Goosebumps, also known as piloerection or cutis anserine, are the small bumps that appear on the skin when it is exposed to cold, fear, or other emotional responses. They occur when tiny muscles at the base of each hair follicle contract, causing the hair to stand up and the skin to pucker.

The primary purpose of goosebumps is to provide insulation. In animals with fur or feathers, raised hair or feathers create a thicker layer of insulation, which can help to keep the animal warm. In humans, however, goosebumps don't provide much insulation because we don't have much hair on our bodies, so their function is more related to a physiological response to certain stimuli.

While the evolutionary function of goosebumps in humans is still debated, some scientists believe that they may have served a purpose in our evolutionary past when our distant ancestors were covered in fur. The theory goes that when humans were covered in hair, goosebumps would have made the hair stand on end, creating a thicker layer of insulation in response to cold or fear. However, over time, humans lost most of their body hair, so the function of goosebumps in modern humans is mostly vestigial.

So, to sum up, goosebumps are a physiological response that occurs in response to certain stimuli, such as cold or fear. While they may have served an evolutionary purpose in our distant past when our ancestors were covered in fur, their function in modern humans is mostly vestigial.