How Our Brains Got Big and Our Penises Lost Their Spines

Lost DNA provides keys to human anatomy and a new way to study evolution

Josh Fischman

The relationship between humans and chimpanzees, thought to be pretty close in evolutionary terms, now comes down to a number: 510. Those are the genetic elements, in key areas of the genome, that differ between the two species. Surprisingly they aren’t 510 pieces of DNA that we have and chimps don’t. It’s actually the other way around.

But our loss, paradoxically, is also our gain. The absence of one piece may have led to a feature that anthropologists have been struggling to explain for centuries—our big, complicated, non-chimplike brain.

The removal of another piece is linked to a different body part that has also received a great deal of attention. It could explain anatomical differences between the human penis and chimp genitals, changes that may contribute to human mating and pair-bonding, behaviors that are decidedly different than in other primates.

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How Our Brains Got Big and Our Penises Lost Their Spines 1

D. Roberts, Photo Researchers, Inc

The human skull (left) houses a brain that’s three to four times the size of a chimpanzee’s (right). Scientists have spotted a stretch of DNA that could have prompted this expansion.


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