Bacteria account for the overwhelming majority of life on earth, perhaps even by weight. With lifespans measured sometimes in hours or even less, bacteria evolve at a much faster rate than plants or animals. Individual plants and animals host many thousands or even millions of bacteria, forming ecosystems unto themselves, and so bacteria can deeply affect how they live and behave. For example, with so much plastic waste left behind by civilization, some bacteria evolved that could metabolize it. When these bacteria joined the gut flora of certain ruminants, they began to seek out plastic waste to eat. The animals did not evolve fast enough to eat plastic themselves. Instead, the bacteria living in their intestines break it down.