The fossil structures bore every hallmark of a prehistoric farm: Crops were arranged according to an intricate, complex plan. Material for harvesting littered the ground. Analysis revealed that the crop was a species that only grows when cultivated.
Instead, the farmers who tilled these ancient plots were termites. And their harvest was fungus.
The fossilized termite gardens, uncovered from exposed cliff sides in the Rukwa Rift Basin of southwestern Tanzania, are the oldest physical evidence of farming on Earth, scientists report in the journal PLOS One this week.
"It captures a record of the evolutionary coupling of termites and fungus ... and allows us to trace back the antiquity of this symbiotic relationship," lead author Eric Roberts, a geologist at James Cook University in Australia, wrote in an email. "The new fossils help us to calibrate our evolutionary clocks and use them to better understand when this symbiosis first developed, which we now think was probably around 31 million years ago."
Across Africa, the massive, edible "termite mushrooms" grown by these tiny insects are famous. But European scientists didn't realize the significance of what was happening inside termite colonies until the mid-20th century. When researchers dissected towering termite mounds, which contain dozens of interlocking chambers and can grow taller than a person, they saw that the insects weren't just eating the fungus that grew alongside them — they were cultivating it.
It turns out that termites are in an "obligate symbiotic relationship" with the fungus, meaning neither can exist without the other. The insects chew up undigestible plant material and form it into tiny pellets, called mylospheres, which they then seed with tiny fungus spores. When the mushrooms are ready, they harvest and eat them. The fungus also breaks down the enzymes in the plant material, allowing the insects to consume what had previously been inedible. According to Nancy Stevens, a paleontologist at Ohio University and a co-author on the study, 90 percent of all dry wood in semi-arid savannah ecosystems gets broken down in this way.
"Today's termites are a formidable force on the landscape," she wrote in an email.
Previous work by molecular biologists had used mitochondrial DNA to trace the evolutionary roots of this relationship back to somewhere between 25 and 30 million years a. But scientists who study the phenomenon didn't have any physical evidence of ancient termite farms to corroborate that date. The oldest fossilized termite mounds were from the late Miocene, less than 10 million years ago.
Then Stevens, Roberts and their colleagues got a National Science Foundation grant to study the evolutionary history of the East African Rift system. The Rukwa basin, criss-crossed by sheer cliffs that expose layers of history, offered up a wealth of information about the region and those who lived there. Fossils from a 25-million-year-old layer showed evidence of a climate that was becoming drier, a landscape that was turning into grassland, and tectonic activity that was ripping apart the land beneath the inhabitants' feet.
That's when they found the two fossil termite farms.
The well-preserved nests bore a strong resemblance to those inhabited by a class of insects known as macrotermitine termites today. Each contained several softball-sized chambers filled with fossilized fungus and the pellets of plant material used to feed it. The gardens were sculpted into the same intricate shapes as those produced by macrotermitine termites, and the condition they were found in suggests that the mounds were the same age as the 25-million-year-old rock around them (as opposed to being a more recent construction).
Farming, Roberts says, was what allowed termites to flourish in a rapidly changing environment. As the ecosystem transitioned from lush rainforest to dry grassland, "the fungus essentially provided termites the ability to utilize woody plant material much more efficiently than they could have on their own," he wrote. After becoming farmers, these termites were much better equipped to succeed in more varied ecosystems, and swiftly spread across the continent.
And they're not the only creatures that have done so. Leafcutter ants in the Americas have been feeding bits of leaves to domesticated fungus for roughly 10 million years. And it's thought that the ambrosia beetle have been dining on fungi they cultivate and carry around with them for five times as long.
"I think this tells us something about the intelligence of social insects," Roberts said. "... Agriculture provided them with an evolutionary advantage and a source of higher quality food."
It only took humans another 24,988,000 years to figure out the same strategy.