On a cool, damp morning in the late 1870s,   in the garden of a large country home in  southern England, the buzz of insects and   chorus of birdsong was rudely interrupted  by the sound of an old man yelling.

The man was Charles Darwin,  now nearly seventy years old,   and the thing he was yelling at…was a worm.

Now, Darwin had nothing  against this particular worm.

This was not a cry of rage or distress  at the earthworm's appearance in his   garden after a night of rain, it  was a deeply scientific scream.

This was one of Darwin's many  strange but meticulous experiments   that he conducted at his home in his later years.

See, after having solved the  small matter of evolution by   natural selection - becoming one of the  most famous scientists in the world in   the process - Darwin focused his big old  brain on a different personal obsession… One that would be the final scientific  side-quest of his long and storied life.

This was Darwin's worm era, culminating  in his worm book - the last book he'd   ever publish, just six months before his death.

But while it was his last,  it was far from his least.

Darwin’s worm book was a runaway  hit, winning rave reviews,   and stimulating the development  of a whole new field of science.

For good reason too, because, as Darwin  argued, these often-overlooked creatures   are among the most important animals  in the entire history of the planet.

And in the century and a half since,   the fossil record has revealed that he was more  correct than he could ever have even imagined… Because, during one of the single most pivotal  chapters of deep time that revolutionized life   on earth, when we go digging for who or  what we have to thank, there we find worms.

Right under our toes the entire time.

If there’s one thing you should probably  know about the kind of guy that Darwin was,   it’s that, at heart, he was simply  a relentlessly curious nerd.

His major contributions to science  didn't stem from some raw brilliance   or unparalleled genius that only he possessed… Instead, his gifts were the luxury of  being able to spend as much time as   he liked studying whatever caught his attention,   combined with the dogged persistence to  follow each side-quest through to the end.

From finches and pigeons to orchids and  carnivorous plants, not to mention the   eight years he spent studying barnacles --  even though they nearly broke him mentally.

It was Darwin's ability to see  beauty and wonder in species that   others overlooked - and to learn  profound lessons from them - that   allowed him to notice things about the  natural world that no one else ever had.

But while all those other quests came and went,   Darwin spent more than 40 years - over  half his life - with worms on the brain.

They first caught his attention in  1837 when he was in his late 20s,   just one year after returning from his fateful  voyage around the world on the HMS Beagle.

His uncle showed him some fields that,  15 years earlier, had been covered in   various waste materials that now were  buried below the surface - probably,   as his uncle suspected, by the work of earthworms.

To most people this would have seemed like  a pretty unimportant throwaway observation.

But to Darwin, the idea that such small  creatures could cause such big changes over time   ignited a fascination with wormkind that  stayed with him for the rest of his life.

Now, in the decades that followed, he got  sidetracked with other things - you know,   figuring out how species originate,  how life evolves over time,   how humanity came to be - minor stuff like that.

But he never forgot worms.

And in the final decade of life, he turned  his laser-like focus on them once more,   determined to finally write about earthworms  “before joining them”, as he put it.

He conducted a range of experiments to  test how they perceived their environment,   like offering them different kinds of food and  exposing them to sources of light and heat.

He’d also yell at them real  loud, as we mentioned earlier,   and he even got his son to play a bassoon at  them, just to see how they’d react to the noise.

But the primary aspect of earthworms that  Darwin tried to figure out was the very   same one that had first captivated  him in 1837: the question of how   they gradually engineered the  ecosystems that they lived in.

See, Darwin recognized that, as worms plough  through the earth, they stimulate a number   of changes – like recycling organic matter  and contributing to the formation of soil.

This process of reworking the ground  would later become known as bioturbation.

And by measuring the rate at which worms  carried out this process, day after day,   year after year, Darwin was able to glimpse  the possible big-picture implications.

Such observations led Darwin to become  the first person to really grasp that   the behavior of these small, unsung  animals must inevitably have dramatic,   planetary-scale effects,  especially over long timescales.

As he put it in his worm book, “It may be doubted;  whether there are many other animals which have   played so important a part in the history of  the world, as these lowly organized creatures.” The only problem was that, during Darwin’s  time, there just wasn't enough fossil evidence   available for him to really point to what that  ‘important part’ might have actually looked like.

Today though, we have the fossil  evidence that Darwin lacked.

