Scientists who study speech in humans think this ability must have arisen in our brains as we evolved from primates, but they don’t know exactly how. These researchers can compare the human brain with other primate brains to see how it changed during evolution to confer human speech.
Previous researchers have suggested structures in the front part of primate brains, like groove patterns, could have helped humans learn to speak. To test whether these and other changes in the human brain were involved in speech evolution, an international team of scientists recently compared speech-oriented parts of human and primate brains. The primates they examined included baboons and macaques, collectively referred to as old-world monkeys, and chimpanzees.
First, the scientists looked at images of human and primate brains generated using scans that help scientists see different parts of the brain in detail. These scans included high-resolution magnetic resonance images from the National Chimpanzee Brain Resource, and high-resolution anatomical scans from the Human Connectome Project database.
They examined these scans to identify specific sections of human and primate brains that were different. They reasoned that since other primates do not speak like humans, sections of their brains that looked different from a human’s brain could have contributed to the emergence of speech.
The scientists looked at one specific part of the brain that controls speech, facial expressions, and language, called the prefrontal extent of the frontal operculum, or PFOp for short. They explained the PFOp is part of a structure that covers the inner parts of the brain, called the operculum. They found the PFOp was fully developed in humans, only partially developed in chimpanzees, and absent in old-world monkeys.
Another difference they found in human brains was that the groove separating the inner part of the brain and the operculum, called the circular sulcus, was closer to the front of the brain on the left side than on the right side. They suggested this meant the PFOp was larger in the human brain’s left hemisphere than in the right. The scientists concluded these differences in the PFOp must have formed more recently in humans since they didn’t exist in the other primates.
Next, the researchers looked at images of the chimpanzee’s brain more closely since chimpanzees are the closest ancestors to humans. They found the size of the chimpanzee’s PFOp was the same on both sides of its brain. They suggested this result provided further evidence that the PFOp had recently fully developed in humans.
They also measured the distance between the circular sulcus and two nearby grooves in the brain that form part of the operculum, called the dorsal and ventral fronto-orbitalis, or D-FO and V-FO for short. Previous researchers had found these grooves were associated with communication sounds produced by chimpanzees, so they wanted to test whether their distance from the circular sulcus had any effect on PFOp development in humans.
The researchers explained that in chimpanzee brains, the D-FO is curved away from the circular sulcus, and can sometimes be Y-shaped. They found the closer the top of a Y-shaped D-FO was to the circular sulcus in chimpanzee brains, the more similar they were to human brains. Additionally, they found chimpanzees with a shorter distance between their Y-shaped D-FO and circular sulcus made more communication sounds and facial movements. Chimpanzees who had brains with a Y-shaped D-FO and a V-FO not connected to the circular sulcus also produced more communication sounds. Based on these results, the scientists suggested the development of the D-FO and V-FO also contributed to speech emergence in humans.
The scientists concluded changes in the operculum and brain groove structures helped humans learn to speak, but questions remain. They cautioned that they could not link any specific function of the PFOp to the development of human speech. Thus, they suggested future researchers should determine how the PFOp functions within the human brain to identify this link.
