Key Takeaway:
New research challenges the hard-steps model of evolution, which suggests that human intelligence is an extremely rare event. The model suggests that intelligence takes billions of years to emerge on Earth, and that it is a result of rare and improbable evolutionary breakthroughs. Some scientists argue that the emergence of eukaryotic cells, oxygen-producing photosynthesis, multicellular animals, and humanlike intelligence are examples of such hard steps. However, alternative explanations suggest that these milestones may not have been as rare as the hard-steps model assumes. The role of Earth’s changing environment may also play a role in the emergence of intelligence. If the hard-steps model is incorrect, scientists can test it by determining when Earth’s environment first became capable of supporting each key evolutionary step, investigating how often each hard step occurred, and studying exoplanets. If intelligence is a more natural outcome of planetary evolution, the search for extraterrestrial intelligence may be more promising than previously assumed.
The question of whether humanity is a cosmic accident or an inevitable outcome of evolution has fascinated scientists for centuries. A prevailing model of evolution, known as the hard-steps model, suggests that the emergence of intelligent life like ours is extraordinarily rare. According to this perspective, Earth was incredibly lucky to have experienced the right sequence of evolutionary leaps that led to human intelligence. However, new research is challenging this notion, proposing that humanlike intelligence may be far more probable than previously thought.
The Hard-Steps Model: A Cosmic Lottery
Some of the most renowned evolutionary biologists of the 20th century argued against the idea that humanlike intelligence could exist elsewhere in the universe. Their skepticism gained support from an unexpected source—physics. In 1983, theoretical physicist Brandon Carter introduced the hard-steps model to explain why intelligent life took billions of years to emerge on Earth.
Carter pointed to what he saw as an extraordinary coincidence: the time it took for human intelligence to evolve—around 5 billion years—is remarkably close to the Sun’s estimated lifespan of 10 billion years. He considered three possibilities to explain this. The first was that intelligent life tends to emerge quickly, within a few million years. The second was that it typically takes about as long as it did on Earth—several billion years. The third was that intelligent life normally requires far longer than a star’s lifespan, meaning Earth was exceptionally lucky.
Carter dismissed the first scenario since life on Earth clearly took much longer to evolve. He also rejected the second possibility, arguing that it would be an unlikely coincidence for the timescales of biological evolution and stellar evolution to align so closely. He concluded that the third scenario was the most plausible: humanlike intelligence is incredibly rare because it typically takes far longer than 10 billion years to arise—much longer than most stars remain stable enough to support life.
Do Rare Evolutionary Leaps Exist?
Carter’s model assumes that intelligence depends on a series of rare and improbable evolutionary breakthroughs—what he called “hard steps.” For an evolutionary step to qualify as “hard,” it must meet two conditions: first, it must be a necessary precursor for human intelligence; second, it must be so unlikely that it would normally take longer than a star’s lifespan to occur.
Many scientists believe that the emergence of eukaryotic cells—cells with nuclei—was one such hard step. Without eukaryotic cells, complex life, including humans, would never have existed. The fact that all eukaryotes belong to a single branch on the tree of life suggests that this evolutionary leap happened only once, reinforcing the idea that it was an exceedingly rare event.
Other potential hard steps include the origin of life itself, the emergence of oxygen-producing photosynthesis, the development of multicellular animals, and the evolution of humanlike intelligence. Like eukaryotic cells, each of these innovations appears to have occurred only once in Earth’s history. If these steps were truly rare, the hard-steps model suggests that intelligence is likely to be exceptionally uncommon in the universe.
Challenging the Hard-Steps Model
However, there may be alternative explanations for why these evolutionary milestones appear unique. The evolutionary biologist Geerat Vermeij has suggested that the apparent singularity of these events could be an illusion caused by gaps in the fossil record. Perhaps these innovations evolved multiple times, but some versions didn’t survive or weren’t preserved as fossils.
Another possibility is that once an evolutionary breakthrough occurred, it altered the environment in such a way that it prevented other organisms from evolving the same feature. For example, if a lineage of microbes first evolved oxygen-producing photosynthesis, it would have flooded the planet with oxygen, fundamentally changing the biosphere. Other lineages that had not yet developed photosynthesis may have lost the opportunity to do so because the environment had already shifted.
If these alternative explanations hold, then the evolutionary steps that led to intelligence may not have been as rare as the hard-steps model assumes. But if these steps weren’t inherently improbable, why did intelligence take so long to emerge?
The Role of Earth’s Changing Environment
The answer may lie not in the difficulty of biological evolution but in the environmental conditions required for certain evolutionary leaps. For instance, for most of Earth’s history—roughly 90% of its existence—the atmosphere lacked enough oxygen to support humans. Similarly, modern eukaryotic cells could not have evolved until Earth had developed the right conditions to sustain them.
Rather than intelligence being the result of rare and improbable mutations, it may have emerged as soon as Earth’s surface conditions became suitable. If the planet had developed these conditions earlier, intelligence might have evolved sooner. This suggests that intelligence is not necessarily rare, but rather contingent on planetary evolution.
Testing the Hypothesis
If the hard-steps model is incorrect, how can scientists prove it?
One way is to determine when Earth’s environment first became capable of supporting each key evolutionary step. If intelligence took billions of years simply because the right conditions weren’t present earlier, then this would challenge the idea that intelligence is inherently improbable.
Another approach is to investigate how often each supposed hard step actually occurred. If new evidence reveals that eukaryotic cells or oxygen-producing photosynthesis evolved multiple times, it would weaken the argument that these were improbable events.
Additionally, astronomers studying exoplanets could look for biosignatures—chemical markers of life—to determine how frequently planets develop conditions suitable for life. If planets with oxygen-rich atmospheres and other key ingredients for complex life are common, then the chances of intelligent life elsewhere in the universe may be higher than Carter’s model suggests.
A New Perspective on Life Beyond Earth
If intelligence is a more natural outcome of planetary evolution rather than a rare cosmic fluke, then the search for extraterrestrial intelligence may be more promising than previously assumed. Rather than being an unlikely lottery win, the emergence of intelligent life might be the expected consequence of planets that follow a similar evolutionary trajectory to Earth.
This shift in perspective has profound implications. If intelligence is common, then the universe may be teeming with civilizations, waiting to be discovered. If intelligence is rare, then humanity may be among the very few conscious beings in existence.
Future research will help determine which of these possibilities is correct. But if Earth’s history is any indication, intelligence may not be a cosmic accident after all—it may be an inevitable outcome of the right conditions, waiting to unfold on countless worlds across the universe.
