Epidemics, neural cascades, power failures, and many other phenomena can be described by a diffusion process on a network. To identify the causal origins of a spread, it is often necessary to identify the triggering initial node. Here we define a new morphological operator and use it to detect the origin of a diffusive front, given the final state of a complex network. Our method performs better than algorithms based on distance (closeness) and Jordan centrality. More importantly, our method is applicable regardless of the specifics of the forward model, and therefore can be applied to a wide range of systems such as identifying the patient zero in an epidemic, pinpointing the neuron that triggers a cascade, identifying the original malfunction that causes a catastrophic infrastructure failure, and inferring the ancestral species from which a heterogeneous population evolves.A sugar piece placed in tea will erode and eventually dissolve. Given the initial shape of the piece, it is trivial to predict its final distribution. However, the opposite problem of determining the initial state, given a final one is extremely difficult. Problems of the latter kind are referred as ill-posed inverse problems [1][2][3].Diffusion taking place on networks, in the forward direction, is well studied. One class of models originally used to describe epidemics is the Susceptible-InfectedRecovered (SIR) model [4,5]. Variations include SI, SIS, SIRS, etc. Others include more realistic delay conditions, such as an incubation period for the infection [6]. Similar models are used to describe neural cascades [7], traffic jams [8] and infrastructure failures [9].Accordingly, a successful method of inverting diffusion on complex networks can help identify patient zero in an epidemic outbreak, pinpoint neurons that trigger a cognitive cascades, remedy the parts of the road network that initiate congestion, and determine malfunctions that lead to cascading failures. In the weak selection limit, evolution can be thought as diffusion on a genotype network [10,11], so diffusion inversion may be used to identify ancestral species.Here we address the problem of identifying the origin of a diffusive process taking place on a complex network, given the its final state. We refer to the influenced nodes as the candidate set C. Any member of C may be the node from which the diffusion originated. We refer to this node as the seed, s, and to the forward model as M .Presently, there are two approaches to identify s. The first uses probability marginals from Bayesian methods [12][13][14][15][16][17][18][19][20][21]. In some cases, it is possible to sample the state space using Monte Carlo simulations [13]. However, this is only feasible for small networks. Message-passing algorithms can approximate the marginals efficiently [12,[14][15][16], however these algorithms are model specific: for every M , one must invent new approximations, heuristic assumptions and analytic calculations.In contrast, the second class of methods works independent of the forward ...