The wonder of children

It's a wonder that the human race has survived this long, with children being as inherently curious as they are. We were over at a friends house tonight, and watching their 19 month old try and do some of the things that our five year was doing was unforgettable (Remind me never to have an elevated/recessed area in any of my houses). Kurtis was running and jumping off of the two-step drop into the living room on and off all evening. All of a sudden, our hosts 19 month old decided to try the same thing. Needless to say, she bit the dust, quite literally. So the question remains, have kids always lacked a sense of self-preservation, or is this something that we have bred into our children?