There are three sources of umami known.
The ones that were first discovered historically were glutamates (present in konbu, parmesan, sun-dried tomatoes, tomato paste.) Others found in short order were various isosinates (present in broths made of dried bonito flakes,) and guanylates (found in high concentrations amongst shiitake mushrooms.)
The key idea here seems to be a sort of synergy. A small amount of any of them enhances the small quantities of natural umami substances already found in hundreds of foods, and the most important point is that when glutamates and isosinates are combined, they have a powerful multiplier effect, which enhances the umami-ness of the underlying food.
This pretty much explains a huge number of historical phenomena, from all over the world.
Consider the Japanese dashi. It's made from both konbu, and dried bonito flakes (katsuobushi.) You're getting a potent mix of glutamates, and isosinates. Sometimes, they add shiitake mushrooms to further enhance the mix.
Same goes for the simple pasta sauce recipe of fried pancetta, tomatoes, and dried porcini mushrooms. Talk about a triple whammy!
We can also explain the powerful spread of the tomato. In a few hundred years, it radically transformed ancient cuisines (Italy and India.) The point is that the local recipes were already very attractive before the tomato, but the addition of the tomato unleashed more of the umami qualities that were already present in minute quantities in the underlying substances.
It also explains certain truths that chefs hold dear -- young peas over older ones, ripe tomatoes over fresh ones, aged cheeses over unaged ones, and oysters in winter over summer (the latter has a health component but one that has been irrelevant for at least half a century.) Same with aged hams, and dry-aged beef, and sun-dried tomatoes (even when fresh ones are available.) Your palate is selecting for the more powerfully concentrated glutamates (and drying is just one way of achieving that.)
It should be clear why fermented fish sauces (garum/liquamen in Roman times, nahm pla in Thai food, nuoc mam in Vietnamese food), fish pastes (allec in Roman times, gkapi in Thai food), dried shrimp, and intense meat products have been popular for millenia. Likewise meat extracts (Bovril), and autolyzed yeast extracts.
This is why pizza (tomato paste and parmesan), and hamburgers, french fries, and ketchup practically constitute two religions in and of themselves.
Free glutamates are most commonly found in vegetables. Isosinic acid most commonly in meat, and animal products. This is responsible for the strong intensification when meat and vegetables are cooked together.
This is why certain Italian recipes start with "fry the pancetta, or the prosciutto" even when the ingredients are all vegetarian, and the instructions call for the pancetta to be fished out after the frying. Even a relatively poor family could afford the modest amount of meat required, but the combination of meat and vegetables is a powerful one.
That's why anchovies have been popular through the millenia, and why Thai curries and pastes can never truly be approximated in vegetarian terms. They are too dependent on the grammar of seafood and its umami. There is simply no mechanism to substitute the intense mouthfulness (= umami-ness) of gkapi (shrimp paste.)