanced,” we are adapting a narrative where life, intelligence, and civilizations proceed through some common, predictable, or directional progression of stages, one logically following another, like an organism going through phases of growth, or a soldier being promoted up the ranks. We don’t know how good an assumption this is. SETI might help us find out.
Could some version of this Anthropocene transition have happened elsewhere?
Now we know that planets are common. We don’t yet know about biospheres, but we suspect they too may be common, and we wonder to what extent they will also fall into classifiable types and follow well-worn evolutionary and developmental sequences.
We imagine that, on the right kind of worlds, there will be an origin of life that will then organize itself into some kind of recognizable cellular and organismic structures. Sometimes we get even more specific and self-referential, imagining that, elsewhere, life will use familiar carbon configurations,1 evolve photosynthesis, poison its atmosphere with oxygen or methane that we might find in our spectrometers, make ozone layers, and/or fill its continents with something like forests that may betray themselves by their coloration.
On Earth, in order to make forests and trees, and animals to swing and crawl among them, life had to advance beyond the stage of individual cells, and organize itself into larger, more differentiated organisms. Here that jump in complexity did not follow closely on the heels of cellular life. Rather, it took life billions of years to make this next leap. Why the wait? Does this long delay mean that such a step was difficult or unlikely, and so should be rare on other worlds?
Based on this, some astrobiologists have concluded that most planets with life might have only microbes. Yet the usefulness of these “timescale arguments” is much debated. How much can we really say about the likely paths and probabilities of life elsewhere based on our one example? We don’t know why life here paused for so long (over a billion years between oxygenating the atmosphere and building the larger animal bodies fueled by that liberated oxygen), so we really shouldn’t overinterpret this fact. Yet we can’t help it. We can’t stand not knowing, so people tend to make up their minds prematurely. For every scientist who is sure the probability of complex animal life elsewhere is close to zero, you can find another who is sure that it is 100 percent.
Then, what about the transition from complex animal life to creatures who tell stories, do science, make art and machines, and struggle with awareness of their world-changing potential? Several biologists have argued, on the basis of life’s history on Earth, that evolution of human-style intelligence is a freak occurrence that would be highly unlikely to be found on other planets. If it is a likely or easy development, why did it take another five hundred million years to go from animals to zoologists?
Not only that, but the brightening sun will make Earth uninhabitable (assuming nobody interferes) after only about another one or two billion years, so it actually took almost all Earth’s useful habitable lifetime for evolution to produce technological intelligence. Given this, it is easy to imagine that it might never have happened here at all. So, some say, big-brained technologists must be rare. However, this conclusion ignores many facts: There are innumerable stars out there with lifetimes much longer than our Sun, many with potentially habitable planets. If Earth were farther from the Sun, out toward Mars, its habitable lifetime would be significantly longer. Finally, we really have no idea what determined the evolutionary timescale for the appearance of intelligent technological life on Earth. Given all this, there is little reason to conclude, on the basis of our single data point, that what has happened here is rare.
It took 4.5 billion years for Earth to go from dead rock to space walk, from molten ball to shopping mall, from sea to me, from goo to you. This is probably the most overinterpreted number in all of science. Timescale arguments are really pretty weak because we must assume our planet is typical. We can’t account for the role of luck. Are we reading huge significance into the results of a careless roll of the dice? If we want to know the likelihood of something like us evolving on other worlds, we have to go out there and find the answers.
And what of the “postbiological” parts of the story starting to unfold on Earth? Here, intelligence of a certain kind made the leap to culture, global civilization, and world-altering technological activity. If we accept that these qualities of human agency (group problem solving, cumulative knowledge, technology, and some small degree of foresight and intentionality*) are now altering our planet’s evolution, we can ask if these could be influencing other worlds as well. It’s a trick question because either way you answer, yes or no, you can be accused of arrogance. Either you’re putting Earth on a pedestal, saying our planet is so special and gifted as to possess potentials and developments found nowhere else in the universe. Or you’re putting humanity on a pedestal, daring to presume that our human qualities are cosmically significant and would be shared by “advanced beings” elsewhere, by the shapers of other worlds, which seems to elevate us to some kind of role model for galactic intelligence.
Crown of Creation?
The idea that our evolution is so much of a freak occurrence as to be potentially unique in the universe, not only in detail but in its overall functional attributes, has most recently been argued by my friend and colleague, the brilliant and reflexively contrarian Australian astrophysicist Charley Lineweaver, with his “Planet of the Apes hypothesis.”2 This revives an argument made many times by eminent biologists, including, among others, naturalist Alfred Russell Wallace in 1904, paleontologist George Gaylord Simpson in 1964, and biologist Ernst Mayr in 1995. All have argued that the fossil record reveals human-type intelligence to be such an unlikely chance occurrence that it has probably never evolved on any other planet.
