  The development Open Source Software fundamentally depends on the
participation and commitment of volunteer developers to progress. Several works
have presented strategies to increase the on-boarding and engagement of new
contributors, but little is known on how these diverse groups of developers
self-organise to work together. To understand this, one must consider that, on
one hand, platforms like GitHub provide a virtually unlimited development
framework: any number of actors can potentially join to contribute in a
decentralised, distributed, remote, and asynchronous manner. On the other,
however, it seems reasonable that some sort of hierarchy and division of labour
must be in place to meet human biological and cognitive limits, and also to
achieve some level of efficiency. These latter features (hierarchy and division
of labour) should translate into recognisable structural arrangements when
projects are represented as developer-file bipartite networks. In this paper we
analyse a set of popular open source projects from GitHub, placing the accent
on three key properties: nestedness, modularity and in-block nestedness -which
typify the emergence of heterogeneities among contributors, the emergence of
subgroups of developers working on specific subgroups of files, and a mixture
of the two previous, respectively. These analyses show that indeed projects
evolve into internally organised blocks. Furthermore, the distribution of sizes
of such blocks is bounded, connecting our results to the celebrated Dunbar
number both in off- and on-line environments. Our analyses create a link
between bio-cognitive constraints, group formation and online working
environments, opening up a rich scenario for future research on (online) work
team assembly.
