Our family has been reading Extra Life by Steven Johnson—a history of early biotech developments—over breakfasts. I got a sinking feeling towards the end of the book, as it related the story of Cecile Steele, a young woman in Delaware who kept chickens. As the book explains:
Each spring, she would order fifty additional chicks from a local hatchery. But in the spring of 1923, a mistake at the hatchery added an extra zero to her order; to Steele’s surprise, five hundred chicks showed up on her doorstep…
She stored them in an empty piano box until a lumberman could build a new shed large enough to house them. Steele fattened them up with newly invented feed supplements, and when they reached two pounds, she sold [the surviving] 387 of them for… a tidy profit. The next year she deliberately increased the order to one thousand chickens and began scaling up the facilities on the farm.
Within a few years, she’d scaled up to 250,000 chickens, and other farmers—noticing her financial success—began to follow suit.

Johnson continues:
By the 1950s, the poultry industry had discovered that feeding chickens vitamin D supplements fortified with antibiotics allowed them to live indoors without exposure to sunlight; before long, industrial-scale coops crowded as many as thirty thousand chickens into wire cages so small that the birds did not have room to spread their wings…
There are now more chickens on earth than all other species of birds combined.
Damn, that was one unfortunate typo in 1923.1
* [Post title is a riff on MacAskill’s ‘The best person who ever lived is an unknown Ukrainian man’. No offense intended to the memory of Mrs Steele, who presumably didn’t anticipate where her innovation would eventually lead.]
Given the economic incentives, presumably someone else would have hit upon the idea eventually. But still.
Aside: something I found a bit off-putting about Johnson’s telling of the story was his insistence that “we can support almost eight billion people on the planet now in part because we have sixty billion chickens to eat each year.” My understanding is that this is actually false: animal agriculture is inefficient, producing fewer calories (and less protein) than “goes in” via the animal feed. So if the crop fields used to grow animal feed were instead used to grow (plant) food for humans, we could feed even more people.
the nitrogen pollution from these chicken farms creates nitrogen blankets in the low atmosphere and pollutes water. All told they are more dangerous to human health than the nutrients they provide---and then we eat other animals and plants similarly overly-nitrogenated.
see https://www.usgs.gov/news/report-finds-poultry-farming-sends-more-pollution-chesapeake-bay-previously-thought
Given how profitable it seems to have been, it’s hard to believe that this wouldn’t have happened sooner or later anyway. But yeah accelerating it is still a pretty massive harm!