Australasian Science Magazine

A 25 million-year-old termite nest with the remains of a fungus garden preserved inside. Credit: H. Hilbert-Wolf

By Eric Roberts & Christopher Todd

New trace fossils from the African Rift Valley reveal evidence for the origins of agriculture, not by humans but by insects.

For the average person, the very utterance of the word “termite” calls to mind the tiny critters that eat away the timber supports of your home. However, this negative association ignores the fact that termites are actually among the most biologically diverse and ecologically important groups of insects. In fact, termites are responsible for as much as 90% of the decomposition of dry wood and nutrient recycling in certain ecosystems, and they have recently been recognised for their role in creating biodiversity and bioproductivity hotspots in African savannahs.

We have now stumbled onto a remarkable fossil discovery that highlights the evolutionary importance of these creatures while also providing an interesting parallel with human development. Over the past 15 years, our team of palaeontologists and geologists have been working in Tanzania, in a little studied portion of the East African Rift System in an effort to document the origin and evolution of vertebrates in Africa. This project, the Rukwa Rift Basin Project, initially focused on the discovery of new dinosaur fossils, but a series of unexpected discoveries pushed the project in many other weird and wonderful directions.

Among the highlights of this work was the discovery of previously unknown fossiliferous sedimentary deposits that helped us to better understand the timing and origin of the East African Rift System itself. In one of these units, dated to ~25 million years old, our team discovered a bounty of new fossils, including the oldest known fossils of apes and Old World Monkeys from Africa. Then, last year, our team discovered an even more unusual series of fossils that provided us with compelling evidence for the origin of agriculture – albeit not the origin that most would expect.

Although agriculture is typically considered a uniquely human endeavour, what we found in these 25 million-year-old rocks was not human or even anthropoid, but termite in origin. The fossils in question are what are known as trace fossils, which preserve evidence of the behaviour of organisms rather than the organisms themselves.

It was in a tectonically uplifted cliff of sandstone that we found the remains of several beautifully preserved ancient termite nests. This is not an unexpected discovery because fossil termite nests are relatively common in rocks going back to the Mesozoic age of dinosaurs. What was unusual was the preservation of a series of specialised chambers with bizarre pebbly, spaghetti-like structures that closely resembled the fungus gardens constructed by fungus farming termites (Macro­termitnae).

However, the fossil record of fungus farming termites only extends back to ~7 million years ago. Using a combination of analytical approaches, our team of specialists was able to confirm that the fossils were indeed ancient fungus gardens.

With this knowledge in hand, we worked together with molecular and evolutionary biologists to recalibrate molecular clock estimates used to estimate the timing of symbiosis between termites and fungi. In other words, these fossils provided strong evidence to support long-held hypotheses that the origin of fungus farming and the symbiotic relationship between termites and fungi – first occurred ~31 million years ago in a central African forest.

This date roughly coincides with the initial opening of the East African Rift, an event that is believed to have significantly altered the African landscape. Such a dramatic environmental shift, and the initial transition from forests to savannahs and grasslands, could have been a key factor that led to the evolution of the fungus farming behaviour in termites and indeed, the origin of agriculture.

The History of Agriculture

The development of human agriculture only occurred 10–12,000 years ago in the Fertile Crescent (what is the modern Middle East) during a period known as the Neolithic Revolution. This was the first time that humans deviated from their nomadic lifestyle and began to intensively cultivate the land around them and domesticate livestock. This has been argued by many as one of the most important developments in human history, which ultimately led to rapid cultural advancements that followed this time period. Similarly, the evolution in certain insect groups of agriculture, in the form of symbiotic mutualisms with fungi, allowed these groups to occupy new niches and become among the most successful groups in the insect world.

Only three groups of insects have derived mutualisms with fungi for agriculture. These include the ambrosia beetles, the leaf-cutter ants and the Macrotermitine termites. In the case of Macrotermitine termites, who we think constructed these 25 million-year-old fungus gardens in Tanzania, the symbiotic relationship offered the fungi a protected growth environment; in return, the fungi produced food for the termites. This is both in the form of degraded plant material and/or enzymes that aid in the breakdown of cellulose, and in the form of fungal biomass.

The termites cultivate the fungi in enormous quantities on a substrate of partially masticated woody plant material. To do this, the termites essentially eat plant material and briefly pass this through their gut before passing it back out and forming the partially digested plant material into a series of small spherical wood pellets called mylospheres. The mylospheres are inoculated with fungus spores to construct the fungus gardens.

While the fungi help to break down the woody plant material in the mylospheres into a more digestible form, they also produce vast quantities of fruiting mushroom bodies that the termites also consume. The mylospheres are consumed by the termites after the fungi in the wood quit producing fruiting bodies and die.

Termites, ants and beetles are the only insect groups known to have evolved such symbiotic relationships with fungi, and estimates on the antiquity of this behaviour range from 24–34 million years ago for termites, 45–65 million years ago for ants, and around 50 million years ago for beetles. Prior to our discovery, fossil evidence of fungus farming behaviour in any of these groups has been scant, with only two reports indicating that the behaviour could be traced back only as far as 7–10 million years ago in both termites and ants. Other workers, however, had hypothesised that certain trace fossils may record evidence of this behaviour as early as 30 million years ago. Despite the observation of fungus farming in beetles, researchers have yet to find unequivocal evidence to trace back the antiquity of this behaviour.

So where do we go from here? The field of continental ichnology is still in its infancy as a science, and discoveries like the one described here highlight the opportunities that still exist in this field of study. When it comes to searching for such things as the antiquity of fungus farming or other behaviours, maybe we just need to change our search criteria.