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The road down the centre of Yorke Peninsula stretches south in a long unwavering line towards a distant vanishing point. Sweeping plains of golden wheat to the east and west overlay bones of white limestone, interspersed here and there with silver salt pans that shimmer, mirage-like, in summer heat. On either side, the sea and sky encircle a blue horizon. The only trees in this landscape cluster along the road verges, thick and scrubby, rarely more than 20 feet tall. I love this place, with its open vistas—it is the landscape of my childhood—but it is hardly the place I’d expect to find a koala.

I was surprised to read that koalas once lived here. When I first read the name, Phascolarctos yorkensis, I assumed this fossil koala must have been found in Queensland – in the lush rainforests of Cape Yorke at the most northerly extreme of Australia.  In fact, it was found much closer to home, not far from the popular holiday fishing destination where my grandparents lived for many years, just a few hours from Adelaide. I had no idea that there were any caves in the area, so I ask Gavin Prideaux, a local palaeontologist, how I could find them. As it happens he knows the site very well – his wife’s family once owned the farm where the caves are located.

‘It’s on private land,’ Gavin tells me, ‘You can only go with a caving club. There’s one at the university that does regular trips. You won’t be able to go where the fossils are found but it will give you a good feel for what the location is like.’

It seems like a good idea. I’m not planning a full-on caving expedition. I just want to see what the landscape is like – try to imagine what it once would have been when the wide open wheatfield were covered in forests. Perhaps I can tag along on a caving trip, maybe peek inside the cave entrance and have a wander around while the serious cavers do their thing. 

I send some preliminary emails, on the off-chance that there might be a trip running at some stage in the next few months and am swiftly invited to come on a caving expedition the next weekend. A colleague generously provides a crash-course in caving basics over coffee, along with an assortment of helmets, torches, kneepads and first aid kits. I eye them warily. It seems I am getting in deeper than I thought.   

‘You can turn back at any time,’ she assures me. ‘Anything you are not comfortable with, just say so and they’ll take you out.’

‘Always look backwards, so you can know how to get out,’ another caving friend advises me solemnly. ‘Always carry two sets of spare batteries.’ 

I find this advice rather unsettling.

The leader of the expedition is Graham Pilkington, a veteran caver in his 70s. On the drive over he tells me cheerily about all the groups of school children he has taken through the caves recently and how the biggest problem was with the teacher. I’m not sure if that has to do with courage or common sense, but I can see that the prospect of being able to bail out with any semblance of dignity is becoming increasingly unlikely. 

The cave entrance is concealed in the middle of seemingly flat farmland. A cleft in one side of a gently sloping hillock opens into a dark chasm that leads to a small steel door, heavily bolted and locked. We walk down dusty steps into a narrowing corridor, our heads soon bumping on the hard roof. If I was expecting a vast cavern to wander through—a welcoming antechamber before the labyrinth—I am quickly disabused. The cave peters out at an unpromising apex in which only a few dark corners and shadows offer slim hope of access. As we wait for our small party to gather, I notice an opening to the side – hobbit-sized at best, but not too small. But Graham turns the other way, and promptly disappears down a small hole in the ground that I am sure even a wombat would view with scepticism. As his feet vanish, I realise with some horror that I am next in line. 

‘You okay to go next?’ asks Sarah, an experienced caver behind me. Best not to think too much about it, I decide.  I take a deep breath and dive in. 

The tunnels are smaller and longer than I expected. Barely big enough for crawling, in many spots they are only big enough for wriggling, arms outstretched, and boots scrabbling for traction. From time to time I have to turn my head to fit my helmet through. Claustrophobia is not an option. I remind myself, over and over, that I am the smallest person in the group, that hundreds of people have worn these tunnels smooth, and that if you can get in, you can get out. There is really no logical reason why you would get stuck. It becomes a mantra to stop me thinking about other possibilities. 

