ant farmers



25 SEP 1998. In the beginning there was agriculture. About 10,000 years ago, people in the Far East and the Middle East stuck the seeds of some tasty grasses in the soil. And faster than you can say "John Deere," they'd domesticated those grasses and become farmers.

A worker ant carries a piece of fungus. © 1998, Palle Villesen. it's a scale thingCenturies of picking the largest seeds from the healthiest plants produced strains of wheat and other cereals that grew abundantly enough to support cities. Soon enough, we had civilization, writing and other good stuff like the invention of empires and cannons.

Not bad for a bunch of people who never read a Dick-and-Jane primer. And while it was no small triumph to invent a food supply that doesn't fight back or run away, your huge primate brain should not swell at our ancestors' achievement.

Fourth farmers of the Americas
Humans, it turns out, were actually the fourth animal to discover farming. Fifty million years ago, not long after the dinosaurs took that last long sleep, lowly ants began farming, growing fungus inside their nests and harvesting it for dinner. It turns out that termites and bark beetles (no heavy thinkers here!) also grow fungi.

And while this "dinner" was not exactly haut cuisine, these ants of the so-called "attine" group were apparently the first animals to deliberately grow their food.

The attines include the leaf-cutter ants, which began eating fungus 5 to 15 million years ago -- before humans diverged from chimpanzees. Leaf-cutter colonies have millions of members that harvest green leaves and grow fungus in football-sized gardens.

Leucocoprinus subclypeolaria, a mushroom in Panama's rain forest, is a close relative of fungus grown by attine ants. © 1996, Ulrich G. Mueller, University of Maryland.

Garden of the fungus-growing ant Cyphomyrmex rimosus, from Florida. This fungus (the small yellow nodules) grows as a yeast on caterpillar droppings (the greenish balls). © 1996, Ulrich G. Mueller, University of Maryland.


Related
Why Files

Agricultural genetic engineering.

eat meAttines also include more primitive ants which do not cut live vegetation, but rather use fungus to degrade dead stuff -- leaves, flowers, and other debris. These ants collect vegetation, return it to their nest, prepare a "garden" that looks rather like a sponge, and add bits of fungus in the group Leucocoprini. Within a few weeks, they have fresh mushrooms.

Not. Actually mushrooms -- the fruiting body of a fungus --never appear in the nest. Instead, ants eat mycelium, fungal tissues made of long tubes of interconnected fungal cells.

And while the ants do drink plant nectar and leaf juice, they don't go out and hunt for small insects, as their ancestors did. And their young eat fungus for breakfast, lunch and dinner.

Ulrich Mueller, an assistant professor of evolutionary biology at the University of Maryland at College Park, has been wondering about our fellow farmers. Specifically, he wants to know how and when they adopted agriculture.

Where did it begin?
And although ants don't keep farmers' almanacs, they do leave a genetic trail of their activities in the cultivated varieties -- cultivars -- of fungus they raise. this looks appetizingThink of it this way: After an ant domesticates a fungus, that fungus changes genetically, becoming different from the wild variety. From the number and variety of genetic differences, a family tree can be constructed.

Using diesel-duty genetic analysis (restriction fragment length polymorphisms and genetic sequencing, if you must know), Mueller tracked 553 cultivars of fungus back to five ancestors.

Writing in the Sept. 25 (1998) issue of Science, Mueller and colleagues Stephen Rehner and Ted Schultz described information on the genetic lineages of fungus collected from ant nests in Panama, Brazil, Trinidad, Costa Rica, Guyana and the United States. Here's what they found:

Distantly related species of ants may cultivate the same or closely related species of fungus.

While each ant nest cultivates only one strain of fungus, one ant species may cultivate many strains in different nests.

One ant-cultivated mushroom was identical to a field-collected strain, indicating that it had been domesticated very recently.

Ants "tamed" wild fungus at least six times -- judging by the dissimilar genetics of the cultivars found.

While Mueller says he's got no evidence exactly how the ants started growing fungus (a remarkable feat for insects just 2 to 4 millimeters long, with fewer than half-a-million brain cells), he does say that this overturns the conventional wisdom -- that domestication occurred only once.

Meanwhile, down on the farm
This new information about these non-human farmers has an eerie resonance with human agriculture. For example, ants seem to distribute crops among themselves just as humans do. And as physiologist Jared Diamond noted in a comment in Science, the ants' reliance on a monoculture echoes a few desert cultures, which eat only a single crop. (Although they eat animals, not fungus, you get the picture.)

Finally, just as ants gather new crop strains, perhaps after diseases killed their existing fungus, humans get new genes from wild crop relatives to help our crops survive new diseases and changing conditions.


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-- David Tenenbaum The Why Files

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