In some ways, bonobos (pygmy chimps) are more similar to humans than to other chimps

According to a recent article by Bob Holmes in New Scientist "Ghost of genetics past shows up in bonobos" (18 April 2008), "The genomes of our nearest kin, chimpanzees and bonobos, are strewn with anomalies where the usual patterns of relatedness break down. As a result,

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Chimps and bonobos are each other's closest relatives, having split from a common ancestor only about 1.3 million years ago. Humans are their next closest cousins, having branched off about 4 million years earlier.

But when David Reich, a population geneticist at the Broad Institute in Boston, and his colleagues sequenced tens of thousands of random snippets of DNA from bonobos and chimps, then compared these to the matching sequences of the complete chimp and human genomes, they found a surprise. In 453 of their sequences, either bonobos or chimps proved to be more closely related to humans than to each other.
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The explanation offered is that our common ancestor carried at least two variants of the gene. If bonobos (pygmy chimps) and humans happened to inherit one and chimps the other, then bonobos would appear more similar to humans than chimps in that particular.

An alternative explanation, suggested by a scientist friend on his coffee break, is "I knew it. The common ancestor of chimps and bonobos is... US!."