I tell myself that primates are, underneath it all, still climbers.  Like koalas, we are adapted for clinging, clambering and climbing: opposable ‘thumbs’, friction-ridged fingerprints, long limbs, a short body and rotating shoulder joints. The adaptations for tree-climbing are surprising similar to rock-climbing. There’s not as much difference as you might think between a tree kangaroo and a rock wallaby. I focus on one step at a time, one tunnel at a time. My sedentary desk-bound body is not accustomed to such exercise but there is a satisfying rhythm to the effort which districts me from the endless dark.

Periodically we emerge into narrow rifts in the rock that open into darkness above and below.  I realise the smaller tunnels have been dug out by hand to join up these different caves – which explains why the tunnels are only just big enough to allow human access. We sit in a rare spacious corner, the beams of our head torches creating a crosshatch of light in the darkness, as we catch our breath and wait for the rest of the group.

I hadn’t expected caves to be shaped like this. 

‘These are dry caves,’ explains Sarah, the geologist on the trip.  She shows me a map of the cave structure: an irregular multi-layered houndstooth pattern labelled with idiosyncratic names like ‘The Walrus’ and ‘Beard Squeeze’. ‘The caves were formed by the water table rising and dissolving the softer rocks that had formed between fracture lines in older harder rock, rather than water moving through them.’

‘There are probably other cave systems underground that we don’t know about,’ she adds. ‘Farmers tend to fill in surface holes so that stock can’t fall in.’

I remember playing hide-and-seek in the limestone potholes in the paddocks where I grew up. The same kinds of holes that could easily have opened into a labyrinth like this one, where countless generations of animals have fallen to their deaths, leaving their bones littering the cave floor.  Only a few caves, with openings to the surface, collect fossils like this. 

‘How far is it to Dreamland?’ one of the others asks. It’s where all the fossils have been found.

‘Two hours in and two hours out,’ Graham replies. ‘Pretty tight in places.’

‘Tighter than Wombat Run?’ someone asks.

‘Worse than Bandicoot Bypass,’ comes a gloomy voice from the darkness.

Graham makes a dismissive noise. This is a man who has opened up large sections of this cave system by digging out the ends of tunnels where he thinks they might connect to other systems, carrying out the dirt in his pockets and lunchbox because the tunnels are too small to accommodate any backfill. He’s squeezed himself into crevices so tight you have to breath out in order get in and scrape out enough dirt to give yourself room to take the next breath. Not that long ago, he discovered that he’d broken some ribs in the process.

‘You have to get through the Letterbox first,’ says Graham ‘then through Alberta which is fairly tight and long, so you get tired by the end.’ He points to a section of the map illuminated by his headlamp. It’s labelled ‘The ##*!’ 

‘And then it opens up to the Portal. That’s the slow spot.’

The Portal is a seven-metre vertical climb through a squeeze between the different levels of the caves. It can only be done slowly, one person at a time. It’s the only way to access the fossil caves – The Bench, Koala Patch and The Graveyard. The latter is accessible through a tunnel so tight that one of the last cavers to enter it had to strip off his overalls to squeeze through the narrow space almost naked.  

In the 1980s, Graham and others found bones in the pockets of red sand here: frog, snake, bird and marsupial rat bones, koala teeth and jaws – all perfectly normal and unexceptional. But 1985, three of them finished off a busy weekend exploring with one final visit to a previously unsurveyed cave. In the cemented red sands that had collapsed within the cave, they found another older koala jaw – at least double the size of modern koala jaws. This creature would have dwarfed the biggest male koala around today. This is the animal now known as Phascolarctos yorkensis.

‘That must have been pretty exciting?’ I ask Graham.

He shrugs. 

‘It’s good when you find something new,’ he says, ‘but fossils are a bit a nuisance really. They stop you from digging new tunnels.’

