The priceless primate fossils found in a garbage dump

Barcelona, Spain — Fewer places are less welcoming than a garbage dump on a frigid night. But that’s where paleontologist Josep Robles found himself in December 2019, on the hunt for clues to human evolutionary history.

For much of the previous few months, he’d spent several nights a week at Abocador de Can Mata, the largest landfill in the Catalonia region of Spain. Seven days a week, 24 hours a day, the excavators plunged their metal claws into the earth in a rush to create yet another deep pit to hold trash from Barcelona and its surroundings. Robles was one of three paleontologists on rotation to keep a close eye on the tons of tawny dirt displaced by the diggers.

During the day, the sickly sweet aroma of rot drew squawking flocks of gulls; the dirt, as soft and fine as powdered sugar, rose in small clouds with every step. At night Robles dressed in heavy layers, a head lamp strapped to his hard hat. Whenever he spotted a mass that had potential, he waved to the excavator operator to pause while he investigated the object more closely. If it looked promising, he covered it with light-catching silver foil for excavation once the sun came up. Then he’d back away, give the all-clear, and the mechanical roar of the excavator would begin once again.

The soil of Can Mata holds a wide array of fossils spanning more than a million and a half years of the middle Miocene, from about 12.5 million to 11 million years ago. Since 2002, Robles and other paleontologists from the Miquel Crusafont Catalán Institute of Paleontology (ICP), located at the Autonomous University of Barcelona, have found more than 70,000 fossils from this period, when the region’s neotropical climate was becoming more arid. Reconstructing its environmental evolution may offer some insights for regional climate change today.

Among the most important finds are fossils of primate species found nowhere else. These are ancestral hominoids—the precursors to lesser apes, such as gibbons and siamangs, and to great apes, including orangutans, gorillas, chimpanzees, and us. These rare bones are helping to fill in the picture of a period deep in our past that in many ways remains mysterious.

But while paleontologists find promise in the landfill, locals have had it with Can Mata—its ripe stench, its endless procession of exhaust-spewing garbage trucks, its ongoing expansion. Nearby municipalities have formed monitoring commissions and filed complaints. In fall 2019, even as Robles and his colleagues monitored the excavators, hundreds of protestors gathered at the dump entrance holding signs written in Catalán. Prou pudors. Tanquem l’abocador. Volem respirar en pau. “Enough stink. Close the landfill. We want to breathe in peace.”

The landfill expansion has been on hiatus since December 2020, but the waste management company intends to resume digging in spring 2021. Whether the dump will eventually close is an open question. If it did, the paleontologists would have to stop coming, too. They’re only allowed access to the site when the landfill expands.

“If they stopped excavating, we would have to stop surveilling the digging activity,” says ICP director David Alba. “Although we would probably continue prospecting the area from time to time, we would never recover as many fossils as we do when they are constructing the dump. I understand no one likes having a dump nearby, but from the viewpoint of paleontological heritage, the dump is making a huge contribution to science.”

From garbage dump to goldmine

Located about 30 miles north of Barcelona, Can Mata landed on the fossil map in the early 1940s when Miquel Crusafont, namesake of the ICP, discovered the mandible and teeth of a Miocene great ape at the site. Subsequent finds helped establish it as a documented—and, later, protected—paleontological site. Despite its status, it has also operated legally as a landfill since the mid-1980s. (Locals began using it as an informal dump in the 1970s.)

In the early 2000s, when site operator Cespa Waste Management wanted to dig new trash-holding cells at least 150 feet deep, the company was obliged under the Spanish Historical Heritage Law to make sure its machines weren’t crunching through fossils or burying them beneath mounds of trash. They hired fossil-spotting ICP paleontologists to supervise the excavations, and the scientists jumped at the chance to access its depths.

In 2002, ICP paleontologists Isaac Casanovas-Vilar, Jordi Galindo, and Alba—who was then a Ph.D. student—began monitoring the dig at Can Mata. Three weeks into the work, they unearthed the tooth of a deinothere, an enormous elephant relative with downward-curving tusks. Investigating the spot more closely, they also found a fragment of a finger bone.

“I was like, man, this looks like a primate,” recalls Alba.

He ran to his car and retrieved a cast of the hand of the extinct ape Hispanopithecus, which was discovered in a nearby valley. They compared the two, but were still unsure what they’d found. Next they found three fragments of a canine tooth, which Alba glued together, and a cache of tiny, fragile bone fragments scattered near a block of sediment. Camera in hand, Alba lay on his stomach to get a better look at the underside of the block.

He was shocked to realize he was eye to eye with an ancient face.

“The three of us, very nervous—we were barely speaking—turned it up,” he says. “And there was the face of Pierolapithecus looking up at us. It was one of the biggest moments in my life.”

Pierolapithecus catalaunicus, nicknamed Pau, is what they’d eventually call the new species of great ape they’d discovered. About 12 million years old, it’s one of the most complete Miocene primate skeletons ever found. The fossil transformed Can Mata from a garbage dump into a goldmine.

Giant pandas and flying squirrels

In the 13 years that ICP paleontologists have actively worked at the site, they've unearthed more than 75 mammal species, including horses, rhinos, deer, proboscideans (elephant relatives), an ancestor of the giant panda, and the world's oldest flying squirrel. There's also a wealth of rodents, birds, amphibians, and reptiles. So far, they’ve documented more than 70,000 fossils in 260 discovery zones.

