I presumed chimpanzees were the closest relatives of us. However, after watching this TED Talk, it seems bonobos are closer to us both in skeleton and behavioral similarity than chimpanzees. I once read in The Magic of Reality that we share quite a lot of common FOXP2 gene letters with chimpanzees, but there isn't any mention on similarity between us and bonobos in that book. Here is the excerpt:
You can tell that FoxP2 is the same gene in all mammals because the great majority of the code letters are the same, and that is true of the whole length of the gene, not just this stretch of 80 letters. Of the total of 2,076 letters in FoxP2, the chimpanzee has 9 letters different from ours, while the mouse has 139 letters different. And that pattern holds for other genes too. That explains why chimpanzees are very like us, while mice are less so. Chimpanzees are our close cousins, mice are our more distant cousins. 'Distant cousins' means that the most recent ancestor we share with them lived a long time ago.
Monkeys are closer to us than mice but further from us than chimpanzees. Baboons and rhesus macaques are both monkeys, close cousins of each other, and with almost identical FoxP2 genes. They are exactly as distant from chimps as they are from us; and the number of DNA letters in FoxP2 that separate baboons from chimps is almost exactly the same (24) as the number of letters that separate baboons from us (23).
So which species is closest to us, chimpanzees or bonobos?