Michael Balter, writing for Discover:
One stark example comes from Tasmania, an island off southeast Australia. Nearly the size of Ireland, it was colonized 34,000 years ago by people with sophisticated toolmaking skills who came across a land bridge from Australia. By the 18th century, Tasmanians used simple technology, hunting with rocks and crude clubs. In 2004 anthropologist Joseph Henrich used a mathematical model of cultural evolution to tackle this mystery [pdf]. He concluded that the island’s population, about 4,000 in the 18th century, at some point fell below the level necessary for complex skills to be passed from generation to generation. Scientists increasingly think population size and density have had a big impact on human development at certain pivotal points.
It goes the other way, too. What appears to be serendipitous co-discovery in the archaeological record may in fact be an artifact of population size and density.