by Mechas
Agriculture triggered enormous changes in human society. More than 10,000 years ago, hunter-gatherers adopted rudimentary agricultural practices that sparked the transition towards a more sedentary existence. Human expansion and population growth are tied to our ability to cultivate diverse and reliable food supplies. But we are not the only species that excel at agriculture.
Fig. 1. Chronogram of phylogenomic data for 288 ant-cultivated and 187 non–ant-cultivated fungi. Orders and important families and genera are labeled. A – D: phylogenetically related groups of ants that cultivate four groups of fungi in the order Agaricales. For more details, see source.
Agriculture can be considered a form of mutualistic symbiosis that benefits the species involved. Yet, finding that it has appeared in over 20 animal lineages still surprises me! These lineages include humans, termites, and ants. In the Americas, ant species developed the capacity to farm diverse fungi for their consumption. Of the four different types of farming ants, those that practice "higher agriculture" are considered the most evolutionarily derived system, i.e., one that evolved from an ancestral trait. Among these are the leaf-cutting ants that bring plant material into their nests to grow the fungal garden on which they feed. These ants also adopted strategies to control contamination of their fungal farms; they associate with actinomycete bacteria that produce compounds to limit the growth of harmful fungi.
In a recent study, Ted Schultz from the National Museum of Natural History and colleagues set out to clarify the history of the fungus-ant coevolution. They used DNA sequence data to reconstruct fossil-calibrated chronograms – dated evolutionary trees – of both fungi and ants. Based on the ant chronogram, the ability to farm fungi arose ~66.65 ± 13.28 million years ago (Ma), at the end of the Cretaceous Period. This event coincided with the Cretaceous-Paleogene boundary marked by the global mass extinction of Mesozoic Era species, including all dinosaurs, due to a large meteorite impact. These conditions gave fungi the opportunity to proliferate more effectively, leading to associations with other organisms and the development of mutualisms and early agriculture in fungus-farming ants.
By combining data from the ant and fungi dated phylogenetic trees, the authors calculated the origin of higher ant agriculture at ~27 Ma. This development occurred after severe global cooling, decreased atmospheric carbon dioxide, and extinction events that took place ~34 Ma. As fungus-farming ants adapted to these new conditions, such as dry habitats in South America, they brought with them fungal cultivars that eventually became isolated from their free-living ancestral populations. This probably resulted in the domestication of their fungal cultivars that now depend on their ant farmers for propagation and survival.
The ability to connect major planetary events with changes in the relationship between ants and fungi is an impressive feat. It helps us understand the effect of natural events on biological interactions and how, in the case of ants, they resulted in a successful non-human agricultural system that serves both partners in their constant struggle for survival.
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