  The process by which new communities emerge is a central research issue in
the social sciences. While a growing body of research analyzes the formation of
a single community by examining social networks between individuals, we
introduce a novel community-centered perspective. We highlight the fact that
the context in which a new community emerges contains numerous existing
communities. We reveal the emerging process of communities by tracing their
early members' previous community memberships.
  Our testbed is Reddit, a website that consists of tens of thousands of
user-created communities. We analyze a dataset that spans over a decade and
includes the posting history of users on Reddit from its inception to April
2017. We first propose a computational framework for building genealogy graphs
between communities. We present the first large-scale characterization of such
genealogy graphs. Surprisingly, basic graph properties, such as the number of
parents and max parent weight, converge quickly despite the fact that the
number of communities increases rapidly over time. Furthermore, we investigate
the connection between a community's origin and its future growth. Our results
show that strong parent connections are associated with future community
growth, confirming the importance of existing community structures in which a
new community emerges. Finally, we turn to the individual level and examine the
characteristics of early members. We find that a diverse portfolio across
existing communities is the most important predictor for becoming an early
member in a new community.
