Every few months, it seems, we’re treated to breathless headlines announcing the discovery of X number of “potentially habitable” exoplanets, followed by declarations that the universe must be “teeming with life.” These statements aren’t just premature – they represent a fundamental statistical error that continues to permeate both popular science and, surprisingly, some academic discourse.

Here’s the uncomfortable truth: as far as we can determine, all life on Earth shares a common ancestor. Life appears to have originated only once in our planet’s 4.5-billion-year history. Despite billions of years of opportunity and countless environments, we have no evidence that life spontaneously emerged from non-living matter more than a single time.

This presents an insurmountable statistical problem. With a sample size of exactly one, we cannot make any reasonable inference about the probability of abiogenesis (life arising from non-life) occurring elsewhere.

When astronomers announce they’ve discovered dozens or hundreds of “Earth-like” planets in habitable zones, many immediately leap to conclude that life must exist on at least some of them. This reasoning is fundamentally flawed. Having identified only a single instance of life’s emergence in the observable universe, we have no baseline probability to work with.

To put it in statistical terms: if you flip a coin once and get heads, you cannot conclude anything meaningful about the coin’s bias. You need multiple flips. Similarly, with only one known origin of life, we simply cannot calculate any meaningful probability for its occurrence elsewhere.

The famous Drake Equation, which attempts to estimate the number of communicative alien civilizations, contains a term for the fraction of suitable planets on which life actually appears. But this is precisely the variable we cannot estimate with our sample size of one.

If life had independently arisen twice on Earth – if we had discovered even one organism with a biochemical makeup suggesting a separate origin – then we would have grounds to believe abiogenesis might be common. But it hasn’t. Despite decades of searching Earth’s most extreme environments, we’ve found only variations on the same biochemical theme.

This perspective aligns with what scientists call the “Rare Earth Hypothesis,” which suggests that the emergence of complex life requires an improbable combination of astrophysical and geological events and circumstances. While simple microbial life might exist elsewhere, complex multicellular organisms may be exceedingly rare.

The Fermi Paradox asks: “If extraterrestrial civilisations are common, where are they?” One compelling answer is simply that they aren’t there – because life itself, particularly complex life, may be extraordinarily rare.

None of this proves we’re alone in the universe. It simply means we shouldn’t make probabilistic claims about abundant alien life based on a sample size of one. Science demands intellectual honesty, and honest scientists must acknowledge that we simply don’t know how common or rare life is in the cosmos.

The next time you read an article claiming the universe must be “teeming with life” because we’ve discovered X number of Earth-like planets, remember this fundamental statistical limitation. Until we discover a second, independently evolved biosphere – whether on Mars, Europa, Enceladus, or beyond our solar system – claims about the abundance of life remain speculation, not science.

In the meantime, this uncertainty makes our search for extraterrestrial life all the more important. Finding even microbial life that evolved independently from Earth life would instantly transform our understanding of life’s prevalence in the universe. It would increase our sample size from one to two – a limited sample still, but infinitely more informative than what we currently have.

Until then, we should approach the question of alien life with appropriate scientific humility. We are working with a sample size of one, and the universe has not yet revealed its secrets.