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The world quietly reached a milestone in the evolution of the human diet in 2011. For the first time in modern history, world farmed fish production topped beef production. The gap widened in 2012, with output from fish farming-also called aquaculture-reaching a record 66 million tons, compared with production of beef at 63 million tons. And 2013 may well be the first year that people eat more fish raised on farms than caught in the wild. More than just a crossing of lines, these trends illustrate the latest stage in a historic shift in food production-a shift that at its core is a story of natural limits.

As the global demand for animal protein grew more than fivefold over the second half of the twentieth century, humans began to press against the productivity constraints of the world’s rangelands and oceans. Annual beef production climbed from 19 million tons in 1950 to more than 50 million tons in the late 1980s. Over the same period, the wild fish catch ballooned from 17 million tons to close to 90 million tons. But since the late 1980s, the growth in beef production has slowed, and the reported wild fish catch has remained essentially flat. (See data.)

The bottom line is that getting much more food from natural systems may not be possible. Much of the world’s grassland is stocked at or beyond capacity, and most of the world’s fisheries are fished to their limits or already crashing. Overstocked rangelands become obvious as the loss of protective vegetation leads to soil degradation, which at its worst can cause punishing dust and sand storms. Overexploited fisheries are less readily visible, but fishing patterns over time reveal that more effort is required to achieve the same size catch as in years past. Boats are using more fuel and travelling to more remote and deeper waters to bring in their haul. Fishers are pulling up smaller fish, and populations of some of the most popular food fish have collapsed.

Historically, people’s taste in eating animal protein was largely shaped by where they lived. In places with extensive grasslands, like in the United States, Brazil, Argentina, and Australia, people gravitated toward grazing livestock. Along coasts and on islands, as in Japan, wild fish tended to be the protein staple. Today, with little room for expanding the output from rangelands and the seas, producing more beef and fish for a growing and increasingly affluent world population has meant relying on feedlots for fattening cattle and on ponds, nets, and pens for growing fish.