Researchers believe humans evolved to have less hair on their faces than their primate relatives so it’s easier to read their moods. Humans are often considered hairless apes, but scientists are still debating why we’re not covered in hair like our primate relatives. No one has pinpointed exactly why this is, but evolutionary reasons include moving from cooler to warmer climates, or to free ourselves from lice and other parasites.
A third reason, proposed by neurobiologist Mark Changizi, suggests we lost our hair so it’s easier for others to read our expressions. Particularly, how our skin color changes and what it means, such as blushing. Humans are trichromats, which means we have three cones allowing us to detect light in the medium wavelength including a red and green mix. Known as color signaling, even the slightest changes in skin color are picked up by another person. These changes happen with varying levels of oxygen in the blood. If our faces were covered in hair, we wouldn’t be able to see these changes.
The fact that humans walk upright means more of our bodies are exposed, which explains why we’re also almost entirely hairless. Most mammals on the other hand, such as dogs, horses and bears, are dichromats with only two cones, and are only able to see short and long light wavelengths from blue and yellow blends. According to Changizi’s research, which involved 97 different primate species, the dichromats were furrier, while the trichromats had much more skin visible.
It’s possible, color signaling wasn’t the original reason humans began shedding their hairy exterior, and instead became a byproduct of it.