Imagine at some future time, two young adults meet on an otherwise deserted planet. They are both heavily freckled. What would this tell us about them, their ancestors and how they had have spent their time? First, all of us learn early in life that skin colour and marks like freckles are unequally distributed across the people of this earth. They are most common in people with pale skin, especially so if they have red hair, and we all know that we get our skin colour from our parents. Second, freckles are most common in those who have spent a lot of time in the sun. So freckles betray both something about our ancestors, and how we ourselves have lived our life.
Skin colour varies across the earth, and the chief determinant of this variation has been the interaction between sunshine (more particularly ultraviolet radiation) and our skin over the last 5 to 50 thousand years. Dark skin is adapted so as to protect against excessive sunshine, whereas we think light skin is better adapted to areas where the sun shines less. As some humans migrated out of Africa, say 50,000 years ago, a series of changes or mutations occurred in many genes to make their skin lighter. Their skin became more sensitive to both the good and the harmful effects of ultraviolet radiation. One way this change was accomplished was the development of changes in a gene called the melanocortin 1 receptor (MC1R), a gene we could also call a gene for freckles.
Skin and hair colour arises from a mixture of two types of the pigment melanin: brown or eumelanin, and red, or pheomelanin. If the MC1R works effectively, eumelanin is favoured; if the MC1R works less well, pheomelanin is favoured. We know that the change to pheomelanin is associated with skin that is more sensitive to sunshine. When people who harbour changes in MC1R are exposed to sun they are much more likely to develop freckles, than those who contain no changes in their MC1R.
And what about the freckles themselves? They are just tiny areas of melanin production. Ironically, to the best of our knowledge the freckles themselves seem to protect against the sun quite effectively. It is the non-freckled areas that are most sensitive to the sun. If in a bid to protect skin against the harmful effects of excessive sun, we were to join all the freckles up, the sensitivity might disappear. Of course, on a planet located far away in time and place, our two young adults might already possess the technology to join all their freckles together. It is just that they chose not to.