When a cell dies, it usually releases its genetic material into the environment. Wild organisms are constantly running across DNA/RNA from dead cells. Some organisms have found that picking up random DNA can be beneficial as it can lead to obtaining genes that will give the organism a special advantage. Imagine this happening millions and millions of times over a period a day. It's not hard to see how some stands of DNA/RNA that make it into cells cause adverse effects and eventually develop into more complex items