One recent find is a Chalicotherium, a tall, knuckle-walking, clawed ungulate that looks like a bizarre mix of giant sloth, bear, horse, and gorilla. Another is a false sabre-tooth cat, so called because it’s not a true felid, the family of animals that includes lions and tigers; it belonged to a family of carnivores that diverged from the ancestors of felids perhaps 40 million years ago. It had died young, its adult teeth just beginning to push its baby teeth out.

The fossils date to an important transitional period from the middle to the late Miocene, when the region’s subtropical rainforests were becoming more arid and grasslands began to dominate for the first time. The scientists are using these finds to reconstruct Can Mata’s environmental changes over about a million years, in time slices of 100,000 years. That resolution is possible because Can Mata has a long, continuous geological record. “It has several hundreds of meters of sediments, all with fossils,” says Casanovas-Vilar.

One method of analysis is simply counting the changing abundance of animals over time. Another is noting the carbon and oxygen isotopes found in the fossils. Carbon indicates what the animals ate, including the carnivores, which retain the carbon of their prey’s diet. Oxygen shows what they drank or, for the tiniest animals, what was in the water they absorbed from eating plants. Both also help to reconstruct the ancient temperatures and precipitation levels at Can Mata.

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While this work is just beginning, the researchers hope to illuminate the local impact of global climate change—both in the past and the present.

“At this resolution, we can tell that climatic perturbations start before the ecosystem starts to change or to crash,” Casanovas-Vilar says. “Ecosystems can tolerate changes for a time, but for how long? That's one question we’ll try to answer for this particular environment and time … that could be related to extant climate change, species conservation, and conservation policies.”

Seeking our origins

Arguably the most tantalizing revelations have come from the site’s parade of fossil primates. Each new fossil clue helps us unravel some of the most profound mysteries of our species: What are we? Where did we come from? And when did we begin to be?

“Understanding how great apes originated and evolved is important to understanding how hominins evolved,” Alba says. Hominins are a taxa that arose after humans split from chimpanzees six to seven million years ago. “[Apes] did not appear out of nowhere. So we need to know where they evolved from.”

In the middle Miocene, there were dozens of hominoids, the precursors to apes. They lived mostly in Africa, but by 12.5 million years ago, they’d also appeared in Asia and to a lesser extent in Europe.

That’s why the team at Can Mata was thrilled to find a new hominoid in 2004, which they called Anoiapithecus brevirostris, dating to roughly 12 million years ago.

While most primate faces protrude, the face of the male fossil, nicknamed Lluc (“one who illuminates”), was intriguingly flat—so much so that it seemed most similar to faces in our own genus, Homo. The researchers proposed that this was the result of convergent evolution, in which similar characteristics evolve in unrelated or distantly related organisms.

Then in 2011 came the fossil of a female Pliobates cataloniae, nicknamed Laia. This newfound species lived about 11.6 million years ago—about half a million years later than Pau. Although she was tiny, weighing about the same as a house cat, she surprised paleontologists with her handful of great-ape features.

"Can Mata has enabled us to show that primates were much more diverse in that timespan than considered before," says Alba. That diversity has been reinforced by recent discoveries of hominoids elsewhere in Europe, including the 11.6-million-year-old Danuvius guggenmosi unearthed in Germany in 2015, and the pelvis of a 10-million-year-old Rudapithecus hungaricus (a species discovered in Hungary in 1967).

“Can Mata is one of the key areas in Europe,” says University of Tübingen paleontologist Madelaine Böhme, who led the team that discovered the fossils in Germany. “It was the single key area before the Danuvius was discovered.”

Last common ancestors

Intriguingly, some of the primate fossils at Can Mata show very early evidence of a characteristic that is unique among apes: an orthograde—or upright—body plan, in which an animal holds its torso vertically.

“This is not to be confused with biped,” Alba notes. “Some people use upright to mean biped. This is plain wrong.” Instead, the orthograde body plan enables vertical climbing, suspending from branches, swinging from tree to tree, and sometimes walking on two feet. While some of these behaviors evolved several times independently, the orthograde body plan might have evolved only once, or perhaps twice.

That’s what makes Pierolapithecus, the face that stared up from the dirt in 2002, so important, Alba says. “It’s the first unambiguous fossil showing an orthograde body plan,” he says. “It’s the oldest indication that orthography had already evolved by 12 million years ago.”

Because this body plan is unique to apes, and Pierolapithecus is the oldest example of it, the last common ancestor of all hominins might have been orthograde, Alba says. If so, that may offer a hint at what gave certain primates an evolutionary edge. However, the “last common ancestor” is an elusive figure in paleoanthropology. Many have been theorized, including the Danuvius fossil from Germany and a 13-million-year-old primate baby found in Kenya in 2014.

“Ancestors are always hypothetical, in the sense that it's very difficult to be able to show that a particular fossil species was actually ancestral using phylogenetic reconstruction methods,” Alba notes. “What matters here is not which one is the first member of this group, but which one is closer to the last common ancestor before [great apes] split.”

The big question now is whether continued digging will be possible in the decades to come, considering the ongoing protests against expanding the landfill. 

Either way, the scientists have plenty of work ahead on the fossils they have already unearthed. Only 20 percent of their finds have been prepared, cleaned of hardened sediment, and chemically preserved. Thanks to additional staff, the pace of preparation has accelerated in the past year, so perhaps new discoveries are on the near horizon. Thousands more are cocooned in brown paper and plastic wrap in cool underground storage rooms. Numbered and labeled, each bundle waits for a curious researcher to unwrap it. Some have been waiting nearly 20 years.

“This is something for the next three or four generations of paleontologists,” Alba says. “I'm sure there are interesting fossils hiding in here.”