We tend too much to think of people and nature as separate, of the plant and animal world as having evolved separately and independently from us. I learned early in my horticultural eduction that there were 'disturbance weeds' that grew in places where the soil was, well, disturbed for some reason. They grow now as our agricultural weeds, and try as we might to spray and cultivate them out of our fields, they grow at the edges and untended places enough to keep seeding in. The thinking seemed to be that they evolved to handle some 'natural' disturbance such as erosion or a tree being uprooted or animals wallowing or digging. But it turns out, the archaeologists are saying, that those plants evolved to fill the openings made by another kind of disturbance entirely. Those plants evolved to fill the edges of the disturbances made by human camps. The centers of our camps we kept clean by our working or playing there, but the edges, we disturbed enough to keep the perennials plants from growing, so these native annuals had to grow abundant seeds and have short life spans to constantly re-green those areas of not quite enough disturbance to keep clear but too much disturbance for the perennial plants of the surrounding ecosystem. So they co-evolved with us and followed us from camp to camp on our cloths and shoes and among the seeds of our crops plants. Turns out we are not so separate from nature after all, but really just one part of it.
Ragweed is one such disturbance weed that many of us are allergic to.