The question is, do all currently living creatures with neurons trace back to a common ancestor who also had neurons, or are there several neuron-having lineages alive today, each of which traces back to a different earliest-neuron-having ancestor. In the latter case, each of those neuron-having ancestors would have shared a common ancestor, but one that did not have neurons.
The lineage of all life on earth certainly creates a more complicated graph than a tree, but at the time scale needed to create a novel feature like neurons, it certainly looks like a tree. To say otherwise, you'd have to point to a case where genetic information passed laterally between species, rather than vertically from parent to child. This probably has happened [1], but is not so common as to make all tree-shaped charts misleading.