Although many of the great apes prefer one or the other hand to perform delicate tasks, Humans are unusual in that righties greatly outnumber lefties. Left-handed people (I am one!) show elevated rates of learning disorders such as dyslexia; tend to perform better on tasks engaging the right hemisphere; and may even have different brain representations of abstract constructs like "kindness." So there has been a sense for some time that left-handedness is a marker for a grab bag of brain differences. Left-handedness is also at least partially heritable, with genetic contributions accounting for about 24% of the variance in handedness in the general population.
variation in the gene PCKS6 as associated with left-handedness in individuals with dyslexia. This gene encodes a proprotein convertase, a type of enzyme which clip many different important signaling molecules. Intriguingly, PCKS6 is known to process an important set of developmental signals known as nodal proteins, and mice lacking PCKS6 develop left-right abnormalities in numerous organ systems such as the lungs and intestines. So it is plausible that similar genes are important in the developing human brain to subtly affect numerous brain functions.
It's important to note that this gene variation was only associated with left-handedness in their subjects who had dyslexia. The same gene variation did not even show the same trend in the general population. Nevertheless, the numerous changes in brain function associated with left-handedness makes this type of gene a plausible candidate for the genetic portion of the difference.
Sunday, December 29, 2013
The Native American site of Cahokia, the central settlement of a large polity on the Mississippi river floodplain near present-day St. Louis, supported up to 13,000 people at its peak, making it the largest pre-Columbian settlement north of Mexico. The inhabitants of Cahokia evidently had extensive trade networks (pdf link), with marine shells and shark’s teeth from the gulf coast; volcanic mica; and copper from the Great Lakes, all found at the central complex.
|The natural range of two species of holly used to prepare|
cassina. The inset shows the several settlement sites which
which are associated with the Cahokia complex.
Source: Crown et al. (PNAS, 2012)
A recent paper in PNAS suggests that the exchange with coastal tribes was not limited to durable goods. Chemical analyses of residues in ritual vessels from Cahokia have found evidence for preparation of cassina, or Black Drink, a ritual drink resembling tea which was prepared from leaves and twigs of holly trees. The species of holly used to prepare cassina grew far to the south of the Cahokia complex, with the nearest sources in Arkansas more than 550 km away. Consumption of this drink at Cahokia suggests, at a minimum, regular trading contact over these long distances. Also, cassina was part of an entire cultural package, including special vessels for its preparation and drinking. Cahokia’s central position on the Mississippi may have been part of the spread of this ritual culture along the Mississippi watershed and beyond. Finally, the Cahokia vessels are the oldest for which boiled holly residues have been detected. This suggests the cassina culture might have even older roots along the Gulf coast.