For decades, many scientists have relied on the "hard steps" model to suggest that intelligent life is rare — the improbable result of a series of unlikely evolutionary leaps. But new research by postdoctoral researcher and geobiologist Dr. Daniel Mills from the University of Munich challenges that idea, and the implications are game-changing for the search for extraterrestrial intelligence.
In a recent SETI Live episode, Senior Planetary Astronomer Dr. Franck Marchis and Dr. Mills discussed how intelligent life actually might be a natural outcome of planetary evolution.
Rethinking the "Hard Steps" Theory
The "hard steps" model, introduced by physicist Brandon Carter, argues that specific critical transitions in life's history, such as the emergence of multicellular organisms or the rise of technological intelligence, are highly improbable. According to this view, humanity represents a lucky accident, arising just before Earth's habitability window closed.
Dr. Mills, however, proposes a different interpretation. He suggests that intelligence is not the product of beating astronomical odds but is a predictable result of how planets like Earth change over time. His research introduces the concept of "windows of habitability" — periods when evolving planetary environments create the right conditions for complex life to emerge and thrive.
In this view, life did not have to overcome improbable hurdles; instead, it developed naturally when the environment became suitable for it.
Earth's Windows of Habitability
Throughout Earth's 4.5-billion-year history, the surface environment has undergone significant changes. One of the clearest examples is the history of atmospheric oxygen. Initially, Earth’s atmosphere contained no oxygen. Then, approximately 2.3 billion years ago, during the Great Oxidation Event, cyanobacteria's photosynthesis partially oxygenated the atmosphere. Eventually, it reached modern oxygen levels, closer to those of the present day.
Major evolutionary innovations align with these environmental shifts. Early organisms that didn’t require oxygen emerged during the anoxic period, when there was no oxygen. As oxygen levels rose to intermediate stages, life forms that could tolerate small amounts of oxygen appeared. Finally, organisms like humans, which need modern oxygen concentrations, evolved once Earth provided those conditions.
In addition to oxygen, other factors also influenced habitability windows, including:
- Earth's temperature
- Nutrient availability in the oceans
- The style of plate tectonics
- Photosynthetic biomass productivity
Changes in these factors over geological time opened different windows, allowing various types of life to emerge and thrive.
The Power of Interdisciplinary Science
One of the most compelling aspects of Mills’ work is his emphasis on interdisciplinary collaboration. Addressing questions about the emergence of intelligent life requires expertise from multiple fields — physics, astronomy, geology, and biology all play essential roles in this endeavor.
Mills' collaboration with astronomers like Jason Wright, winner of the 2019 Drake Award, and Adam Frank highlights the importance of combining different scientific perspectives. Only by integrating knowledge of planetary environments, evolutionary biology, and astrophysical conditions can we hope to fully understand the pathways to intelligent life, both on Earth and beyond.
A New Hope for SETI
This new model offers a much more optimistic outlook for the search for extraterrestrial intelligence. If intelligent life is indeed a natural product of planetary evolution, then SETI’s chances of success are significantly higher than the hard steps model would suggest.
The Future of Life on Earth
Looking ahead, Earth's biosphere has a limited future, with estimates suggesting that life may only persist for another billion to 1.5 billion years. Yet within that time frame, evolutionary potential remains vast. Complex life forms have already existed on Earth for over half a billion years, and with so much time still available, the potential for new intelligent species to arise remains strong.
Even if humanity were to go extinct, evolution would not necessarily come to an end. Other lineages could develop cognitive abilities comparable to ours, provided the right conditions are met.
Daniel Mills’ Next Steps
Dr. Daniel Mills shared that he would soon start a new postdoctoral position at the University of Düsseldorf, working with Professor Bill Martin to study the origin of life — a field he hasn’t formally studied before but is excited to explore.
Beyond this new role, he plans to build on his research into windows of habitability, aiming to define the specific environmental conditions necessary for key evolutionary transitions, such as the origin of life, the emergence of photosynthesizing bacteria, the evolution of eukaryotes, and the development of animals. By collaborating with other geologists and Earth system modelers, he aims to determine when these environmental conditions first emerged on Earth.
His ultimate goal is to help bridge the gap between planetary science and astrobiology, providing new tools to the search for life beyond Earth.
To learn more, check out the research paper, watch the video below, or find Is Intelligent Life Easy? Human-Like Life Probably Evolves “Right on Time” on our YouTube channel.
