Timing may, ultimately, be everything.
It seems a rather odd coincidence that in the 4.6 billion years since Earth formed, humans have emerged now. For us to be here, first life itself had to get started, of course, and then develop more complexity. Then enough oxygen had to accumulate in the atmosphere. And habitability had to continue for a further 2 billion years or so while complex animals evolved. But here we are, now, thinking about such things, on a world that seems uniquely hospitable to us.
Good thing it didn’t take too much longer, however—for in around 5 billion years’ time, the sun will start to run out of fuel and will swell and fry the Earth, before shrinking to a much-cooler white dwarf. It’s hard to imagine any kind of life surviving that.
The odds of all of these things lining up to allow intelligent life to evolve on the same kind of timescale as the life of our planet—neither much sooner nor much later—seemed vanishingly small in the late 20th century to theoretical physicist Brandon Carter.
Scientists had been bandying about the question of whether complex life exists elsewhere since even before it was famously posed by the Italian physicist Enrico Fermi in the 1950s (“Where is everybody?”). But at a Royal Society meeting in London in 1983, Carter, who had studied alongside Stephen Hawking at Cambridge, presented a new way to think about that problem in a talk on “The anthropic principle and its implications for biological evolution.”
The presentation, to luminaries that included Hawking himself and Nobel laureate Steven Weinberg, came at the end of a long day and, as cosmologist Paul Davies, now at Arizona State University, remembers it, was not the most engaging of talks. Carter “overran by many minutes,” he recalls. “His overhead slides were incomprehensible. I remember thinking ‘Whatever is this all about?’ ”
But what ultimately sunk in was that Carter was offering an argument for why beings as cognitively complex as us are likely to be very rare indeed in the cosmos. Even if we are not truly alone, he implied, the chances of other intelligent life forms are so low that we might as well be. “The essence of Carter’s coincidence is that humans evolved on a timescale approaching Earth’s total habitable lifespan—we have evolved close in time to the ultimate extinction of all life on Earth,” says geomicrobiologist Daniel Mills of the Ludwig Maximilian University in Munich.
Why didn’t we appear on Earth much sooner?
So what? Well, the apparent coincidence has three possible explanations. One is that the two phenomena really do have inherently similar timescales. But there seems to be no logical reason why they should, because they have nothing to do with one another: One timescale is determined by stellar physics, the other by biological evolution. A second is that the evolution of intelligent life is typically much faster than it was on Earth, but for some reason it took an unusually long time here. But, again, there’s no obvious reason why that should be so.
The third possibility is that the appearance of human-like intelligence on a habitable planet is usually slow compared to stellar lifetimes, and so on most planets it never gets the chance to happen. We just got lucky.
Lucky how? Carter argued that, in order to reach human-like intelligence, life here had to clear several hurdles that involve rare, chance events: a gauntlet of “hard steps” along the way. These hard steps are “evolutionary singularities,” leaps that occurred perhaps only once. Otherwise, they wouldn’t be hard. (The evolution of the eye, for instance, can’t be one, as it is thought to have occurred many times independently in various species.)
Carter proposed two candidate hard steps. First, the origin of the genetic code (by means of which DNA sequences can encode proteins). Second, what he called “the final breakthrough in cerebral development,” meaning the appearance of human-level cognitive capabilities, such as language. Other researchers have since suggested other hard steps, including the origin of life itself (abiogenesis), oxygen-generating photosynthesis, the appearance of eukaryotes, and animal multicellularity.
Carter’s “hard steps” picture remains a pervasive and influential framework for thinking about intelligent life beyond Earth. Earlier this year, though, a team of scientists challenged his argument for why the evolution of intelligent life is cosmically unlikely.
Mills and colleagues took these last four steps, as well as complex cognition, as the best “hard steps” candidates—and considered how unlikely each of them really are. Their conclusion: Perhaps none of them are. “We are raising the possibility that hard steps do not exist at all,” Mills says.
Maybe, they say, the emergence of beings like us is then not hindered by hard steps but is simply slow. Perhaps the universe is teeming with alien civilizations—or will be before very much longer. Whether you find that reassuring or unsettling might depend on which sci-fi movies you’ve watched, but the new work is making some think again about the course of evolution on our planet.
One problem in determining the likelihood of a given evolutionary step is that, of course, we only have one example to study: the trajectory of life on Earth. And even here we have a very incomplete record of all things that ever lived: Evolution has a high rate of information loss over geological time.
For example, a transition such as the appearance of photosynthetic, oxygen-making bacteria might not have been truly unique but only looks that way now because only the ancestors of one of several such events have survived or left any traces. Alternatively, two such events might have happened independently in closely related lineages that subsequently became genetically indistinguishable enough that they look now like a single event.
