Wandering seeds

Towering bunya trees were a source of nutritious nuts and the focus of Aboriginal festivals.

Millennia before Europeans arrived in Australia, humans helped shape the distribution of the continent’s plants

A version of this story appeared in Science, Vol 381, Issue 6658.Download PDF

On shady, densely wooded riverbanks in eastern Australia, the black bean tree is easily missed. It can reach 12 stories tall, but Castanospermum australe mostly blends in with its rainforest peers. Each November, however, it catches the eye as sprays of large red and orange blossoms adorn its dark, glossy leaves. Six months later, cylindrical pods longer than a banana encase three to five large seeds.

The seed pods are buoyant, so those that end up in the water can drift away, allowing the seeds to germinate far from the parent tree. But each seed weighs about as much as a mouse—too heavy to be carried off by the wind or easily dispersed by birds and most rodents.

Therein lies a mystery: Black bean trees sprout not just along waterways and coastal areas, but also along ridges high above rivers, far from the water. A few years ago, Maurizio Rossetto, an evolutionary ecologist at the Royal Botanic Garden Sydney, began to wonder how they got there.

One potential answer was that these unusual patches were the remnants of forests created long ago by large, now extinct animals that once helped spread black bean seeds across eastern Australia. Rossetto knew that most large-seeded plants thought to have lost their ancient distributors had retreated to small areas, yet the black bean trees are widespread. So, he wondered about another possible disperser: prehistoric Australians, who arrived on the continent at least 50,000 years ago.

Rossetto was experienced in using genetic data to reconstruct the historic distributions and movements of other Australian plants. But to link the distribution of the black bean tree to people, he would need help from today’s Indigenous Australians, who use the tree for food and ritual purposes. He turned to ecologist Emilie Ens of Macquarie University, who is a pioneer in the emerging field of cross-cultural ecology, in which stories told by Indigenous grandmothers and their ancestors can carry as much scientific weight as studies of DNA. By developing collaborations with Indigenous people, Ens and Rossetto have been able to combine oral histories, Indigenous knowledge, and scientific data to unravel how the histories of Australia’s plants and people are often closely intertwined.

“Increasingly, there is an acceptance that it would make a lot of sense to consult the people who have lived here for tens of thousands of years” when trying to reconstruct Australia’s ecological evolution, says Tony Hughes-d’Aeth, a cultural historian at the University of Western Australia (UWA).

Ens’s and Rossetto’s cross-cultural study of the black bean tree isn’t unique. A second Australian team is working closely with Indigenous partners to understand how other culturally important plants arrived at their current distributions. Together the two groups are finding that the seemingly natural distributions of key food plants likely reflect the habits and travels of the first Australians. “We are increasingly aware that what we thought of as ‘wild’ ranges of species did not take into account traditional activities,” Rossetto says.

The findings could aid efforts to conserve plants and help them adapt to climate change. They also echo similar revelations about Brazil nut trees and other plants in South America. And the research speaks to the underrecognized sophistication of the people who occupied landscapes long before the arrival of Europeans. In Australia, the work has helped further undermine colonial-era assumptions that native people were aimless wanderers, as well as claims of terra nullius—that the land belonged to no one when the first British settlers arrived.

“Humans have [long] been part of Australia’s ecosystems,” says ecologist Alison Lullfitz of UWA. “But it’s only in the last decade that ecologists have started to meaningfully try to consider that.”

To trace the journey of the black bean tree, Ens and her colleagues first scoured the literature for accounts by early European settlers that mentioned how Aboriginal people used the species or recorded their stories about the plant. She contacted Oliver Costello, a Bundjalung man who runs the Jagun Alliance, a nonprofit organization advocating for culturally informed resource management. He said he was glad to be involved. His parents planted his placenta under a black bean tree, and he gave his daughter seeds from that tree so she could plant her own tree. “I had a personal connection to it,” Costello says.

Costello helped interview five Indigenous “knowledge custodians.” Among other things, the researchers learned of a ritual that involved mashing the seeds, or nuts, and then soaking the mash for several days in a river to detoxify it so it can be eaten. In some cultures, “the processing of the nuts was part of the initiation of becoming a woman,” says Monica Fahey, a molecular ecologist at the Sydney botanic garden and a Macquarie Ph.D. student who works with Rossetto and Ens.

The custodians also recounted “songlines,” narratives memorized by Aboriginal travelers to help them navigate from place to place. One songline, recorded in the 1950s, described how an ancestral spirit carried a bag of black beans as it traveled from Australia’s east coast to mountains in the west. Using their intimate knowledge of the region, Costello and a “traditional pathway” expert, Ian Fox, predicted that this route included ridges near the Nightcap and McPherson mountain ranges. Some ridges mark the boundary between the states of Queensland and New South Wales, where black bean trees now grow. In other conversations, the researchers learned that Aboriginal people often buried black bean seeds as they moved from camp to camp.

