Grannis Island served as a summer oasis for the Quinnipiac Tribe and their ancestors for more than 4,000 years. Evidence indicates that Native Americans visited the island in summer to take advantage of abundant seasonal seafood and hunting opportunities. In the 1940s, an archaeological excavation uncovered many Native American artifacts there, including at least 26 hearths, five of them stone fire pits used for baking clams. Archaeologists also found a wide variety of animal bones, arrowheads, knives, and drill-like implements, along with dog remains buried on their sides and surrounded by artifacts, suggesting a ritual practice. Researchers believe dogs were part of the community, helping guard the tribe and assist in hunting, and that they held spiritual importance to many tribes.