In 1964, Simpson published, in Science, “The Nonprevalence of Humanoids,” a scathing diatribe against both SETI and planetary exploration. He railed against belief in extraterrestrial intelligence, for reasons that closely recapitulated the Earth-centric views voiced six decades earlier by Wallace, codiscoverer with Darwin of evolution by natural selection. They argued that the evolution of human intelligence required too many accidental and lucky factors to represent a process that could occur anywhere else in the universe.
Wallace wrote, in 1904:
The chances against such an enormously long series of definite modifications having occurred twice over, even on the same planet in different portions of it… are almost infinite…
Simpson’s 1964 paper concluded:
I shall close this chapter with a plea. We are now spending billions of dollars a year… on space programs. The prospective discovery of extraterrestrial life is advanced as one of the major reasons, or excuses, for this. Let us face the fact that this is a gamble at the most adverse odds in history. Then if we want to go on gambling, we will at least recognize that what we are doing resembles a wild spree more than a scientific program.
To some it seems that the reward could be so great that facing any odds whatever is justified. The biological reward, if any, would be a little more knowledge of life. But we already have life, known, real, and present right here in ourselves and all around us. We are only beginning to understand it. We can
learn more from it than from any number of hypothetical Martian microbes.*
Lineweaver has renewed this argument. By comparing them to the authors of the schlocky but fun film Planet of the Apes, he is offering a mocking critique of those who feel that evolution of smart, inventive creatures may be possible on other planets. Yet one does not have to literally believe that creatures on other worlds would be hairy humanoids wearing medieval costumes, riding around on horses with guns and whips, speaking English, and acting like evil fascists to think that other biospheres might eventually find their way to producing some sort of life-forms that are clever, social, sophisticated thinkers who develop technology.
Brains are good survival tools. If you evolve to sense your environment and behave in response to it, then you will need some sort of nervous system to process this information and coordinate behavior. Better-functioning nervous systems will be selected, and so some species will get smarter. How far this can go is anyone’s guess. It seems irrational to think that it can’t in some locales go a lot farther than it has on Earth. So rather than argue over whether “human-level intelligence” can ever be reached elsewhere, it might be more reasonable to wonder to what degree it would be surpassed.
What does the fossil record say? It depends whom you ask. It is easier to make the case that human intelligence is a convergent feature, one that might represent some common or inevitable trend in evolution, if you accept the published evidence that brain size has increased over time within groups of species. There is a history of disputes over the quality and significance of this evidence. In 1995, biologist Ernst Mayr publicly feuded with Carl Sagan on this question, with Mayr arguing that SETI programs were an irresponsible waste of money,
since it can be shown that the success of an observational program is so totally improbable that it can, for all practical purposes, be considered zero.
Sagan countered that
the notion that we can, by a priori arguments, exclude the possibility of intelligent life on the possible* planets of the 400 billion stars in the Milky Way has to my ears an odd ring. It reminds me of the long series of human conceits that held us to be at the center of the universe, or different not just in degree but in kind from the rest of life on Earth, or even contended that the universe was made for our benefit. Beginning with Copernicus, every one of these conceits has been shown to be without merit.
Charley’s arguments are not a priori—meaning not based on any relevant data or experience. He uses data from Earth evolution and argues that isolated populations of prehumans on several continents had plenty of time to make the transition to human intelligence, but only in East Africa did this actually happen. He feels that this shows that our “cognitive revolution” was a rare and unlikely type of development.
Charley’s an awfully smart guy who has made many important contributions in cosmology and astrobiology, but I think here he overstates the case, because the reality is that our evolutionary history is still quite poorly known to us. The details of what happened to make us what we are today are largely still buried in the dirt, lost in the murk of time. Just in the couple of years while I’ve been writing this book, several major discoveries have been made that require us to rewrite substantially the provisional drafts of our human memoir. A 2.8-million-year-old jawbone discovered in an Ethiopian hillside is the oldest human fossil known to date, and pushes back the origin of the genus Homo by nearly 500 million years. A cutting tool found under a 15,800-year-old ash layer in Oregon may be the oldest evidence of human occupation of North America. A new discovery of stone flakes near Lake Turkana, in Kenya, may push back the known date of toolmaking and use by 700,000 years, back far before the evolution of the genus Homo, meaning that our nonhuman ancestors manufactured tools. Also, we have discovered that, very recently in Earth history, there were in fact many different human species living simultaneously. These previously unknown contemporaries of modern humans belie the notion that Homo sapiens has been the only tool-making human species on Earth since the Neanderthals died out.
