For decades, the prevailing scientific view held that humanlike intelligence was astronomically rare?perhaps even a cosmic accident. But a growing number of researchers are challenging that idea, proposing instead that the coevolution of life and Earth?s environment may make intelligent life more likely than once believed.
This reevaluation focuses on the popular ?hard steps? model, which argues that intelligence arises only after a planet undergoes a series of highly improbable evolutionary milestones. Developed in part by physicist Brandon Carter in 1983, the model was based on the observation that the emergence of humans took nearly 5 billion years, suspiciously close to the 10-billion-year lifespan of the Sun.
Carter concluded that this timing wasn?t a coincidence, but rather evidence that evolving intelligent life requires extremely rare, low-probability events, or ?hard steps.? These might include the origin of life itself, the appearance of eukaryotic cells, photosynthesis, multicellularity, and eventually, intelligence. Since each of these innovations only occurred once in Earth?s history, the model assumed they were extraordinarily unlikely.
However, new research suggests these steps may not be as improbable as once thought. For example, extinction, fossil record gaps, or environmental bottlenecks could explain why key evolutionary leaps appear only once. Perhaps these innovations emerged multiple times but left behind no trace?or were simply outcompeted by earlier lineages.
What?s more, geobiologists now believe that Earth?s evolving atmosphere and surface conditions may have played a central role in determining when these steps could occur?not whether they were likely to happen at all. For instance, Earth lacked sufficient oxygen for most of its history, making the rise of complex, oxygen-dependent life impossible until relatively recently.
This environmental framing challenges the hard-steps model. It implies that intelligence did not arise late because it was improbable, but because the planet wasn’t ready. Earth?s increasing habitability over billions of years may have steadily opened new doors for evolution to explore.
So how can scientists test whether the hard steps are real? According to researchers, it will take a cross-disciplinary approach:
- Earth scientists can study how planetary environments evolve and when they become capable of supporting complex life.
- Evolutionary biologists and paleontologists can look for independent instances of key evolutionary innovations.
- Astronomers can study exoplanets to identify how often conditions for complex life emerge elsewhere.
If this perspective holds, then the evolution of humanlike intelligence may not be a rare cosmic fluke but rather a common outcome on life-friendly planets.
As researchers continue to explore the deep interplay between biology and planetary science, the chances of finding extraterrestrial intelligence could be higher than previously imagined.