New Scientist: Why bonobos make love, not war
… Pygmy chimps or bonobos are both literally and metaphorically our kissing cousins. If you know them at all, it is probably as the most highly sexed of all the apes, but they are also considered by many to be our closest living relative—closer even than the common chimp.
This is incorrect. Bonobos are not more closely related to us than are chimps, nor vice versa; bonobos and chimps are equally closely related to us.
As the (subscriber only) article goes on to explain:
Somewhere between 6 and 8 million years ago, our ancestors split from the line that would become today's two species of chimps. Then around 2.5 million years ago, bonobos and common chimpanzees went their separate ways.
Two generations ago, my immediate Carter ancestor (my father) split from the line (his brother's) that would become my two cousins. Ignoring our maternal lines for the sake of the analogy, to say that I am more closely related to one of my cousins than the other is ridiculous, as they shared a common ancestor (my uncle) more recently than they shared a common ancestor (my paternal grandfather) with me. The same argument goes for chimps, bonobos and humans: if bonobos are our kissing cousins, then chimps are their (less affectionate, equally closely related to us) siblings.
The rest of the article is, however, very interesting. It argues that living in areas with more abundant, nutritious, protein-rich plants meant that there were not the same selective pressures on bonobos to evolve/devise methods of food prepartion—including, perhaps, the use of tools. Instead, the plentiful supply of food has made bonobos' feeding time more of a social activity than a competitive one, which could explain their more peaceful and sociable (to put it mildly) lifestyles.
It's a nice idea, but I'm not sure how you could test it.