Did life on Earth have to turn out this way? Oh, I’d like to think it was all an accident.

In “Last Hominin Standing,” a bright and eloquent Aeon piece, Dan Falk weighs whether the semi-intelligent life we call Homo sapiens simply had to be or if we’re the result of a series of lucky bounces in distant history. Did we depend entirely on a certain spiny eel-esque creature surviving extinction? Without a course-altering asteroid making an impression, would it still be a planet of dinosaurs? The late paleontologist Stephen Jay Gould believed our existence a contingent one, though he had no shortage of willing debaters on the topic. My particular favorite part of Falk’s essay is the passage on a theoretical world, free of Homo sapiens, that became dominated by Neanderthals. An excerpt:

If we re-played the tape of evolution, so to speak, would Homo sapiens – or something like it – arise once again, or was humanity’s emergence contingent on a highly improbable set of circumstances?  

At first glance, everything that’s happened during the 3.8 billion-year history of life on our planet seems to have depended quite critically on all that came before. And Homo sapiens arrived on the scene only 200,000 years ago. The world got along just fine without us for billions of years. Gould didn’t mention chaos theory in his book, but he described it perfectly: ‘Little quirks at the outset, occurring for no particular reason, unleash cascades of consequences that make a particular future seem inevitable in retrospect,’ he wrote. ‘But the slightest early nudge contacts a different groove, and history veers into another plausible channel, diverging continually from its original pathway.’

One of the first lucky breaks in our story occurred at the dawn of biological complexity, when unicellular life evolved into multicellular. Single-cell organisms appeared relatively early in Earth’s history, around a billion years after the planet itself formed, but multicellular life is a much more recent development, requiring a further 2.5 billion years. Perhaps this step was inevitable, especially if biological complexity increases over time – but does it? Evolution, we’re told, does not have a ‘direction’, and biologists balk at any mention of ‘progress’. (The most despised image of all is the ubiquitous monkey-to-man diagram found in older textbooks – and in newer ones too, if only because the authors feel the need to denounce it.) And yet, when we look at the fossil record, we do, in fact, see, on average, a gradual increase in complexity.

But a closer look takes out some of the mystery of this progression. As Gould pointed out, life had to begin simply – which means that ‘up’ was the only direction for it to go. And indeed, a recent experiment suggests that the transition from unicellular to multicellular life was, perhaps, less of a hurdle than previously imagined. In a lab at the University of Minnesota, the evolutionary microbiologist William Ratcliff and his colleagues watched a single-celled yeast (Saccharomyces cerevisiae) evolve into many-cell clusters in fewer than 60 days. The clusters even displayed some complex behaviours – including a kind of division of labour, with some cells dying so that others could grow and reproduce.

But even if evolution has a direction, happenstance can still intervene.•