Rossetto and his colleagues soon found that their genetic data aligned remarkably well with these oral histories. Black bean tree leaves collected at a number of sites, including traditional camping spots just below the mountain ridges, yielded chloroplast and ribosomal DNA sequences that were notably similar, the team reported in 2017 in PLOS ONE. That supported the idea that even trees found in widely separated watersheds shared a common ancestor, and that the species had spread across the region in just a few millennia. The pattern “makes sense,” Costello says.

In contrast, DNA sequences taken from black bean trees growing in wetter, more tropical areas farther north—where Indigenous people appear to have weaker cultural connections to the tree—showed less similarity and ancient divergences, suggesting prehistoric people played a lesser role in moving seeds across the landscape.

“It was exciting to see new genomic techniques being combined with traditional knowledge, songlines, and biogeography,” recalls Jennifer Silcock, an ecologist at the University of Queensland who was not involved in the work.

Costello himself was surprised by the results. He grew up in Bundjalung country, along Australia’s east coast, thinking black bean was native there. Now, knowing how these and perhaps other plants were introduced “ helps us think through how we might do that in the future,” perhaps to aid conservation efforts.

Lullfitz embarked on her own cross-cultural journey at about the same time. She grew up on a farm next to a large national park in southwestern Australia, but she had thought little about that area’s human history until she met Shandell Cummings and her mother Lynette Knapp, members of the Noongar Aboriginal group native to that area. The women “took me under their wing,” Lullfitz says. At times, they would meet to gather food plants in the bush, including the tuber Platysace deflexa, which has various names, including yoowak, youlk, and youaq. Lullfitz was impressed by the mother’s and daughter’s knowledge and passion about what they called their “country,” by which they meant not just the land and its plants and animals, but also their links to it through traditions and stories.

When Lullfitz started her Ph.D. in 2014, she embraced these connections, both personally and professionally. She decided to study the distribution of yoowak and two related species—and to recruit Cummings and Knapp as close collaborators.

That was a wise approach, says Alan Williams, an archaeologist at EMM Consulting Pty Unlimited. “Researchers are generally only briefly in country and therefore may miss the nuances and longer changes in the environment that Indigenous people will be aware of,” he says.

Lullfitz’s work focused on Australia’s southwest floristic region, a biodiversity hot spot that includes a United Nations Biosphere Reserve because it’s so rich in unique species. The landscape is dotted with heathlands, eucalyptus woodlands, and granite outcrops that include water sources Indigenous people visit for ceremonies and use as way stations during long-distance travel.

For her study, Lullfitz collected three Platysace species. Historically, Indigenous people harvested two of them: yoowak, which grows on sandy plains; and P. trachymenioides, which Lullfitz dubbed rock youlk because it grows on outcrops. The third, P. effusa, is not eaten because it lacks tubers.

Lullfitz and her colleagues analyzed DNA isolated from the three species at dozens of locations. The yoowak sequences were remarkably similar, even though the highly variable soils might be expected to isolate populations and lead to genetic divergence. The same pattern held for rock youlk—but only in the southeastern part of its range. In contrast, there were large genetic differences among the P. effusa samples, Lullfitz and her colleagues reported in 2020.

The yoowak pattern suggests humans were moving that plant—but not the inedible P. effusa—from place to place, Lullfitz says. And it fits with the traditional knowledge related by Cummings and Knapp, who said their ancestors often carried yoowak on trips because it holds a lot of water and remains edible for a long time, particularly if stored underground.

The women also believe they can explain why rock youlk is genetically homogenous in only a small region. That plant was considered less desirable than yoowak, so they and Lullfitz think it was not used very much— except in the southeastern part of its range, where other, tastier edible tubers are much rarer. Such findings add to the “increasing evidence that some occurrences or populations of species may be cultural,” Silcock says.

Incorporating Indigenous knowledge into research projects can produce valuable insights, but it can require ecologists to adopt new ways of working. Ens, for example, notes that it can be “very hard to access” Indigenous sources of information—particularly for academic scientists juggling grant deadlines, teaching obligations, and other commitments. So, when her group wanted to trace the history of another culturally important tree in 2018, she recruited Patrick Cooke to help build ties. Cooke, a member of the Gungalida Aboriginal group, had experience working with native communities on health and social issues.

Noongar member Shandell Cummings (left in second photo) works with researcher Alison Lullfitz to unravel the history of rock youlk (first photo), a tuber traditionally important in Cummings’s culture.

