With a heart on the left, why are most of us right handed?
Why almost 90% of humans are right handed is a mystery. The hypothesis presented here is that having your heart on the left resulted in a survival advantage for right handers in very ancient times...
In the 19th century, Thomas Carlyle, Scottish philosopher and essayist, proposed that right hand dominance …“probably arose in fighting; most important to protect your heart and its adjacencies...” Almost concurrently, Pye-Smith launched the warfare shield theory suggesting that, with the development of the shield, individuals carrying it in the left hand with a weapon in the right would experience mortal wounds less often than those fighting with the left hand, and hence “...a race of men who fought with the right hand would gradually be developed by a process of natural selection. Such a race would naturally use the right hand also when they discovered how to draw and to write...”
The first to suggest this advantage of right handers was probably the fencing master Roland (1824): “In actual combat the left-handed person labours under a serious disadvantage, as many wounds of the lungs alone have been known to do well, which, if carried to an equal depth on the left side, would immediately have produced fatal consequences by wounding the heart.” i.e. the left-handed fencer would be likely to pierce a right-handed rival’s right side, striking the right lung, usually a non-mortal wound, while the injury for the left-handed would be on the left, piercing the heart.
The problem with this hypothesis is that the shield was developed long after right handedness became the norm. Examination of flint implement splinters from 1.8 million years ago (mya) reveals that they were probably made by right-handed individuals (McManus 1991).
However, the usual argument against the warfare shield theory, that the shield is a recent invention, may be an oversimplification. The heart and the aorta are situated mainly within the left thorax. The hypothesis raised here is that, regardless of shield use, early ancestors with a preference for using the right forelimb in combat may have had reduced risk of a mortal wound.
The rationale is that use of the right hand or forelimb will rotate the right side of the body towards the opponent, reducing the exposure of the left hemi-thorax. This particular advantage of right-handers would not have selective value in a peaceful society, or a society without sharp implements. However, literature indicates that homicide and duels were common in early hominins. In combat with sharp implements, handedness may influence the relative level of exposure of left and right thorax. While fighting with sharp tools, a left hand unilateral grip will rotate the left hemi-thorax towards an opponent.
I've just published a study that attempts to quantify the degree of thoracic/cardiac asymmetry in humans and estimate the difference in risk of injury from a sharp implement attack to the left or the right human thorax. CT-scans of 37 men showed that, on average, two-thirds of the heart volume are situated in the left hemi-thorax.
I also asked nineteen physicians (who were unaware of the hypothesis) to estimate the outcome of weapons penetrating the left and right thorax or abdomen at random points. The difference in their estimated mortality for left and right thorax was significant, p<0.001.
Together, these results suggest greater vulnerability of the left side of the body in combat, and, accordingly, an adaptive value of right-handedness.
So we may have our hearts to thank for our handedness, it seems...