Ecologists have suspected for quite a while that the landscape, and especially the vegetation, remembers the way things used to be, when the western hemisphere had an intact mega-fauna, and Homo sapiens had not yet arrived to re-arrange and delete natural communities. The evidence is that fruits such as (wild) avocado, papaya, and Osage orange all evolved to be eaten (and dispersed) by such vanished giants as gomphotheres (an extinct elephant lineage), ground sloths, and glyptodonts (see Connie Barlow’s book The ghosts of evolution for an entertaining introduction to this subject).
Just in time for Thanksgiving, a paper has come out in the Proceedings of the National Academies of Science (PNAS) discussing the relationship of mastodons (more elephants!) and such favorites as pumpkin and squash. Science Daily has the story (I obtained the pre-pub version of the original article through investigator Logan Kistler’s ResearchGate page).
The cucurbit fruits which are the ancestors of our cultivated varieties are bitter (owing to defensive compounds) and indeed toxic to animals below a certain size. Humans count as large mammals, but not large enough to eat those fruits. Mastodons and their ilk, however, can tolerate them, and we know that they did because seeds have been found in mastodon dung.
Humans must’ve noticed that the big animals ate these things, and investigated their possibilities as food at some point, beginning the long labor of selection for digestibility and other horticultural virtues — but meanwhile, many of the wild ancestors of the pumpkins etc. have gone extinct, perhaps because they were no longer being dispersed by their big symbionts (who would have fostered their continued success by abrading the seeds during all that chewing, and further improving germination during their passage through the mastodonic digestive tracts).
Without elephant-sized animals to distribute seeds, wild plants will grow only where the fruit drops — as far as the pumpkin rolls. At the same time, the disappearance of megafauna altered the landscape from one of a patchwork of environments to something more uniform. Cucurbita [sic] are weedy plants that liked the disturbed landscape created by the megafauna, but faired less well in the new landscape of the Holocene.
Makes you think. Ecology is such a cool science. Happy Thanksgiving!