Cooke says he was eager to use his connections—and develop new ones—to help reconstruct the distribution of Araucaria bidwillii, a massive conifer whose common and Aboriginal names include bunya-bunya, the monkey puzzle tree, Bonyee, and Bonyi-Bonyi. Striking for its dome-shaped symmetry and now often planted by landscape designers, it comes from a lineage that dates back to the dinosaurs and has relatives across the Southern Hemisphere. In its homeland, the tree grows wild only in a few places in southeast and north Queensland.

Bunya cones, which can weigh more than a small dog, are a rich source of nutritious seeds. The tree was so important to Aboriginal people that they would travel hundreds of kilometers to stands in the Bunya and Blackall ranges for periodic festivals. Some older trees bear foot- and handholds created by climbers who harvested cones. These ceremonies ended at the turn of the 20th century, when loggers started to cut bunya.

At first, finding people willing to share their knowledge of bunya trees “was very hard,” Cooke recalls. “No one knows who you are.” Always, the initial outreach simply involved “yarning”—making small talk about friends and family that barely touched on the project at hand. Often, however, people became interested in sharing their bunya stories after Cooke described the earlier black bean project. The process of properly developing collaborations, he says, “is like peeling back the layers” of an onion. (In 2022, Cooke and colleagues described in Ecological Management & Restoration eight “layers” of protocols and guidelines that scientists should consider when forging Indigenous collaborations, ranging from expectations set by international treaties to rules for one-to-one conversations.)

As with the black bean tree, it turned out “there is strong agreement between Indigenous knowledge and genetic and distributional data” for the bunya, says Rossetto, who was involved in the work. In an upcoming doctoral thesis, Fahey notes that the evidence suggests that before Europeans arrived in Australia, bunya stands were scattered across southeastern Queensland, but only two small populations grew in northern Queensland. Those southern stands are genetically similar, Fahey says, suggesting humans had a hand in their appearance. But the two northern pockets are genetically distinct, suggesting little human influence.

Indigenous knowledge shared with the team supports this picture. In the north, for example, there were no stories about people using or celebrating with bunya. And archaeological evidence suggests prehistoric people in the north consumed other nut species. As a result, bunya “didn’t take hold [in the north] as it did in the south,” Ens says.

Even in the south, bunya tree stands are scattered, which Fahey says is consistent with reports that families controlled bunya trees and who got seeds—and therefore how far they were distributed. Only after European colonization, when Aboriginal families were forcibly scattered to missions far from their homes, did bunyas became more widespread, as displaced families often planted trees on mission lands.

When Fahey publishes her work, Indigenous collaborators will be among the authors. And that, Ens says, represents a significant step toward acknowledging the value of cross-cultural research collaborations. “Having people involved and being able to talk about their scientific knowledge in a scientific space is very important,” she says. In addition, demonstrating that genetic data back up stories told by Elders gives others “the confidence to talk.” Hughes-d’Aerth agrees: “For an Indigenous child to hear and know that their culture is a source of valuable knowledge literally means the world.”

The researchers involved in reconstructing Australia’s past interplay of plants and people say there is plenty of work left to do. Fahey and colleagues, for example, are continuing to gather knowledge about the black bean tree and collect samples from more sites. Fahey has sequenced the bean’s whole genome and hopes these data will enable researchers to refine the tree’s history and perhaps even pinpoint the origins of the existing populations.

They have also launched a systematic effort to identify more examples of human-dispersed wild species. In a paper published in Genes in 2022, the researchers identified 15 species with nutritious, edible seeds too large to be easily dispersed by other means. Five of those species have known cultural significance, and Fahey is now working with Indigenous collaborators to decide which ones to study further.

The findings from these kinds of studies could be of more than just academic interest. They could also offer practical insights into how to make conservation projects more effective. Research in South America and elsewhere has shown that Aboriginal populations have helped maintain biodiversity over long periods, says Maria Beatriz N. Ribeiro, a plant ecologist who works with the Socio-Environmental Institute, a Brazilian Indigenous organization. “In past decades, it was believed that the protection of ecosystems without human presence would be essential to save biodiversity,” she says. “But today, in many places, this paradigm is being overthrown.”

In Australia, understanding the bunya tree’s human-influenced past could help ensure its future. The species is threatened by a soil pathogen, and translocating it could provide refuge. “Understanding how plants reacted to past redistribution can help with restoration efforts today,” Fahey says.

For the moment, however, “For most species, inferences or suspicions of [Aboriginal] dispersal remain untested,” Silcock says. But Scott Mooney, a paleoecologist at the University of South Wales, expects that to change. The recent cross-cultural studies, he says, are “part of a trend. There will be more. … These are at the front of that wave.”