Mills and colleagues say that the first such scenario—multiple occurrences of which only one left a trace—might have happened for the appearance of eukaryotes (cells with a nucleus, distinct from bacteria). Some fossil records of single-celled organisms from the Proterozoic era 2.5 billion to 0.54 billion years ago (during which eukaryotes are thought to have arisen) look kind of eukaryotic but can’t be definitely ascribed to the lineage of today’s eukaryotes. They might in fact be the traces of entirely independent eukaryote-like fusions of simpler cells—in which case, far from being rare, such events could have happened several times.
What’s more, just because we now see only one instance of a key evolutionary transition doesn’t mean it was an unlikely event. Evolutionary biologists have long also recognized a winner-takes-all aspect in evolution: The first lineage to make a particular innovation may lay such tenacious claim to the evolutionary niche it opens up that other lineages never get a chance to compete. Mills explains: “The first evolutionary lineage to successfully complete the transition in question is the one that ultimately endures.”
Life began almost as soon as the environment became conducive.
Take phototrophic bacteria. These convert light energy into chemical energy, the first step toward oxygen-producing photosynthesis. There are two groups of phototrophic microbes today, which use different light-absorbing molecules and two different gambits for making the process efficient. Between them, these two groups might have basically cornered the market, leaving no options for other potential phototrophs to seize. It’s possible that human-like cognition involved such a priority effect, too.
In such ways, evolution pulls the ladder up behind it: By its very occurrence, rather than by its inherent improbability, a major transition limits what can come after it. Most organisms today had better be able to cope with an oxygen-rich atmosphere, for instance. Once a step is taken, there’s no going back—and so it looks as though the step is intrinsically unique and unlikely, merely because it happened at all.
Mills and colleagues say these two illusions might apply to all the candidate hard steps.
“I think the case is far from proven either way,” says Davies, “but I do like how Mills and colleagues discuss the ways that apparently singular evolutionary events may appear intrinsically improbable, even though they are not.”
If Mills and colleagues are right that there are indeed no truly hard steps to intelligent life, then why didn’t we appear on Earth much sooner? Why have we only arrived about halfway before the inevitable end of our planet’s habitability?
The researchers say this need be no mystery. “It is not hard at all for deep-time paleontologists, geochemists, and Earth system modelers to think of reasons why our arrival could have been so ‘greatly delayed,’ ” they write. While it’s tempting to think of the key evolutionary steps as being all about biology, in fact they could only happen when the environment itself was permissive. It’s not that the crucial steps were hard in themselves but that they had to wait until the time was right.
The final three candidate hard steps, at least here on Earth, required an oxygen-rich atmosphere. But of course life doesn’t simply adapt to a pre-existing environment. Rather, life and environment coevolve. We are adapted to breathe oxygen not because the Earth’s atmosphere was inherently oxygen-rich but because earlier living organisms made it that way. This coevolution of the biosphere and its environment was advocated in the Gaia theory of British scientist and inventor James Lovelock. “Lovelock’s fundamental insight,” says marine and atmospheric scientist Andrew Watson of the University of Exeter in the United Kingdom, who was not involved in the work, “is that Earth history is a co-production between the environment of the Earth’s surface and the life developing on it.”
Obviously humans need oxygen—but in fact we need it at a concentration that, according to the geological record, wasn’t established stably until about 400 million years ago, in an episode known as the Paleozoic Oxygenation Event. In other words, says Mills, “for over 90 percent of Earth’s existence, oxygen levels in the atmosphere were likely too low to support long-term human settlement.” And once the atmospheric conditions were right, it only took hundreds of millions of years for humans to appear, not billions. “So, relative to the onset of the permissive conditions, the origin of humans was relatively quick,” says Mills.
The other two most recent candidate hard steps on Earth—animal multicellularity and the appearance of eukaryotes—also demand a fair amount of oxygen, because they force cells to burn up a lot of energy. It’s not easy to figure out exactly what the threshold oxygen concentration was for multicellular life here. But it seems likely to have been higher than the level that existed before the so-called Great Oxygenation Event (GOE) of around 2.4 billion years ago, when oxygen went from being a mere trace gas in the atmosphere to comprising about 1 percent of it (likely thanks to a combination of photosynthetic microbes and a shift in the way oxygen reacted with the planet’s surface environment).
Perhaps the emergence of life was neither hard nor improbable after all.
So for at least half of Earth’s history, the three final putatively “hard” steps were out of the question anyway, simply because the environment wasn’t right. “I agree that the basic hard-steps model doesn’t take into account the delays that might be inherent in the evolution of the Earth system,” says Watson. He has previously pointed out that eukaryotes seem to have appeared rather soon after the GOE, suggesting that the event responsible for them wasn’t so inherently unlikely once there was enough oxygen around.
