What came first? Backpropagation or Sigmoid? Backpropagation came out around 1974 I believe (paper by Werbos). Looking at the paper, there is no mention of the sigmoid activation function.

When did the sigmoid function become so popular in NNs?
I imagine the sigmoid function would have made its way into ANN's "since the beginning" since it is used in computational neuroscience to approximate the observed activation function of biological neurons. See for example this 1972 paper.