The Tollund Man, Denmark. Photo by Christian Kober.Corbis
(Dan Falk is a science journalist based in Toronto. His books include The Science of Shakespeare (2014) andIn Search of Time (2008). He also co-hosts BookLab, a podcast that reviews popular science books.)
Was human evolution inevitable, or do we owe our existence to a once-in-a-universe stroke of luck?
In the movie Sliding Doors (1998), a woman named Helen, played by Gwyneth Paltrow, rushes to catch a train on the London Underground, but just misses it, watching helplessly from the platform as the doors slide shut. The film explores two alternative universes, comparing the missed-train universe to a parallel reality in which she caught the train just in time. It wasn’t a cinematic masterpiece – the critics aggregated at Rotten Tomatoes give it only a 63 per cent ‘fresh’ rating – but it vividly confronts a question that many of us have asked at one time or another: if events had unfolded slightly differently, what would the world be like?
This question, applied to the history of life on our planet, has long beguiled thinkers of all stripes. Was the appearance of intelligent life an evolutionary fluke, or was it inevitable? This was one of the central themes in Stephen Jay Gould’s book, Wonderful Life: The Burgess Shale and the Nature of History (1989). 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. Most disruptive are the mass extinctions that plague Earth’s ecosystems with alarming regularity. The most catastrophic of these, the Permian-Triassic extinction, occurred about 250 million years ago, and wiped out 96 per cent of marine species, along with 70 per cent of land-dwellers. Gould examined the winners and losers of a more ancient mass extinction, the Cambrian-Ordovician extinction, which happened 488 million years ago, and found the poster child for biological luck – an eel-like creature known as Pikaia gracilens, which might be the precursor of all vertebrates. Had it not survived, the world could well be spineless…