In a new paper published in the journal Life, William Bains and I make the case that complex, macroscopic life should be common in the universe. We added one caveat, however: The conclusion only applies if the origin of life is not a rare event—if it is rare, then chances are we live in a rather empty universe.
In our paper we analyzed the various transitions, or key innovations, of life on Earth throughout its history, including the transition from single-cell life to multicellular life, the rise of photosynthesis, the evolution of macroscopic life, and the rise of intelligent life. We examined whether these transitions were due to a random event, whether they required a critical, unique path, or whether they happened many times through different pathways (the “Many Paths” model). For most of these transitions we found that the key innovation was “invented” several times. For example, photosynthesis originated independently at four different points in life’s history, and multicellularity arose several times in different clades of organisms.
The results show that once life originates, the march to increasing complexity naturally follows. This does not mean that most of the biomass will be complex—most terrestrial life is not. However, a certain fraction of organisms will reach higher levels of complexity given a suitable environment and sufficient time. Other planets might not necessarily develop plants and animals like on Earth, but they would have something similar in function and complexity.
The main transitions for which we could not find many paths were the origin of life and the rise of technological intelligence. Only one species (humans) in Earth’s history has developed technology, making it impossible to say whether this should be a common occurrence on other worlds, a very rare event, or something in between. Our conclusions therefore aren’t much help in solving the Fermi Paradox. Our research does, however, let us place Robin Hanson’s “Great Filter”—something that apparently prevents the rise of intelligent civilizations—at the origin of life, just prior to the ascent of technological intelligence, or perhaps in our future, but not in between the origin of life and the rise of intelligent animal-type organisms.
Our conclusion also has implications for the search for life on other worlds. Not only should we expect to see microbial biosignatures on a planet with life, but also signatures that depend on large, complex, multicellular organisms such as vegetation’s red edge. Thus, SETI searchers should consider using tools that are capable of finding signatures of a global and diverse biosphere on other worlds.