You will never look at a mango or avocado the same way again, once you've read "The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms" by Connie Barlow.
Barlow experienced a "eureka!" moment when she realized that many North and South American plants must have evolved in tandem with vanished mega-fauna from the Ice Ages and before. Just as wild blackberry vines depend on birds to eat the fruit, and excrete the seeds to plant new vines far from the parent, so plants like Osage oranges, avocados, and honey locusts must once have had non-human seed dispersal partners. But what kind of animal could possibly gulp down whole avocados, and pass the pits undamaged?
Before the arrival of human hunters armed with Clovis-tipped spears and atl-atls, the Americas heaved with enormous creatures. Some, like camels and horses, migrated west from their ancestral continent, and waited out the onslaught in Asia. Others, such as bison and sloths, evolved new behaviors and took to new habitats to elude the hunters. But many, including mammoths, mastodons, glyptodonts, and gomphotheres, went extinct. This history of Ice Age mammalia has been known for decades; what hasn't been widely considered before, though, is the fate of the plants that coevolved with them.
That's where Connie Barlow's insight comes in. Brave woman that she is, Ms. Barlow actually samples many of the fruits that we today consider to be inedible. She makes very convincing arguments to explain which prehistoric creatures could have browsed on the lofty fruit of saguaro cactus, gulped down grapefruit like we do strawberries, and milled the flesh from around that huge, fibrous pit at the center of a mango. She also explains how the plants could have hung on, once their ecological partners have vanished. It's a poignant story; since plants have longer generations, and tend to evolve more slowly than animals do, these species are still producing the same fruit they did 20,000 years ago. They flower, fruit, add some spikes onto their branches, and then wait for that gomphothere to lumber up for a snack. But the gomphotheres are ghosts; they're never coming back.
I highly recommend "The Ghosts of Evolution." After reading it, you may well find yourself walking around your neighborhood, or browsing through the produce section of the grocery store saying, "Oh, of course! Wow... Only a mastodon could swallow this whole... Amazing."