Large-scale interacting human activities underlie all social and economic phenomena, but quantitative understanding of regular patterns and mechanism is very challenging and still rare. Self-organized online collaborative activities with a precise record of event timing provide unprecedented opportunity. Our empirical analysis of the history of millions of updates in Wikipedia shows a universal double-power-law distribution of time intervals between consecutive updates of an article. We then propose a generic model to unfold collaborative human activities into three modules: (i) individual behavior characterized by Poissonian initiation of an action, (ii) human interaction captured by a cascading response to previous actions with a power-law waiting time, and (iii) population growth due to the increasing number of interacting individuals. This unfolding allows us to obtain an analytical formula that is fully supported by the universal patterns in empirical data. Our modeling approaches reveal "simplicity" beyond complex interacting human activities.human dynamics | online collaboration | double power law | multibranching Q uantitative understanding of regular patterns in human dynamics is of great importance but fairly challenging because they are driven by complex decision-making processes, involving competing choices under limited time and cost, interactions with social peers, influences from the external environment, and so on. Previously, when detailed and precise records of human activities were rare, individual activities were assumed to follow random Poissonian processes with exponential distributions of interevent times (1). In contrast, recent analysis and experiments on deliberate human behaviors, such as communication through emails (2), surface mails (3), cell phones (4), instant messages (5), text messages (6), social contacts (7), and online activities including web browsing (8), movie watching (9), searching (10), and shopping (11), showed that individual activities usually embody the bursty nature featured by fat-tailed, power-law-like distributions of interevent times. Several models have been proposed to explain the possible underlying mechanisms, including a task competition model driven by individual decision (2, 12, 13), a nonstationary Poisson process driven by daily and weekly circadian circles (14-16), and adaptive interest (17). Besides the complexity in individual decision making, these works do not take the interaction between individuals into explicit consideration. The interaction was theoretically explored by placing a task competition model into a network of two (18) or more agents (19)(20)(21)(22). Only recently, it was explicitly demonstrated that interaction can lead to a bimodal distribution of the interevent times in short message communication, which is mainly a two-person interaction system (23). Thus far, how interactions happen in a real system organized by a large number of individuals is still unknown.Modern information technology has allowed many new types of human inte...