Late last year a team of scientists published a study in Nature comparing the biomass of wild mammals on Earth with the biomass of humans and our domesticated mammals—cattle, pigs, sheep, etc. Biomass in this sense—totaling the weight of organisms by species or habitat—is a relatively crude measure, and estimating it gets harder and harder as you go back in time and the observational record thins. The study cautiously proposes the year 1850 as a point of parity, when the biomass of humans and our domesticates roughly equaled that of wild mammals.

No one in 1850 could have sensed this conjectural state of balance, of course, and not only because the data was missing. That year came and went as most years do: humans doing their humanly thing with vanishingly little regard for other species. 1 Which brings us to the present. The proportion of mammalian biomass on this planet is no longer fifty-fifty. It’s now 95 percent humans and our domesticated mammals—and only 5 percent wild mammals. How did we get here?…