The medieval settlement of Unguja Ukuu, on the Zanzibar archipelago off the coast of Tanzania, was a key port in a vast Indian Ocean trading network that linked East Africa, southern Arabia, India and Southeast Asia.

Our archaeological research shows how human activities between the 7th and 12th centuries AD irreversibly altered the coastline around the site. At first these changes may have contributed to the development of the trading colony, but later they may have contributed to its decline and abandonment.

former sailor

For millennia, the Indian Ocean has been the maritime setting for a first form of globalization. Large trading networks operated across the vast ocean, prefiguring modern global maritime networks. Unguja Ukuu was a crucial location in this early trade and an important node in the burgeoning slave trade out of mainland Africa.

Read more: From war elephants to cheap electronics: Modern globalization has its roots in ancient trade networks

Unguja Ukuu was an active settlement from the middle of the first millennium until the beginning of the second millennium AD. Archaeological evidence and historical accounts suggest that Unguja Ukuu was one of the earliest known trading settlements on the Swahili Coast.

The rise and fall of trading ports

To understand how and why early ports flourished or declined, it is important to know how the coastal landscape influenced the way traders operated. This includes their choice of mooring locations and their connections to interior locations.

But the question of how these commercial activities in turn changed the coastline has received less attention.

Satellite image of the location of Unguja Ukuu and the surrounding landscape. Insets: A) the extent of the tidal channel leading to the village; B) satellite view of the settlement site; c) the Uzi channel leading to the stream. Illustration by Julien Lubeek.
GoogleEarth, Author provided

Unguja Ukuu thrived in an ecologically marginal area, wedged between the sandy back-reef shoreline of Menai Bay and the mangrove-fringed coves to the east.

Menai Bay provided shelter from monsoon storms and waterways through the shallow interior shelf to shore. It also provided food and other materials from the mangrove habitat.

This landscape allowed the emergence of the agricultural, fishing and trading colony of Unguja Ukuu.

Sediments, sand and shells

We studied sediments, backshore sand and shells at Unguja Ukuu to understand how the colony had affected its own environment. We have found that the accumulation of coastal sediments over the centuries has caused significant changes in the landscape.

Colony detritus, such as food scraps, fire pits, and other household waste, helped the beach spill into the sea. Our analyzes show how human waste and compaction of ancient surfaces caused the shoreline to change , favoring the emergence of an important commercial site.

Photograph of the northern part of trench UU14 with a schematic representation of the facies. and interpretations of anthropogenic signatures in sediments. Author provided.

As more land was used for urban life and agriculture, more sediment moved from the land to the sea. This contributed to the rapid growth of beach fronts, physically altering the coastal landscape and the ecological conditions of the adjacent seascape.

These changes could in turn have led to habitat changes and siltation of the lagoon, which may have contributed to the decline of Unguja Ukuu.

First human impacts

Human-caused processes may also be involved in the decline and eventual abandonment of Unguja Ukuu during the second millennium CE. This was an important period in the socio-political and economic transformation of African coastal societies, marking the emergence of maritime Swahili culture.

But to suggest a purely environmental cause for abandoning the colony would be too simplistic. The interaction of coastal villages and ports with their dynamic landscapes may have played a role in this regional reorganization of human settlements, ports and trade flows.

Read more: How the term ‘Anthropocene’ went from geoscience to hashtags – before most of us knew what it meant

New advances in the techniques of archaeological science, combined with systematic archaeological analyses, increasingly allow us to distinguish the factors of natural events from those of human origin. Such work often reveals much earlier human impacts than initially envisagedshedding light on the earliest roots of Earth’s current geological epoch: the Anthropocene, in which human activity is a key force reshaping the planet.

man-made soil

Our work records snapshots of the evolution of a natural coastal system on the margins of an early settlement.

The river sediments were covered with beach sand containing increasing amounts of accumulated human waste from the mid-seventh century AD. This backshore activity area was used for small-scale subsistence activities (including the processing of shells for meat), trade and the dumping of industrial waste.

Previous urban development shaped the soils of Unguja Ukuu over the long term and through periods of decline and settlement abandonment from the 12th century AD. A black earth “anthrosol” (artificial soil) continues to evolve over these archaeological deposits today, supporting culture in and around the modern city.

Read more: Soil: it’s what keeps us clothed and fed

Man-made dark soils such as these, formed by the rapid decomposition of the colony’s organic and phosphate-rich waste, can be used as markers for as yet undiscovered archaeological sites on the east coast of Africa. Their distinctive dark color makes the soils easily identifiable on satellite images and other remote sensing datasets.

Understanding the past to shape the future

Our study clearly shows how human modification of natural environments affected the coastal landscapes of an East African island more than 1,000 years ago. These discoveries remind us that humans have been altering our environment for thousands of years, sometimes for better and sometimes for worse.

Studying history and archeology is not simply learning from the mistakes of our ancestors so as not to repeat them. It is also about ensuring that scientifically rigorous data that shows how human activity in the past has often altered the landscapes and environments in which people lived is effectively communicated, both to governments and to the public.

If we can do this, we may be able to make more informed sustainable choices for the future of our planet.