The limestone that underlies Yorke Peninsula periodically breaks through the shallow soils in gleaming white sheets. It comes to the surface in misshapen lumps, under the plough, flung in piles that line the edges of wheatfields like bleached bones in the sun, more rubble than dry stone walls. This labour of past generations reminds me of just how modified this landscape is, how different it must have been before Europeans cleared the trees, dug up the rocks and drained the swamps.

It's only on the southern tip of the peninsula that I appreciate how vast this difference is. A row of brackish swampy lakes isolates the toe at the foot-shaped tip of the peninsula. The remains of old drainage works and gypsum-mining activities mark the efforts of former residents to develop the area. But the harsh, coastal conditions, poor soil and extensive lakes thwarted their efforts and in the 1970s the area was declared a National Park. 

Looking at the swampy lakes, I remember a PhD student who took one of my classes in academic writing. He described the fragile nature of the freshwater aquifers under the land and the incursions of saltwater from the ocean. The ground water beneath these lakes is increasingly salty, at risk from bores that disrupt the delicate tensions holding the water in place, like the surface membrane of a droplet of water balancing on a tabletop.

It had never occurred to me before how much this underground water shaped the landscape and how important it has been. Once, when the water table was higher, it had dissolved the soft fingers of rock that had infiltrated cracks in the ancient Cambrian limestone creating the Corra Lynn cave formation. Earlier still, it had filled the low-lying areas of the peninsula with shallow freshwater lakes that had supported the forests that once covered the southern areas of Australia and the prehistoric koalas that inhabited it.

These long vanished swampy forests keep recurring in the koala story. Across different ages, and different locations, wherever there are koala fossils there were swamps and forests. It is tempting to assume that this purely because swamps are good for making fossils. If koalas were found in drier forests they would not have left fossils for us to find. 

But I can’t help noticing that even today the favourite food trees of the koalas – like manna gums, swamp gums and river red gums – are all what they call ‘riparian’ species. They are trees that grow in damp moist areas and along the banks of rivers. As a result, koalas today are almost entirely distributed along watercourses, floodplains, inland rivers and streams, just as fossils of their predecessors were. Annual rainfall seems to be the single biggest predictor of koala abundance, with more rain linked to better tree condition, more freestanding water and smaller home ranges. Both koalas and eucalypts may be models of drought adaptation, defined by their remarkable ability to cope with little surface water, yet the more I look into their evolutionary history, the more I realise that their origins are in a damper, swampier world. Like most Australian plants and animals, koalas are descended from the ancestral ‘mesic’ or moist forests that once covered much of Australia, not the dry ones we are more familiar with today. 

When we came out of the cave, the world was suddenly full of light, sound and movement. A stiff breeze blasted across the flat landscape, strangely disorienting after the dark, muffled stillness of the caves below.  I emerged with a whole new respect for the words I so often read in palaeontology papers when they briefly mention in passing that ‘cavers found this fossil…’  

‘Did you find the fossils?’ a friend asks me excitedly on my return.

‘No, that wasn’t the plan,’ I explain but she still looks puzzled and crestfallen. ‘I just wanted to get a better understanding about where the koala fossils were from and how everything fitted together.’

‘Reckon you could have done that without spending five hours crawling around in a hole,’ she replies. 

Maybe. But I get a feeling that the answers to the questions I have about koalas are not going to be easy to answer. Some things have to be done the hard way.

The science inspiring the piece:

Understanding our past, including our evolutionary past, requires an exhaustive amount of research. I read a lot of scientific papers (which don’t always reveal the latest, or the most interesting, part of the story) and I also try to talk to scientists about their research and ask them to check what I’ve written about their work as well as talking to other knowledge holders. Doing ‘fieldwork’ or location research can often reveal surprising new insights how all the knowledge fits together – and definitely helps blow out some cobwebs from sitting at a desk!

Read more:

'Australian Mammal Ecology, Evolution & Extinction' via Flinders University

'Keen cavers explore the dark secrets of SA' via ABC News