 Ancient butter clam shells showing the animals grew larger in different periods. A 11,000-year-old shell (left) and a shell about a thousand years younger (right). (Ginevra Toniello) | | | By studying shells, archaeologists recently constructed the history of clams and people in northwest Canada. They coaxed a long story out of the shells: It began more than 11,000 years ago, after the ice sheets retreated and opened shoreline habitat to people and shellfish. Clams flourished on the gravelly seabeds left in the glaciers' wake. Over time, the bivalves lived longer and grew larger. The researchers excavated layers of sediment at five sites on the Salish Sea, where "kilometers of deep and ancient" shell deposits line the coast, Simon Fraser University archaeologists wrote this week in the Proceedings of the National Academy of Sciences. The archaeologists concluded Native Canadians began to harvest clams regularly around 9,000 years ago, adding the nutritious, abundant shellfish to their diets. Then, at about 3,500 years ago, the clam-human tango kicked up a step, thanks to the invention of clam gardens. Aboriginal Canadians built the gardens — enclosed rock barriers — in the tidal zone. Shellfish thrived under human care. High tides kept the animals submerged, and the terraced walls also protected clams from fish and other predators. Low tides gave the cultivators easy access. Young clams more quickly grew large. Roughly a thousand years after the invention of gardens, clams were so frequently collected their shells all but disappeared from the water. The researchers found huge quantities of shells inland, discarded in trash heaps. The gardens were abandoned with the arrival of European settlers to British Columbia. Silt from logging settled over the gravel and caught in the rock walls, degrading the habitats once primed for shellfish. Clams left to modern untended beaches grow more slowly, the archaeologists say. But the ancient gardens may not stay lost. First Nations people are trying to revive clam gardens and other ancestral methods, which the scientists predict will boost shellfish populations. | 
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