My wife and I sat at the dinner table last evening trolling trough memories of almost 50 years together. We were astonished at some of the trivial stuff we remembered, but aware too of whole epochs of experience that have evaporated into forgetfulness.
"Too much life, not enough disk," said my wife. She suggested that the human brain evolved when an average lifetime was a only few decades. Now that we live two or three times longer, we just don't have the gigabytes to store it all.
There may be something to that, since remembering past experiences -- crocodiles in the river -- can clearly have survival value. But it's hard to see how natural selection would work to keep every little thing in the archives. Just before my mother died at age 92 she could still recite long poems by Longfellow, Whittier, Riley, Lowell, and the rest. Not much Darwinian advantage there, but it gave her considerable pleasure. Amazing that all those musty poems were somehow squirreled away in a tangle of her neuronal snapyses.
The human brain contains 100 billion neurons, and each neuron is in contact with a thousand others, more or less. If we think of each connection as being "on" or "off" (a crude simplification), then we can say that the human brain stores roughly 5,000 gigabytes of information (the equivalent of 5,000 billion keyboard characters). I'm not sure I did the calculation right, but that's more than enough capacity to store every poem you ever learned plus Proust's Remembrance of Things Past.
Still, memories slip away. And, when the power goes off, it is lost forever. Which is why we resort to diaries, scrapbooks, photo albums, souvenir collections. More publicly, we have memoirs, autobiographies, homepages, blogs. The internet has become willy-nilly the collective memory of our species. How vunerable are our wikiselves to evaporation? I have piles and piles of floppy disks around the house that will never be read again.
There was one more thing my wife said last evening that I wanted to add. Now let's see, what was it? It's just on the tip of my tongue...