Brain In A Bag

When news sources first picked up a bizarre story about a plastic bag containing a brain found near an apartment complex, nobody knew if the specimen came from a human or an animal. Now, the medical examiner’s office in Richmond, VA has reported that the brain does not come from a human. But this begs the question, how do you know what species a brain comes from?

Size, for one thing. The average human brain weighs in at about 1.35 kg. Despite the fact that humans have the highest brain size:body weight ratio (also known as the encephalization quotient, or EQ), we don’t actually have the largest brains. For instance, a sperm whale’s brain weighs in at a hefty 7.8 kg. That having been said, the only animals with brains that could be confused with a human’s are the dolphin and walrus, whose noggins weigh in at 1.55 and 1.07 kg respectively.

However, unpreserved brains tend to shrink in size, and general morphology may no longer serve as a useful indicator of the species of origin. In this case, biochemical methods can provide more accurate information. These assays have been especially useful in the food industry, where it is important to know which animals’ tissue are present in the final product.

While protein content can provide important clues regarding the identity of the species in question, DNA analysis is preferable. Specifically, the rapidly evolving mitochondrial genome (mtDNA) differs widely between species. In 1989, Kocher et al. showed that universal primers could be used to amplify mtDNA coding for the cytochrome b protein from a variety of animals. Treating these segments with restriction enzymes yields fragments of characteristic lengths that can be used to deduce the species to which the tissue belongs. While the fragments are meaningless by themselves, they can be compared to reference samples from different species in order to find a match.

So the next time a brain turns up on your doorstep, you’ll know how to find out if it’s human.

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