Humans have been practicing agriculture for about 10,000 years. But the attine ants of South America (which include the well-known leafcutters) have us beat by a long way.
According to a new paper co-authored by entomologist Ted Schultz, curator of ants at Smithsonian's National Museum of Natural History, attine ants, which farm on an industrial scale similar to humans, have been carefully cultivating gardens with a complex division of labor to grow an edible fungus. Schultz's team found that the ants have been doing this far longer than previously believed—up to 65 million years—and that we have much to learn from them.
Schultz and his co-authors, led by by Sanne Nygaard, Guojie Zhang and Jacobus Boomsma of the University of Copenhagen, conducted an analysis of the genomes of the various species of attine ants as well as the fungus that they cultivate. Their results answer some long-standing evolutionary questions.
The 210 species of attine ants, including the 47 species of leafcutters, forage through the forests of Central and South America in search of leaves and other vegetation, which they carve into pieces using their powerful jaws and carry back to their nests. But they never eat the leaves directly. The plant matter is used as a growth medium for certain varieties of edible fungi which Schultz's team says have been cultivated and passed on by generations of ants going back tens of millions of years.
When a group of hunter-gather ants in South America eons ago switched to eating fungi, ant farming began to evolve.
Schultz says that the ancestral switch from a hunter-gatherer lifestyle to agriculture was probably irreversible. Many genes disappeared from the ants as they made the transition.
“It looks like possibly shortly after the ancestor of these ants started cultivating fungi, the ant lost the ability to make one of the amino acids, arginine. So the idea is that it depends on the fungus for that amino acid. . . .They probably can't go back.”
While arginine is found in meat and in many plants, the need to find sufficient wild sources of the amino acid could make it difficult for the ants to give up their dependence on the fungi that uses the ants just as much as the ants use the fungi.
“When I first started doing this, the way we thought of it was ant farmers are like little humans,” Schultz says. “And we're pretty sure that we're in charge of our crops. When I started working with mycologists, they had a different take on it. The fungal colony is sending out the ants to bring back food. I tend to think of it as nobody's in control. It's a symbiosis and selection is acting on the combination.”
Attine ants began their symbiotic relationship with their fungi starting somewhere between 45 million years ago and 66 million years ago. Zeroing in more closely on the timing may teach us a lot about why they changed their way of life. At 66 million years ago, the dawn of fungus-farming would coincide with the period right around the asteroid impact that most scientists believe wiped out most of the dinosaurs and a great deal of the other life on Earth.
“That would be a time when a lot of things you would have relied on in the plant world would have crashed,” Schultz says. “It would probably be a big boom for things like fungi and things that fed on decaying material. Then it could have been a boom time for things that fed on those organisms.” Many plants produce chemical defenses that prevent insects from eating them. But the leafcutters and other attines managed to dodge around that problem by feeding poisonous leaves to edible fungi. In a post-impact world that probably resembled our concept of nuclear winter, it was probably useful to not be picky about what plants you harvest.
But why have these ants thrived, diversified and continued to grow fungus during tens of millions of years that aren't a post-apocalyptic nightmare? On the surface, they seem to have a risky, over-specialized survival strategy. Completely dependent on a single food source that is subject to disease and climate change.
“It appears that ants expend as much energy being agricuturalists as being hunter-gatherers,” Schultz says. “There have been studies showing that primitive agricultural humans were less well nourished than hunter gatherers. So what would the advantage be, in humans or in ants? Stability? A lot of being an ant is about buffering yourself from the environment. …if you're feeding your young based on what the scavengers bring in and the prey supply fluctuates, that can be bad. With the [leafcutters], at any given time the colony tends to have more food than the colony can eat. A few weeks' supply, at least. ”
Attine ants will starve to death if their fungus dies out and they are unable to find a sample of a similar strain to cultivate. Some of the strains of fungi used by certain species of ant are no longer found anywhere in the wild (all of the fungi cultivated by ants are descended from a single ancestor). They may produce mushrooms occasionally, but the spores do not appear to be successful. Both fungus and ant are completely dependent on one another for survival.
Humans may have important lessons to learn from the attine ants. We have struggled to protect the survival of our crops for only about 10,000 years. “We're constantly coming up with herbicides or antibiotics to control pests. And the pests are constantly evolving countermeasures against those things,” Schultz says.
The most economically important variety of banana became functionally extinct in the 1960's and another variety is heading in the same direction. “Somehow this system with the ants has been in an equilibrium for millions of years,” he adds.
Shultz notes one important difference between the evolution of human versus attine agriculture.
“At least technically, humans are not biologically dependent on any given crop. If we wanted to become hunter gatherers again, biologically, we could.”