Remnants of Ancient Life in Sumatran Caves: Words from the Scientist

In West Sumatra, Indonesia, the Padang Highlands is surrounded by kilometres of thick rainforest with a rich and diverse population of animal species. Among the dense foliage is an abundance of natural caves. During the Pleistocene epoch, these caves and their surroundings were teeming with biodiversity, resulting in a wealth of material for palaeontological research. The abundance of fossils in the caves throughout West Sumatra drives scientists, such as Palaeontology PhD candidate Holly Smith, to venture into the Padang Highland's rugged terrain. After completing her Masters at Durham University, UK, Holly made the cross-globe move to Brisbane, Australia, where she is currently a member of the Australian Research Centre for Human Evolution at Griffith University. We had the opportunity to interview Holly and delve deep into the nature of her work.

Padang, Sumatra. Credit: Holly Smith, Griffith University

With stacks of equipment in tow, Holly and her colleagues begin their adventure by flying to Jakarta to meet their Indonesian collaborators. Next is another hop to Padang city, where Holly takes in the striking architecture of the traditional curved-roof buildings. Then the team travel to the Padang Highlands, where they will spend five weeks studying the paeleohistory of three Sumatran caves; Lida Ajer, Ngalau Gupin, and Ngalau Sampit.

To the sound of morning prayer playing from loudspeakers, Holly awakes at 4:30 am and begins her fieldwork preparation. In full trekking gear, she and her team have breakfast whilst discussing the day's activities. With the plan set and ready for action, the team drive up to two hours to meet with the local workers, distribute equipment, and begin their trek to the elected cave entrance. En route to the cave site, the team sees terraced rice paddies and endless rows of palm oil and tea plantations. The caves are hidden within the vast rainforest that slopes down into the valley basin before steeply climbing into further hillsides. The cave's mud-strewn floor has accumulated foliage and tangled vines settled lazily over collections of limestone rock. Inside the cave, large limestone Speleothem columns hang from the cave's jagged ceiling. Embedded in the walls are dense calcareous clastic rocks called breccia deposits. And it is these that Holly and her team are here for because these deposits may contain Pleistocene remnants such as mammalian teeth. Holly describes her surroundings:

“When in the mountains, I experienced dense, tall rainforests, and huge volumes of thunderous rainfall, that I had only seen in movies.”

Ngalau Sampit, Sumatra. Credit: A. Prof Julien Louys, Griffith University

"In Lida Ajer, there are three main chambers, the first of which is barred by a steel cage door that originally was locked to deter edible bird's nest thieves. These caves often contain bird nests created by edible-nest swiftlets using solidified saliva, which are harvested for human consumption. Scarring on the cave walls indicate prior nest sites. Once through the steel cage door, there is little natural light, and so reliance is on head torches. The furthest chamber from the entrance contains fossiliferous breccia deposits and leads to a deep, caramel-thick muddy sinkhole with a pungent damp smell. Ngalau Gupin is comprised of one large chamber, and there are countless bats on the cave ceilings. The squeaking is pandemonium, and below the ground is covered in around four feet of guano droppings that smell of ammonia and sting your senses. One or two rats scurried between the stalactites along with all manner of insects. Fossiliferous clay-rich sediment and soil are spread across the cave floor below the breccia deposits. We would trowel these sediments into large white bags and carry them down to the river, where we would filter them through a make-shift sieve and then lie by the river sifting through the specimens for cataloguing. Ngalau Sampit entrance was a series of very long, narrow tunnels that you had to squirm through whilst either pushing the gear ahead or pulling it behind tied to your ankles. The tunnels open into two large chambers, one that is below water and the other that stands a couple of metres above water level. There are two short passageways containing breccia deposits.

Outside you can almost feel, rather than hear, the heavy rainfall hitting the mountainside and filtering down into the cave system. There is no natural light at all, and so when head torches are turned off its pitch black. Local volunteers would hold up lamps and help us carry equipment in deeper areas where we were doing research, and they would chat happily and smoke strong cigarettes while we worked at the entrance."

"It is dark, muddy, and sweaty at the sites, so you must be willing to get your hands dirty."

Upon entering their elected cave, Holly uses her GPS to pinpoint her position and navigate whilst taking photographs and notes. A detailed true-to-scale map is created with her team, highlighting key locations and sites where deposits and fossils where excavated.

