A major evolutionary leap took place with the evolution of the brain in mammals. Mammals–which appeared in the fossil record after the first fishes, amphibians, and reptiles–are the only vertebrates to possess the evolutionarily recent, outermost portion of the cerebral cortex—the neocortex. The neocortex of the more primitive monotremes–duck-billed platypus and several species of spiny anteaters–as well as that of marsupials–kangaroos, koalas, opossums, wombats, and Tasmanian devils–lack the convolutions—sulci and gyri—found in the neocortex of most placental animals–eutherians, including humans), the more recent and more advanced animals. Within placental mammals, the size and complexity of the neocortex increased over evolutionary time. The area of the neocortex of mice is only 1/100th that of monkeys, and that of monkeys is only about 1/10th that of humans, the latest evolving primate. In addition, rats lack convolutions in their neocortex, in part related to their smaller size, whereas the neocortex of cats, the later evolving animal, has a moderate degree of convolutions. The human neocortex exhibits very extensive convolutions.
Most invertebrates have reached evolutionary dead ends. The great leap forward–another of those greatest milestones–that propelled vertebrates into domination and produced ever improving interaction with the environment and other creatures, was the development of a true central nervous system. Having a CNS is a decidedly anomalous capability compared to what invertebrates have—to the great invertebrate failure. The development of specialized sensory organs, especially with their connection to an effective CNS, added to the intellectual dominance of vertebrates over invertebrates. Nevertheless, there is a remarkable similarity between fly bristles and the vertebrate octavolateral system (inner ear + lateral line), so much so that there is little doubt that both are derived from a common ancestor—another insight into the parsimony of genes in evolution.
The usual example exemplifying increase in complexity is the eye, which originated as a very simple photosynthetic organ in invertebrates whose initial function was to use light as a source of energy, but which benefited from mutations and through selection pressure over eons to evolve into photoreceptors using light to provide a source of information. Sight, even primitive light vs dark detection, played an extremely important role in the advancement of vertebrates over time. Fossil records record a ripening over time of an increasingly enlarged visual portion of the brain. Mutations contributed advantages to certain fish enabling them to use improved eyesight to move faster and with more agility, to survive and to reproduce better in water than other fish and other species. The sense of smell became a rival somewhat later and added a refinement to the evolution of earth life; the tubes that were devoted to the sense of smell were present in fish about 300 MYA.
Competition between senses of sight and smell continued for millions of years. It is likely that the tetrapods which left the seas and invaded land lacked sufficient brain to cope with two dimensional vision, hence they continued with one dimensional sight. Smell–a one dimensional sense–was within the power of their small brains, and it was more practical for them. The fossil record shows how the occipital lobe shrank, and the cerebral hemispheres enlarged over ensuing generations. Gradually–in the evolutionary record–as mammals became larger, the sense of vision regained heightened usefulness. There is some question as to which was the earlier or greater influence on the acquisition of increased vision—was it improving eyes that ramped up brain growth in size, sophistication, and speed, or did the mutations that enhanced mammalian brains allow, even push, the development of eyes? Whichever is the case, the larger brains of mammals could accommodate and process the increased demands created by improving sight. In mammals, the neocortex–including the occipital lobe /visual cortex–becomes disproportionately large as absolute brain size increases, whereas most other regions become disproportionately smaller; larger brain size comes largely at the expense of olfactory lobes. As in all evolution driven by natural selection, the creatures with larger brains and better sight were better suited to survive and to reproduce in a changing terrestrial environment even though their sense of smell diminished. Once again, however, obvious similarities have been noted between sense organs of arthropods and of vertebrates, highly suggestive of a common origin and of conservation of genes which perform different functions up the evolutionary ladder.
Enlargement of the mammalian brain came at a cost. As with 
previous steps, the need for energy and energy efficiency was core and increased in mammals until it became a self-limiting evolutionary factor. “The nervous system has a unique position as the interface between morphology, physiology, and behavior, but at the same time is subject to costs related to the amount of energy it consumes. Characterizing this trade-off between costs and benefits is essential to understanding the evolution of nervous systems, including that of humans.” [Jeremy E. Niven and Simon B. Laughlin from the Department of Zoology, University of Cambridge, and Smithsonian Tropical Research Institute, República de Panamá, Energy Limitation as a Selective Pressure on the Evolution of Sensory Systems, Journal of Experimental Biology, 211: 1792-1804, 2008]. They went on to conclude from their extensive studies, “The high energetic costs associated with neural tissue favor energy efficient coding and wiring schemes, which have been found in numerous sensory systems.” This drove the acquisition of genetic systems to make larger and more effective brains and also a better gut, circulatory and respiratory system to better oxygenate and to deliver nutrition to accommodate the remarkable advancements in the mammalian→primate→human brains. There are about a dozen neurochemicals involved in the brain’s electrochemical processes. The human brain is one of the most complex aspects of all evolution, and it should come as no surprise that it consumes more energy than any other organ—burning up 20% of all of the energy taken in by humans.