Africa is way below the world average. However it's quite worrisome in middle parts of the continent. I think it's mostly due to their poor living standards and corrupt governments.
Here's nation wise life expectancy in Africa.
Yep, I think that the food supply is the most important component in these countries. Which unfortunately often goes hand in hand with corrupted govts.
Corrupted health care system inaccessible to millions is just another hurdle. As an example, I posted a chart of prevalence of obesity and death rates from obesity in some countries some days ago, and while the likelihood of being obese is very low in Africa, it is way more likely to die from it once someone is obese.
And there are regional conflicts and wars that I believe also contributed to the result.
Here's nation wise life expectancy in Africa.