Almost 90% of humans are right handed, but why is unclear. It has been suggested that right handedness evolved in the context of escalating motor and cognitive demands related to tool use. Literature indicates that homicide may have been common in early hominins. Since, in combat with sharp implements, handedness may influence the relative level of exposure of left and right thorax, the hypothesis presented here is that thoracic anatomic asymmetry resulted in a survival advantage for right handed individuals. While fighting with sharp tools, a left hand unilateral grip will rotate the left hemi-thorax towards an opponent. The aims of this study were to quantify the degree of thoracic/cardiac asymmetry in humans and to estimate any difference in risk of injury from a sharp implement attack to the left and the right human thorax. CT-scans of 37 men showed a mean of 73% (SD 7%) of the heart volume to be situated in the left hemi-thorax. Nineteen physicians unaware of the hypothesis estimated the outcome of weapons penetrating the left and right thorax/abdomen at random points. The difference in estimated mortality for left and right thorax was significant, p\0.001 (Wilcoxon- signed-ranks-test for two related samples). These results suggest greater vulnerability of the left side of the body in combat, and, accordingly, an adaptive value of right-handedness. Thoracic asymmetry may have contributed to the development of right hand preference in humans.