In 2003, remains were found of extinct species of humans on the Indonesian island of Flores. Homo floresiensis stood three and a half feet tall, used sophisticated stone tools, and lived until at least twelve thousand years ago. In 2010, a forty-one-thousand-year-old finger bone was found in Denisova Cave in Siberia. It belonged to a young female who was human (genus Homo) but not Homo sapiens. DNA analysis indicates that these Denisovans ranged widely over Europe and interbred with both Neanderthals and modern humans, and that traces of their DNA can be found in several modern populations, including Melonesians and Aboriginal Australians.
Yet another recent and distinct species of humans, the Red Deer Cave People, lived in China until at least around eleven thousand years ago. Hints of other recent human species have also been found, and it’s clear that we don’t really know how widely peopled Earth was, until very recently, with alien humanoids. Twenty thousand years ago, Earth may have been home to a wide range of distinct human species. We don’t know what happened to all these close cousins, but it seems likely that in various ways, they fell victim to the great success of our species as we spread around the globe. What a strange and different world it would be if multiple species of humans had survived to the present day. Perhaps in such a world we would be less prone toward thinking of ourselves as the crown of creation, and thinking of our own intelligence as something so unique and rare in the universe.
It’s easy to fall into the illusion that modern science has figured most things out and just the details need to be filled in. Every generation of scientists thinks so, always falsely. The pace of discovery suggests we have much to learn about our own origin story and its meaning.
Charley feels that, as we chart the paths of evolution, and speculate on what we might find on other worlds, we put too much emphasis on searching for “human type intelligence,” and specifically, too much emphasis on the evolution of big brains. He regards this as a form of self-aggrandizement, suggesting that we fixate on brains out of pride, simply because they are our own most unique anatomical attribute. As a thought experiment, he suggests that a race of intelligent elephants might instead measure and chart evolutionary progress based on the size of noses of various species, and speculate on the existence of advanced species with massive trunks elsewhere in the universe. This seems to me to be a silly argument, and not just because of the delightfully goofy image of a bunch of intelligent elephants sitting around discussing evolution. The phrase “a race of intelligent elephants” contains the flaw in the argument. How are these putative intelligent elephants able to have this imaginary conversation? What gives them the ability to do science, to know something of biology, to study their planet’s natural history, to make graphs of evolutionary progress and discuss them with one another? Wherever these abilities come from, these elephantine evolutionary biologists seem to have a great deal of “human-type intelligence” which they use to study life on their own planet and speculate on its paths elsewhere. Whatever it is that gave Charley’s imaginary elephants the ability to do science, they are doing something that, on Earth today, is being done only by human beings. Anybody who can ask such questions about evolution, whatever their anatomy, is demonstrating enough “human-type intelligence” to put the I in SETI.
No, brains are not just some random feature that could be swapped out for any other in an evolutionary analysis, and the Planet of the Apes hypothesis seems to willfully ignore what is actually happening to our planet right now. It is our nervous systems, our cognitive abilities, amplified by our social skills and our propensity for toolmaking and storytelling, that makes us such a powerful and unprecedented force of planetary change.
I always enjoy talking to Charley and reading his provocative papers and talks. It’s valuable to be reminded to question our assumptions, and that we don’t know what we mean when we use words such as intelligence. He reminds us to “check our privilege” as cognitive creatures searching for like minds elsewhe
re, and to be aware of geocentric, egocentric, or homocentric assumptions. Once every few decades or so, some well-spoken individual comes along and refurbishes this argument. It is worth listening to. Then, having done so, we can nod our outsize heads and turn our attention back to searching the cosmos.3
Just as astrobiology has challenged our ideas about the universal qualities of life, and SETI has provoked much thought about the nature of intelligence and technology, we can ask, assuming these cognitive qualities do exist elsewhere, how they may be interacting with planetary evolution. By seeing the Anthropocene as a particular example of something that can happen to a planet, we are forced to imagine a view beyond the random particulars of terrestrial history.
Accuse me of being bio-, ethno-, geo-, or egocentric all you want. There is something so strange and beautiful about the presence of human minds on Earth, about the fact that this one bit of biosphere is awake and aware and curious about everything—stuck in the gutter, perhaps, but looking up at the stars. Yes, we are perhaps unduly impressed with ourselves. But how can we help but wonder if any other part of the universe has this same sense of aliveness, this same urge to look out at the stars, and also to wonder if somewhere under a different sun some other growing things began sensing and responding to their environment, evolved memory, cognitive faculties, and—who the hell knows—maybe social behavior and language and culture? And technology? Have they, at some point, been surprised and troubled by their ability to change their world, or doomed by their own cleverness?
Earth in Human Hands Page 29