In 2008, workers searching for diamonds off the coast of Namibia found a different kind of treasure: hundreds of gold coins mixed with timber and other debris. They had stumbled upon Bom Jesus, a Portuguese trading vessel lost during a voyage to India in 1533. Among the 40 tons of cargo recovered from the sunken ship were more than 100 elephant tusks.
More than a decade after the ship’s discovery, a team of archaeologists, geneticists and ecologists have pieced together the mystery of where the tusks came from and how they fit into the overall picture of historical ivory trade. The researchers’ analysis also revealed that entire elephant lineages have likely been wiped out since the Bom Jesus set sail, shining a light on the extent to which humans have decimated a species once found in far greater numbers across large parts of the African continent.
“The cargo is essentially a snapshot of a very specific interaction that took place at the formative stages of globalization,” said Ashley Coutu, an archaeologist at Oxford University, and co-author of the study, published Thursday in Current Biology. “The power of doing historic archaeology is the ability to link those findings to modern conservation.”
Despite spending nearly half a millennium in the ocean, the tusks recovered from the ship were surprisingly well preserved. For that stroke of luck, the researchers credit the exceptionally cold waters off Namibia. “The state of preservation of the organic material in an archaeological tusk makes a huge difference in terms of what you’re able to extract and do with the sample,” Dr. Coutu said.
The researchers extracted genetic material from cells preserved inside the tusks. This allowed them to identify the ivory as having come from forest elephants rather than the species’ larger, more well-known savanna-dwelling cousins.
Next, the researchers isolated mitochondrial DNA, which is passed by mothers to their offspring and can be used to identify the provenance of elephants. They identified tusks from 17 unrelated elephant herds, only four of which they could confirm still exist today.
“Some of these lineages were possibly extirpated over time from ivory trade and habitat destruction,” said Alfred Roca, a geneticist at the University of Illinois at Urbana-Champaign, and co-author of the study.
In addition to this insight, the DNA sequences recovered from the historical herds “substantially adds to the relatively scarce genetic data available for forest elephants,” said Alida de Flamingh, a postdoctoral researcher at the University of Illinois at Urbana-Champaign and lead author of the study.
By comparing the recovered mitochondrial DNA to modern and historical genetic data sets, the researchers also found that the tusks had come from forest elephants that lived in West rather than Central Africa. A chemical analysis of carbon and nitrogen isotopes in the tusks additionally revealed that the animals must have lived not in deep rain forests, as most forest elephants do today, but in mixed woodland and grassland savannas, of the types present near major 16th century maritime trading posts in West Africa.
While a few forest elephants still live in savanna-like habitats today, scientists have wondered if they migrated to these spaces only after West Africa’s savanna elephants were decimated by the ivory trade in the early 20th century. The new study suggests that some forest elephants have always lived outside of the deep rainforest, Dr. Roca said.
John Poulsen, an ecologist at Duke University who was not involved in the study, said the “incredible detective work” undertaken by the authors demonstrates the importance of interdisciplinary collaboration. “The conclusions of the study are important for understanding human history, elephant genetic diversity and ecology and biodiversity conservation, while also innovating a methodological framework to analyze museum collections of ivory,” Dr. Poulsen said.
From a historical point of view, insight into the Bom Jesus’ tusks is important because experts have almost no records about ivory trade patterns from this early period, said Martha Chaiklin, a historian who studies the ivory trade. The researchers’ findings about the tusks’ geographic origins and that they came from different herds are especially enlightening because they “can be a tool for better understanding Portuguese trade in Africa and the impact ivory trade had on elephant populations in premodern times,” Dr. Chaiklin said.
Samuel Wasser, a biologist at the University of Washington, Seattle, who was not involved in the research, is skeptical, however, about the authors’ interpretation of what caused the forest elephants to dwell in a savanna-like habitat.
“The ivory trade took off in West Africa prior to and during the first slave trade, which was in the 16th century, right when the ship went down,” he said. “These elephants were likely experiencing considerable disruption to their movements, presumably because they were seeking safer havens to escape from heavy poaching.”
Dr. Wasser and his colleagues previously reported that a high occurrence of hybridization of savanna and forest elephants in northeastern Democratic Republic of Congo could be partly explained by historical poaching that drove the two species together. “The same thing likely happened in West Africa when the ivory trade was booming,” Dr. Wasser said.
Centuries later, forest elephants are far from out of the woods when it comes to harm inflicted on them by humans — from poaching and deforestation to climate change and habitat fragmentation. From 2002 to 2011, forest elephants experienced a 62 percent decline in population, with fewer than 100,000 animals estimated to remain today.
“Elephants provide numerous ecosystem services from which humans benefit, and this study emphasizes that elephants are also a part of our history,” Dr. Poulsen said. “We should respect and conserve that.”