What about the earliest two candidate hard steps? Little is known for sure about the origin of life, but one thing seems clear: It likely couldn’t have happened during most of the Hadean era, the period up to around 4 billion years ago. For much of that time, the Earth was still seething with volcanism from the fury of its formation, and was strafed by giant meteorites, the debris left over from the formation of the solar system. For most of this time, the planet would have been too hot to host substantial bodies of water on its surface.
Some think that the Earth would have been inhospitable until about 3.9-3.7 billion years ago, the age of the earliest fossil evidence for primitive life. But recent work to reconstruct the last common ancestor of all life on Earth today, based on comparisons of their genomes, has suggested this ancestor might have been as old as 4.2 to 4 billion years. That would imply that life began almost as soon as (if not indeed apparently before!) the environment became conducive. Perhaps the emergence of life was neither hard nor improbable after all.
As for the origin of photosynthetic bacteria: Today’s cyanobacteria can’t survive in water warmer than about 163 degrees Fahrenheit, and it might have taken a long time for the Hadean seas to cool to something close to that temperature.
All in all, then, Carter’s puzzle—why have we turned up so late in Earth’s history—needn’t imply a solution involving improbable bottleneck events that don’t happen on most Earth-like planets. Rather, you just can’t rush something like us. Our planet needed billions of years before it could become human-friendly. “Intelligent life could require much less time on other planets that are able to achieve permissive conditions more quickly,” says Mills.
Michael Wong, an astrobiologist at the Carnegie Institution for Science in Washington D.C., thinks that the researchers “don’t make a slam-dunk case” but that their analysis is “probably as good as one can do right now.”
“I am quite sympathetic to this reasoning that we can only understand the timing of these key inflection points through the coevolution of our biosphere, geosphere, hydrosphere, and atmosphere,” Wong says. “Planets change, and the probability of the emergence of something [on them] is not uniform throughout a planet’s history.”
Admittedly, it does rather look at the moment as though we are a solitary outpost of intelligence floating in an inhospitable cosmos. Decades of searching for signs of intelligence far afield in our galaxy have consistently elicited nothing but what some scientists call the Great Silence.
Others might reply that this is a classic case where absence of evidence is not evidence of absence. The silence is hardly surprising given that we don’t really know what we should be searching for, we’ve been searching for barely a blink of cosmic time, and there’s a lot of space to cover. Maybe it’s just too early to conclude anything at all about the chances of intelligent life on other worlds from our quest so far to find it.
Carter appeared to give reason to think that the search for extraterrestrial intelligence is doomed from the outset. But the not-so-fast response of Mills and colleagues is, they say, a potentially testable idea. For one thing, they argue that we need to develop a better understanding of how the candidate hard-step evolutionary transitions actually happened, being open-minded about how unique they really were. This includes getting a better understanding of the environmental conditions, such as oxygen levels, temperature, ocean acidity and salinity, and so on, needed for these steps to be possible at all.
You just can’t rush something like us.
But perhaps the most discerning test will be what we find in the atmospheres of planets orbiting other stars, some of which can now be studied by astronomical instruments such as the James Webb Space Telescope.
For example, if the development of oxygen-producing photosynthesis was truly a hard step, we’d be unlikely to find Earth-like exoplanets with significant amounts of oxygen in their atmospheres. Some astronomers say that we might even read the signature of intelligent life in alien atmospheres, for example if they contain relatively high concentrations of complex gases such as chlorofluorocarbons, for which no known natural source exists.
If Mills and colleagues are right, says Watson, “then when we are able to spectrally analyse exoplanet atmospheres, there is a good chance of detecting relatively modern Earth-like biosignatures.”
That time might not be very far off. In early 2025, a team from the University of Cambridge in the U.K. claimed (controversially) that they had detected the fingerprint of a compound called dimethyl sulfide, made naturally on Earth only by living organisms such as marine plankton, in the atmosphere of a planet called K2-18b orbiting a star 120 light-years away. More certainty about such claims might have to await better instruments. “I look forward to the day we build and launch telescopes that can get us the answer,” says Wong.
While the researchers’ arguments pose a challenge for most of the putatively hard steps, the biggest unknown, Davies notes, is the first: the genesis of life itself. “We have no idea how non-life became life, how many steps were involved and what the prevailing conditions needed to be,” he says. “And you can’t estimate the odds of an unknown process. This is in contrast to all the subsequent putatively hard steps, because at least we know the process involved: Darwinian evolution. Nobody knows the process whereby non-life became life.”
For this and other reasons, Watson thinks that Carter’s notion of hard steps might still hold—even if the slow pace of environmental change puts the brakes on how soon intelligent life can arise. In a forthcoming paper, he presents a model in which such delays are incorporated into Carter’s earlier picture. “The hard-steps model remains a useful and viable framework for understanding the evolution of complexity in the Earth’s biosphere,” he says.
“If I’m right, we will find only Archean-like biosignatures [in exoplanet atmospheres],” says Watson. “Sometime this century, we should find out who is right.
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