For hundreds of thousands of years, we had no soap. We did have water, but swimming and washing in a summer stream isn't hot water, and it's hot water which strips away the oils. And even pre-20th century, people did NOT bath in hot water daily.
Often it was just armpits + groin in a sponge bath.
What if our skin was protected by our own oils? I can imagine that even a small amount of oil, would reduce the amount of UV that gets to the skin.
I know sunblock has minerals(zinc, etc) in it to reflect the sun, but I just googled there are oil based sunscreens too:
"Chemical absorbers: Chemical (or organic, meaning carbon containing) sunscreen ingredients form a thin protective film that absorbs UV radiation before it penetrates the skin."
I can easily imagine that our own oils could provide protection, and block UV but let good frequencies of light through. Yet now we wash a lot of that oil off, with hot showers daily, and shampoo and soap which runs down our body as we shower.
Would be an interesting study.
edit: another thought! The oil on our skin is often policed by bacteria. The bacteria eats that oil, and also any organics (skin flakes, or stuff that sticks to the skin).
That's another level of shield. I imagine that without hot showers, and loads of soap, that layer of oil has bacteria multiple layers deep. And that bacteria moves. A moving, sunshield.. in addition to the oil.
And of course, bacteria exudes waste. That waste is going to have carbon in it, too. So our skin oil would be combination of the oil + living shields + their waste.
I imagine the waste would build up after a while too.
edit2: A harsh response from KennyBlanken, who seems to disagree.
However, this poster quite literally ignores things I stipulated, or gets things mixed up. He then ends with harsh criticism, which seems very un-HN like (eg, personal attacks).
That said:
* Skin oil contains a myriad of ingredients, fatty acids, salt, complex acids, bacteria, their leavings, and more.
All of this is simply ignored by Blanken, who just says "skin oil doesn't block UV, because it doesn't have the same precise ingredients as over-the-shelf sunblock". Further statements about "Google this" are provided, yet clearly I did a bit of quick research, as I provided a quote about sunblock. Again, very un-HN like.
* Weird statements about how long soap has been around for, when all I indicated is that for "hundreds of thousands of years" we had no soap, which is entirely true, and obviously stated in terms of how we evolved. This fact seems to have been missed in the reply, however.
* Discussions about how soap has been around for a while, meanwhile missing the entire point that most humans pre-20th (as I stated) typically just did the sponge bath thing. People didn't shower, and most of humanity did not have running water until the 20th, regardless of rare rich Romans.
None of these rebuttals seem to even consider that the scientific process starts with random thought, eg as I said "A theory just popped into my head.", then consideration, then debunking via actual, real research.
As opposed to some 19th century British-Science-Guild like huffing, including personal attacks and so forth.
KennyBlanken, perhaps you could debate more politely? And follow the guidelines a bit more closely?
The oil in sunscreen is to make it spreadable and dilute the UV-absorbing ingredients, which are things like avobenzone, oxybenzone, and so on.
No offense, but you could have googled "active ingredients in sunscreen."
Further: just because something is called an "oil" doesn't mean it has any similar physical properties compared to another oil.
Recipes for soap date back something like 5,000 years, by the way - and probably were passed down by oral history or demonstration even earlier.
You don't seem qualified to be making (or evaluating) the theories you are coming up with. Not in terms of basic scientific knowledge, critical thinking skills, or research skills.
Your "people didn't wash back then so they had natural sunscreen" is about as well-reasoned as the people who think keto is the best diet because "it's what cavemen ate" (hint: people ate whatever was available to them in their area.)
Anyway, it's good to think outside the box, even if this theory is probably a miss.
We saw a mass migration of eastern Europeans to the middle east mid 20th century and the result was the catastrophic increase in skin cancer rates.
We saw a mass migration of middle Easterners to great lakes region in the same period and we saw a catastrophic increase in depression rates.