They are also reversals of previous trends in human evolution, which were earlier influenced by the warm, sunny climate of the African plains. The chin serves as an attachment site for jaw muscles, partially compensating for the weak bite produced by our reduced snout and jawline. Four primary skeletal differences between modern humans and chimpanzees have been assessed, and support a model of successive increase in the potential for articulate speech within our lineage. This region is flat in other mammals and in humans younger than two years old. In other mammals, food from the mouth takes a path that is consistently lower than the top of the larynx.
Food migrates around the sides of the larynx and into the esophagus.
In humans, the descent of the larynx provides extra room, a chamber of air, above the larynx that allows humans to produce a greater variety of vowel sounds than is possible in other mammals. However, the descent of the larynx also produces a greater risk of choking in humans, as the food and air pathways are crossed within the throat. Breathing or speaking while simultaneously swallowing produces a great risk of food becoming lodged in the trachea. The adaptive benefits of complex speech must have been great to overcome this hazard. The hypoglossal foramen, a passage through the base of the skull for the hypoglossal nerve, which services the tongue, became larger starting approximately 400,000 years ago within Homo heidelbergensis .