Published online by Cambridge University Press: 24 September 2020
Here, I start by discussing Stephen Jay Gould’s famous thought-experiment of ‘replaying the tape of life’. If we could wind back to the early days of evolution and reboot, would the tape play out in a similar way? Gould thought not, but his hypothesis was untestable since a real version of his thought-experiment is impossible – at least on Earth. However, other inhabited planets represent independent playings of the tape of evolution, and when we can observe enough of these we will know to what extent evolution is repeatable in a broader context than the one that Gould considered. We can hypothesize in this broader context, confident in the knowledge that our hypotheses will ultimately be testable. Plausible hypotheses are: (1) most life is based on carbon (not carbon chauvinism – the assertion that all life must be based on carbon); (2) most life is based on cells; (3) many features of large life-forms will recur often across different inhabited planets, including skeletons and muscles; (4) intelligence will be absent from some inhabited planets, just as it initially was on Earth – where it occurs, it will be the exception rather than the rule, just as it is here.