science Watch / Saul Scheinbach
The Disaster That Created Farming
“Extinction events can be huge disasters for most organisms, but it can actually be positive for others.” – T. Schultz
About 250 species of ants living in the Americas cultivate various species of fungi that they harvest for food. They began this practice tens of millions of years before humans even existed and became the first farmers. Four related ant groups cultivate four types of fungi. The fourth, most advanced group, contains 52 species of leaf-cutter ants that practice “higher agriculture.” In the same way that humans feed domesticated livestock hay, these ants harvest leaves they use to feed their domesticated fungi, primarily one species, Leucoagarigus gongylophorus. And like human farmers the ants use antibiotics to protect their precious crop (See: 1999, Leaf Cutting Ants - The Original Organic Farmers).
Photo by Adrian Pingstone
L. gongylophorus is completely dependent on its ant caretakers. It still produces a mass of threadlike vegetative hyphae, but the hyphae no longer generate spores for reproduction like the fungus’s free-living relatives. Instead, they produce a profusion of swellings called gongylidia that contain a liquid rich in lipids and carbohydrates that the ants eat and feed to their larvae. Now a study published in the October 4, 2024, issue of Science reveals when these ants first became fungus farmers and explains how it happened.
Compared to the evolutionary history of fungus-farming ants, the lineage of their fungal crops had not been well studied because there just weren’t many samples of free-living and ant-cultivated fungal samples in the hands of scientists. This knowledge gap meant that no one had determined exactly when fungal cultivation by ants began. So, a research team lead by Ted Schultz, National Museum of Natural History, Smithsonian Institute, Wash., D.C., collected samples for genetic sequencing from 475 fungal species, 288 of which are cultivated by ants, and 276 species of ants, 208 of which cultivate fungi. "To really detect patterns and reconstruct how this association has evolved through time, you need lots of samples of ants and their fungal cultivars," Schultz said.
The team focused on “ultraconserved elements” (UCEs) of the genomes. UCEs are DNA sequences that remain unchanged throughout the evolutionary history of a group and can lead investigators back to its earliest ancestors. “In this case we were interested in the regions close to these elements. They showed the most recent differences between species and allow us to trace a fairly accurate evolutionary line,” said coauthor Pepijin Wilhelmus Kooij. The huge database enabled the researchers to develop evolutionary trees for both ants and fungi and yielded a surprising conclusion. The ant and fungal taxonomic groups involved in farming both arose about 66 million years ago.
That is the exact time an asteroid hit the earth, sending massive amounts of dust and debris into the atmosphere, blocking the sun and stopping most photosynthesis. It not only killed off all the dinosaurs, but wiped out half of all plant species, creating huge amounts of decaying plant material that could feed proliferating fungi. “At the end of the Cretaceous, dinosaurs did not do very well, but fungi experienced a heyday,” said Schultz.
Prior to the asteroid strike the ants already collected fungi to feed on and were primed to take advantage of this newly abundant food source. “But the fungi were not an essential part of the ant’s diet. The pressure exerted by the meteor impact may have [eventually] turned this relationship into an obligatory mutualism, in which these fungi come to depend on the ants for food and reproduction, while at the same time the ants depend exclusively on the fungi as a food source,” said team member André Rodriguez.
The team also found that it took nearly another 40 million years for ants to then develop higher agriculture. Until then both ants and fungi lived independently in wet, forested areas. But the partnership between ants and fungi, primarily L. gongylophorus, became even more tightly knit due to a second selective pressure. The earth became cooler and drier, and the leaf-cutter ants adapted to the new habitats, bringing their main staple with them. But now the fungi could only survive in the moist ant nest. “This resulted in the fungi losing contact with their gene pools and triggered domestication, basically total dependence on their ant farmers,” said Schultz.
"The ants domesticated these fungi in the same way that humans domesticated crops," Schultz said. "What's extraordinary is now we can date when the higher ants originally cultivated the higher fungi."
Saul Scheinbach