It is also a fact that these people were black. Consider this. These people wore no clothes nor did they live in caves. This all came later and thus they were in the sun a lot of the time. It is a fact a person with black skin will take up to five times longer to absorb the same amount of vitamin D as a white person.
A black person will absorb about 10,000 IU ( international units) of vitamin D in about 2 to 2.5 hours. So how much time did these early people actually spend in the sun. There is not way to find out, but I think it was difficult to avoid the sun. One study I read suggest it takes over 30,000 IU a day to induce toxicity. So it not illogical to assume that skin color was evolved, in Africa, to control vitamin D levels at a level that was proper and healthy by this feed back mechanism of skin color. Conversely white skin evolved to allow more absorption of vitamin D at latitudes where there is less available from the sun.
As people moved out of Africa away from the equator, and a constant year around sun that was full of vitamin D, there skin became lighter. This was a survival necessity. As the skin lightened they were able to absorb more vitamin D quicker and more survived to reproduce. (One of the extreme things that happens with very low vitamin D is rickets. Rickets will cause the female pelvis to be misshapen and vaginal delivery to be impossible). Surely you have noticed that those in southern Europe are darker than those in Norway.
Where I live,there is very small to inadequate amounts of vitamin D obtainable September to May. To receive vitamin D from the sun you have to have a shadow length that is shorter than your height. If it is longer it maybe possible to tan, but you will not receive significant amounts of vitamin D. Tanning sun wavelengths and vitamin D wavelengths are not of the same frequency.
This lack of the vitamin, in areas away from the equator was made up by eating foods that are high in vitamin D and some summer sun. Believe me you do not want their diet. Basically they ate the whole animal and a lot of roots and fruit in season. Animal organs are high in vitamin D. Farming came later.
Dosage. I recommend 1,000 IU per 25 lbs body weight, everyday for three months. It takes that long for the level in the blood to stabilize. Get tested and adjust your dosage to get a level of 80 ng/ml year around. As the number of ng goes up so does pain relief, but I would stay below 100ng unless there is a very good reason to go above. More on that subject later, Only the 25(oh)D test is valid for this purpose. There are other test for vitamin D that are not useful in otherwise healthy people.
If you want to read more on this subject, I recommend www.vitamindcouncil.org They are non profit organization and there are many links to studies there.
One more thing; vitamin D obtained over the counter varies in quality. One brand, according to a study, contained only one third of the amount marked on the label. Changing brands may adversely effect your ability to effectively control your vitamin D level. So find a good brand and stick with it.