Pemba Island, Tanzania (ISTOCK)

An international team of researchers have made some tantalizing discoveries on an African island that may help inform our understanding of how humans have shaped the natural world throughout the ages. Fossils recovered from a cave on the island of Zanzibar, off the coast of Tanzania, have given scientists a rare look into not only what types of animals existed there over the past 20,000 years, but also when they disappeared — and how their disappearances may have been driven by human occupation of the island. The research was published this week in the journal PLOS ONE.

The study site itself — a cave called Kuumbi Cave — presents a unique opportunity for research, according to the study’s lead author, Mary Prendergast, a professor of anthropology at St. Louis University’s Madrid campus. It contains a fossil record going back 20,000 years. “If you look at the east African coast, there is no published site with this long of a sequence,” she said.

The researchers were interested in looking at the cave’s fossil record in the context of the island’s ancient history. Thousands of years ago, the island of Zanzibar was actually attached to the east African coast — but over time, rising sea levels separated it from the mainland. So the researchers decided to compare their archaeological findings with the history of the sea-level changes that shaped the island, as well as their knowledge of human habitation in the area.

First, the scientists used a model to reconstruct sea level changes off the east African coast over the past 20,000 years and create a timeline of Zanzibar’s conversion into an island. As far back as 20,000 years ago, the island was still very much a part of the mainland. Gradually, the rising seas created a deeper separation between the two, and by about 10,000 years ago, the island was fairly distinct.

The researchers also conducted excavations of a trench in Kuumbi Cave. The trench included layered deposits from five different time periods: the first from around 20,000 years ago, the second from 17,000 years ago, the third from 11,000 years ago, the fourth from a little over a thousand years ago and the last from as recently as 500 years ago.

Because the deposits in the cave are from distinct time periods, “we don’t have this beautiful continuous record spanning 20,000 years,” Prendergast said. “But we get these time slices that enable us to look at change over time.”

The excavations retrieved thousands of animal specimens. As the researchers worked to identify them, a pattern emerged: some smaller animals tended to persist through the fossil record, but the larger ones disappeared over time, suggesting that they went extinct from the island. There are a few possible explanations.

First, the island’s gradual separation from the mainland, illustrated by the researchers’ model, could have a lot to do with the bigger animals’ failure to thrive. There’s a huge body of literature on the biological impacts of island formation, Prendergast pointed out, noting that species diversity tends to decrease and local extinctions are common. The types of animals that disappeared from Kuumbi Cave, such as buffalo and zebra, are known for needing large, open habitats. As Zanzibar became separated from the coast, they simply might have been cut off from the kinds of resources they needed to maintain their populations.

However, it seems likely that human influence had a lot to do with the disappearances as well, likely through a variety of factors including hunting, habitat destruction and competition for resources. Many of the animal bones found in Kuumbi Cave were slashed, broken or burned in ways that could only have been inflicted by humans. And human artifacts found in the cave indicate that humans were definitely present on the island starting around 1,000 years ago, which is also the point after which many species went missing from the cave’s fossil record.

In other words, it looks like the large animals persisted for thousands of years — before humans arrived. 

The disappearance of large animals as a result of human influence is an effect that’s been suggested in other parts of the world as well. In North America, many experts have pointed out that the continent’s largest mammals — for instance, mammoths, giant beavers and saber-toothed cats — were mostly lost by around 10,000 years ago. A prime theory is that the arrival of human populations was the major cause of their demise.

In Zanzibar’s case, the question that remains is whether this is the only period in the island’s history during which humans were present. It’s a difficult question to answer because of the fragmented collection of human artifacts scientists have to go on. Some other studies of the island have suggested that humans may have been present on the island thousands of years earlier. Some scientists believe that these humans abandoned the island for a period of time and returned later, while others believe that the island was always occupied.  

Because nobody’s 100 percent sure about the history of human occupation on Zanzibar, it’s impossible to say  exactly how and to what extent humans shaped its ecology, although it’s very likely they played a significant role in the way wildlife populations waxed and waned on the island. Additionally, there are other factors not considered by this study that probably played a role in the island’s history. Historical climate change is one prime example.

Despite all the uncertainties, however, the study provides some useful insight for both our understanding of the past and our management of the present. Conducting case studies on other islands around the world, and then comparing them with the Zanzibar study, can increase overall understanding of the kinds of environmental factors or historical human influences that have had a hand in shaping the natural world.

And on a more immediate note, biologists and conservationists can use the kinds of information found in Kuumbi Cave to make better management decisions for ecosystems they want to protect.

“One thing that people who are in charge of managing diversity often have to deal with is what is the ideal balance in any given ecosystem between different animals,” Prendergast said. “Archaeologists can help provide some historical context for figuring out what that balance is and what an environment used to look like before a major human impact.”

Such tailored management techniques are becoming all the more relevant in a world that many scientists believe is heading for another mass extinction — the sixth in the Earth’s history and the first to be caused mostly by human influence. Turning to the past for help is one way archaeologists and biologists can work together to make better decisions for the future — and Zanzibar is not likely to be the last place in which this kind of work is valuable.

“I like to think that archaeology is making the contribution to the future by looking at the past,” Prendergast said.