In 1950, physicist Enrico Fermi put forward his famous paradox during a lunchtime conversation. If life in the universe is common, where are all the alien civilizations? Over the last 70 years, we have tried to answer the question in a multitude of ways. A new study now re-affirms a popular hypothesis: maybe, there’s nobody out there.
The work is published in the journal Astrobiology. Andrew Snyder-Beattie and colleagues have provided strong statistical backing to the idea that intelligent life is exceptionally rare in the universe. The caveat is that this intelligent life must have evolved through the same biological transitions that life on Earth has gone through to eventually arrive at modern humans.
The first transition is abiogenesis, the creation of organic molecules from forces that are not living organisms. Then there’s eukaryogenesis, the formation of cells with distinct internal structures. That’s followed by sexual reproduction, the development of multicellular organisms, and eventually intelligence. Each transition requires the preceding ones to have occurred but the opposite is not true. There is no evidence to suggest that intelligence is the inevitable result of sexual reproduction in multicellular organisms.
The work focused specifically on the timing of these transitions. The researchers made the assumption that each transition is equally probable at any time in Earth’s history. This might not be the case in actuality but it requires the least amount of biological assumptions, so it is an appealing shortcut. Based on this, the team finds the complete series of transitions is likely to take a longer time than terrestrial planets are capable of supporting complex life like our own.
“It took approximately 4.5 billion years for a series of evolutionary transitions resulting in intelligent life to unfold on Earth. In another billion years, the increasing luminosity of the Sun will make Earth uninhabitable for complex life. Intelligence, therefore, emerged late in Earth's lifetime,” the authors wrote in the paper. “Together with the dispersed timing of key evolutionary transitions and plausible priors, one can conclude that the expected transition times likely exceed the lifetime of Earth, perhaps by many orders of magnitude.”
The team shows that this work also has two interesting consequences. One has to do with the possibility of intelligent life around stars smaller and dimmer than our Sun. Discoveries have shown that terrestrial planets are common around these red dwarfs, but this model suggests that these worlds are 10,000 times less likely to be habitable. The other prediction is actually more encouraging for those wanting to believe in aliens. The model favors the possibility that life on Earth might have started elsewhere, maybe Mars.