And   it shows that his quite literally  groundbreaking ideas were right… This really is a worm’s world, the rest  of us are just living in it - and if it   wasn't for their bioturbation of the ancient  earth, we might never have evolved at all.

Let me explain.

One of the most consequential chapters in the  entire history of life was the Cambrian period,   between around 539 and 487 million years ago… Perhaps you’ve heard of it and the so-called  Cambrian Explosion that happened in it - the   spread of complex animal species that were some  of the first to be well, recognizable as animals.

They replaced the generally much  weirder lifeforms that preceded   them in the Ediacaran period, which we often  struggle to even place on the tree of life.

Those older Ediacaran species  were simpler, squishier,   and mostly immobile - anchored  to the hard and solid seafloor.

And this evolutionary transition from strange  Ediacaran life to familiar Cambrian life   has been one of the most mysterious and  hotly debated topics in natural history.

What changed after billions of years that led to   the relatively sudden rise and spread  of complex animal life as we know it?

Well, while many ideas have  been proposed over decades,   in recent years, some scientists  have argued that a large part of   the credit should go to - you guessed it -  ancient worms digging through the sediment.

See, around the tail end of the Ediacaran,   right before the Cambrian began, the earliest  worm-like animals appear in the fossil record.

And they appeared in such diversity and abundance   that this slice of the Ediacaran is  often known simply as ‘wormworld’.

The fossilized burrows these  ancient worms left behind show   that they were the first lifeforms  routinely digging into and burrowing   through the seafloor - the very first  bioturbators that the world ever saw.

And just like their earthworm equivalents  today, the ancient species of wormworld had   a massive impact on their ecosystem as the late  Ediacaran transitioned to the early Cambrian.

By churning up the seafloor for the first time,  they opened up a whole new ecological dimension,   altering geochemical cycles, changing  the nutrient availability in the water,   and generally making the previously  hard seafloor much softer.

This softer, looser, and increasingly  multifaceted new seafloor, which had been   created by the evolution of bioturbating  worms, was a catastrophic environmental   shift for Ediacaran species that were adapted  for anchoring themselves to solid surfaces.

A shift that may have contributed  to their decline, while helping set   the stage for the Cambrian explosion  of much more familiar animal groups.

The new ecological niches and resources opened  up by those ancient worms ploughing their way   through the seafloor would have been a massive  opportunity for the more complex Cambrian species.

These more mobile species were well positioned to  take advantage of this increasingly dynamic world.

And the legacy of their rise in the  aftermath of wormworld continues to   this day in the form of you,  me, and all other animal life.

We are the direct descendants of those early   Cambrian lineages that got their  start over 500 million years ago.

Darwin had no way of knowing any of this of  course, or that his backyard experiments and   observations on bioturbation by worms  were basically replicating one of the   single most pivotal environmental changes  in the entire history of life on Earth.

And while he did have a hunch  that worms had been far more   important over the course of natural  history than anyone else assumed,   he was wracked by self-doubt and  insecurity about his work– classic Darwin.

Instead of chasing scientific fame or  validation at this late stage of life,   he was simply trying to scratch his humble  worm itch that had lasted over 40 years.

And he was convinced that no one  would really pay any attention to   what he described as his “curious  little book…of small importance.” He even referred to the entire topic of   bioturbation as a subject “I have  perhaps treated in foolish detail.” But the world disagreed.

Darwin’s worm book was a spectacular success,   selling more copies in its first year  than On the Origin of Species had.

…despite that book tackling one of the  most profound questions humanity had   ever asked, and this one addressing,  well…what the deal was with worms.

It also won critical acclaim across  the world, with one reviewer saying, “Darwin confers upon the despised  and humble earth-worm an interest   it never possessed before…portions of  his book read almost like a romance,   for there is much in his revelations  of surprising strangeness and novelty.” Less than six months after publishing his worm   book and finally completing his last  and longest-lasting side quest that   had occupied his mind for more than  half his life, Darwin died at age 73.

To honor his life and work, he was buried  beneath Westminster Abbey in the heart of London.

A spot reserved for only the  most renowned of scientists,   where he lies to this day alongside such  names as Isaac Newton and Stephen Hawking.

It was his capacity for recognizing the big  impact of even the smallest of factors – be   it tiny creatures or miniscule change over  time – that landed him among the greats.

And so, instead of fulfilling his prediction  and eventually joining the earthworms that   he had obsessed over, Darwin’s own hole in the  ground turned out to be much, much more special.