The protostome developmental sequence arose just once in evolutionary history, resulting in two main subgroups—Lophotrochozoa and Ecdysozoa. What does this finding most likely suggest? The protostome developmental sequence arose just once in evolutionary history, resulting in two main subgroups—Lophotrochozoa and Ecdysozoa. What does this finding most likely suggest? The protostomes are a polyphyletic group. Division of these two groups occurred after the protostome developmental sequence appeared. The lophotrochozoans are monophyletic. These two subgroups have a common ancestor that was a deuterostome.