Non-primate mammals like rodents, pigs, cats, and ferrets show frequent axon-carrying dendrites in layer II/III pyramidal neurons. Primates including humans and macaques show rare axon-carrying dendrites in the same neurons. Axon-carrying dendrites allow direct action potential generation bypassing somatic integration. High-resolution microscopy revealed micrometer-level differences invisible to standard light microscopy. Over 34,000 neurons from multiple species assessed using five staining methods. Quantitative differences exist in axon-carrying dendrites for inhibitory interneurons between cats and humans. No differences found across macaque cortical areas with sensory vs higher functions. Axons typically originate from neuron cell bodies in textbook knowledge. Dendritic axon origins termed axon-carrying dendrites. Species difference marks primates vs non-primates in neocortical architecture. Reason for primate rarity and processing advantage unknown.