Through technologies like paper, photographs, and computers, humans have augmented their memories so that information can be stored more permanently and precisely, and is easier to share. One could argue that the tendency to externalize one's memories is a defining characteristic of Homo Sapiens. Probably the earliest example of extra-somatic memory is oral tradition. When a song or story is told to others, its memory is no longer dependent on one person for its continued existence. With oral tradition, a tribe or group can have a memory independent of its members.
As art and writing developed, memories' continued existence no longer depended on people at all. We can dig up clay tablets written by peoples whose cultures are long dead, and still get an inkling what was on their minds. In addition, we no longer have to give things a catchy tune or poetic quality to make them more memorable.* You can now get everything you set out for at the supermarket,** or remember what you planned to do today, even if you're the type of person who would forget his own head.
Extra-somatic memory also makes collaboration easier. Thanks to the miracle of the whiteboard, the tribe can now plan out their attack on that herd of antelope using pictures rather than just gestures and words. Computers take ESM one step further, and actually do some of your thinking for you, and let you record and develop your memories in easier and more creative ways.
Because it causes intra-somatic memory to be used less, some of that memory's power has probably atrophied. I bet paleolithic people could remember what they went for at the grocery store.
* I'm not saying that catchiness or poetry are without value. I think they're part of our mechanism for evaluating the structure of ideas. Some of that structure is intangible and can only be touched through our emotions. But I digress.
** I'm aware that writing pre-dates supermarkets. Work with me here.
Note: I didn't make up the term, somebody else did.