The biographer of Robert Moses, Robert A. Caro, refers to the bridges and underpasses of the famed New York State parkways. These bridges and underpasses are quite low, intentionally specified by Moses to allow only private cars to pass. All those who traveled by bus because they were poor or black or both were barred from the use and enjoyment of the parkland and its "public amenities" by the technical design of the bridges. Even at the time of Robert Moses, a political statement of the form "We don't want them blacks in our parks" would have been unacceptable in New York State. But a technological expression of the same prejudice appeared to be all right. Of course, to the public the intent of the design became evident only after it was executed, and then the bridges were there.
We might be closer than we think to cures for depression, hypertension, and yes, even obesity.
The answer to scurvy was just one thing, plus a few wrinkles — mostly “not all citrus has the antiscorbutic property” and “most animals can’t get scurvy”. This was only difficult because people weren’t prepared to deal with basic wrinkles, but we can do better by learning from their mistakes.
This means don’t give up easily. It suggests that there is lots of low-hanging fruit, because even simple explanations are easily missed.
Lots of theories have been tried, and lots of them have been given up because of something that looks like contradictory evidence. But the evidence might not actually be a contradiction — the real explanation might just be slightly more complicated than people realized. Go back and revisit scientific near-misses, maybe there’s a wrinkle they didn’t know how to iron out.