In a different form than they do now. We have, and still do, genetically modified them to their current and future state. We've been selectively choosing and breeding plants for many millennia. Corn, beans, and tomatoes didn't much look like what you buy in the store today.

So, how did they survive? Plants grow, plants die, and the rotting plant material gets returned to the soil. Add in a few dead critters, a bit of fire, and some rain, and Baby you gotta stew going!

We still do that even today. Gardeners often compost things like food scraps to grass clipping to create small scale "natural" fertilizers and work that into their gardens.

If you are a 'Murican you will have been taught that Native Americans taught the Pilgrims how grow food and hunt. Because you know, them Pilgrims was a bunch of City Slickers that didn't know how to survive in the "wild". There weren't no Piggly Wiggly's or Aldi's around. They were taught how to grow the Three sisters - Corn, beans, and squash together for best yields. The beans, (legumes), fixed some nitrogen into the soil, the squash plant provided shade cover to limit the growth of "weeds" that could choke out the plants you did want and to hold moisture and keep the soil cooler so the corn, and beans would grow better. The Native Americans also understood, that adding some dead animal matter will also boost your yield. As did most any early farmers.

These days, we need to grow food crops on a very large industrial scale. And yields need to be vastly increased to provide enough for everyone to eat. To do that we need to create better varieties of plants that can withstand the high growing stresses from high density planting with more disease and bug resistance all while producing greater and greater yields. This does require the added use of fertilizers and even pesticides to reach the desired yield goals.