We read more and more about how the various organelles in our cells, like mitochondria, were free living creatures that became incorporated into other creatures, rather than simply being eaten and consumed. Mitochondria are an excellent example since mitochondrial DNA remains separate from our nuclear DNA, and so are only passed on as entire units in our mother's eggs.
This article, http://www.whoi.edu/oceanus/feature/the-amazing-acquired-phototrophs , discusses certain species that can incorporate the chloroplasts of algae they eat into their own metabolism, rather than destructively consuming them, and thus gain the ability to photosynthesize.
It's a fascinating window on how evolution works, and involves processes other than simple point mutations to drive speciation and the radiation of living creatures into diverse ecologies.
Hope you enjoy!