Ngalau Gupin, Sumatra. Credit: Dr Gilbert Price, University of Queensland

"My colleagues and I were in full field gear; Gortex walking boots, weather/water-proof trousers, long-sleeved shirts, cave helmet, head torch, and gloves. I would use a hammer and chisel to take large block samples from each remnant breccia deposits in the cave. Unconsolidated sediment samples are coated in plaster of paris during excavation to retain integrity or transported to the nearby river to be sieved for fossil remains. Isolated fossil samples were chiselled from some deposits. All samples are bagged, labelled and boxed for transport. If the research were completed with daylight to spare, we would comb the nearby hillside to search for any further cave sites with potential fossiliferous deposits for analysis. Should the daylight be waning, we would transport all the equipment and samples back to the car and transport it back to the hotel."

After a trying day in the humid Padang Highland caves, Holly and her team return to town and unload their equipment into their designated rooms before washing in traditional mandi baths consisting of a large water tank and scoop. The night continues with dinner at a local food market and completion of the day's field notes. By 11 pm, and with their equipment prepared and supplies replenished, the team retires for the night, awaiting the next day's work.

Holly's research focuses on a branch of Palaeontology called taphonomy, studying how organisms decay and become fossilised. In South-East Asia, the palaeohistory is extensive; however, as Holly explains: determining the mechanisms of site formation, faunal accumulation, and deposit history is challenging. Many of these challenges are due to complexities in dating and preservation of samples. Working with breccia deposits is considered difficult and are often overlooked due to their homogenous nature. Typically, researchers excavate fossils by destroying the breccia, however within the breccia is important paleontologic information. Instead, Holly established a method to use the calcareous breccia deposits that accumulate in limestone caves to ascertain the dominant factors responsible for the accumulation and preservation of the vertebrate remains trapped within the rock by leaving the breccia intact. For her method, Holly uses Neutron Tomography to scan her samples from the Padang caves and conduct an extensive geological survey. This technique uses scans to create planer images that researchers can use to generate 3D reconstructions of their samples.

“I keep the breccia whole and can see the internal geometry of the sample using neutron scanning as one would see bones in an x-ray without breaking the skin.”

With Neutron Tomography, Holly can extract important geological and palaeontological information that she can use to interpret the history preserved in her breccia samples. Holly can then directly associate the measurements with the taphonomic history of the fossil remains incorporated within the rock.

“This is the first time that a geological survey has been completed (multiple breccia scanned) using neutron scanning for palaeontological purposes.”

Lida Ajer, Sumatra. Credit: Dr Gerrell Drawhorn, California State University

In addition to the remnants of the past, the Padang Highlands are home to an abundance of animal species. It is common for visitors to come across one of its many residents during their visit:

"In the caves, there were numerous atypical insects. I remember freezing over in fear after noticing a huge Scutigera centipede on my shoulder. Luckily, I found out they are harmless, but it looked terrifying! Swifts nested on the cave walls and flitted in and out of the cave entrance. There were diverse species of bats from smaller cave-dwellers with wingspans of thirty centimetres to fruit-eaters hanging from the trees with wingspans of over five metres! By the rice paddies, children herded huge water buffalo and massive monitor lizards sunbathed by the water. While walking in the rainforest, we often saw vibrant birds, frogs and butterflies. The busier roads skirting the rainforests' peripheries were often filled with confident macaques looking to beg or steal food. The most amazing moment was during a survey for new cave sites with colleagues, we heard the tree branches cracking and looked up to see long, dark silhouettes leaping over our heads, and as our eyes adjusted, we realised it was a large group of gibbons. It felt special to share a space with them, even for a few minutes."

For British born Holly, working in the Padang Highlands and travelling through Indonesia gave her a trove of novel experiences and stories. Whether it be when she was introduced to the infamous pungent Durian fruit, or her newly discovered favourite fruit: Mangosteen, or perhaps when her team discovered a WW2 bomb in Lida Ajer cave, Holly embraces her experiences.

Holly explains that the logistics of carrying such a project was complicated due to the large quantity of equipment and the nature of their samples. At airports, it was common for the team to be stopped at security checkpoints and X-ray machines.

"Whomever carried the gamma-ray spectrometer was guaranteed to be taken to one side by security. We had zip-tied this box at another airport, and Indonesian customs asked us to open it and handed my colleague a Stanley blade. He cut his thumb open in the attempt and was bleeding. The first aid kit was in the suitcases as we expected no injuries in the airport of all places, so we had to wrap it up in a clean sock at the time. While all this was going on, people with a large camera turned up, and a presenter stood in front of us, talking into the lens. It turns out we were prime material for the Indonesia Border Security Tv show.

Due to its abundance of breccia deposits, Holly describes Indonesia as a prime destination for taphonomic researchers seeking to extract untapped palaeontological data. Through her experiences, she has undoubtedly contributed to her personal growth and helped build her global network. "Cave studies are certainly not suited to anyone who can experience claustrophobia. It is dark, muddy, and sweaty at the sites, so you must be willing to get your hands dirty! The walks are intense, and it certainly pushed my limits physically."