Probing the sanctum sanctorum: New eyes and approaches explore the diversity of vertebrate brains


The spring of 1861 was one of the most explosive and formative in modern history. For those of us in the Western hemisphere, a cataclysmic war was being unleashed as a small fort in Charleston harbor sat besieged by cannon fire. The shelling of Fort Sumter by troops of the South ushered in the American Civil War, a scar that has defined much of our history ever since. While the Blue and the Gray unleashed their generations of built-up animosities, another battle was commencing on the other side of the pond. The setting for this conflict was also a fortress, though one neither built of brick nor manned by cannons, but by the barricades of years of intellectual blockades erected to ward off new, progressive, and iconoclastic ideas. The “Fort Sumter” for this battle turned out to be the seemingly innocuous meeting of the Society of Anthropology of Paris, and the provocateur not a general but a surgeon and part-time anthropologist.

By all accounts, Pierre Paul Broca had the planning ability of a great military strategist and the flair of a galloping cavalry officer rolled into one. He also had the pent-up anger of a genius whose ideas had been pushed to the side and shunted to the back burner by members of the “old school” who sat on the boards and academies of a French hierarchy that he was not a part of. Although only 37 by 1861, Broca was already recognized as an immense talent in anthropology and science, yet his ideas on the functions of portions of the brain had not yet claimed center stage.

To understand fully the effect of the spring of 1861 on the course of anatomy in general, and neuroanatomy in particular, one has to comprehend the great battle that had gone on for years over the question of whether the brain—the sanctum sanctorum or holy of holies of the human body—could be partitioned off into discrete components or had to be viewed as a complete entity that served as the site of the mind and soul. The question was a heavy one, and one that pitted staunch religious-backed conservatives on one side and academic firebrands such as Broca on the other. The conservatives, such as the great experimental physiologist Jean Pierre Marie Flourens (1824), had long-held that the brain and mind could not be broken down into just so many boxes, but retained an integrated, almost sacred, “oneness.” Others, such as Broca, his contemporary Jean-Baptiste Bouillaud (1825), or his controversial predecessors Franz Joseph Gall and Johann Spurzheim (1810–1819), believed that there were discrete areas related to specific functions.

While the battles in these “brain wars” ebbed and flowed over the years, the tide dramatically turned in 1861 (Broca, 1861a, 1861b, 1861c). It was in early spring of that year in a momentous meeting of the Society that Broca presented his findings on the brain of a recently deceased 51-year-old patient known as “Tan,” who was so named as he could only utter that one word. Broca, reportedly with his proud father looking from the packed galleries, adroitly demonstrated the presence of a lesion in the third frontal convolution of the cerebrum. Voilà! The clearly visible, damaged area of Tan's brain was a powerful first barrage onto the walls the opposition. Broca's charge picked up momentum, and over the following months he presented more arguments and another speechless patient with an identical lesion. The faculty of speech, arguably the hallmark of our loquacious species, had been shown to reside in a specific, localized area.

While many subsequent studies have hotly debated the idea of the circumscribed location of language and speech in the brain, the major, global victory of Broca was establishment of the principle that functions could indeed be localized. Like the walls at Jericho, with Broca as a neurobiological Joshua, the age-old ramparts against exploring the different functions of different parts of the brain came crashing down. The “compartmentalizers” were liberated; the controls of centuries of religious prohibitions and fearful eyes were gone forever. After 1861, the world of neuroanatomy and neurobiology would be free to explore new horizons in a way that it could not previously. While comparative anatomists had long commented on differences among the brains of animals, their assessments pre-Broca were at best superficial and without understanding of potential brain area homologs. How could one explore the nature of language among species, for example, if you did not have an idea where to start?

And start they did. Comparative neuroanatomy in the almost 150 years since that explosive spring has brought a wealth of investigations to the anatomical dissecting table. This special issue of The Anatomical Record entitled “Nature's Experiments in Brain Diversity” is a wonderfully robust example of the current richness of the field. It is an extraordinary compilation of cutting-edge science and scholarly reviews exploring the paths that have been taken during the evolution of seminal brain regions and functions during vertebrate evolution. The guest editors for the issue are Lori Marino and Patrick Hof, two of the foremost comparative neuroanatomists in the field today. Frequent coworkers, particularly in their shared explorations of the brains of cetaceans and primates, Marino and Hof have based this issue in part on a symposium they organized on the evolution of neurobiological specializations in mammals at the 2005 meetings of the American Association of Anatomists in San Diego.

Marino and Hof's experience as comparative anatomists, neurobiologists, and evolutionary morphologists have given them a wonderful perspective on the breadth and diversity in their fields and allowed them to select work that portrays vividly the different paths that species have taken in addressing the particular demands and needs their environment and evolution placed on them. From the brain of tiny insectivores (who knew moles could be so interesting, especially my new favorites, the extraordinary star-nosed moles) to those of mammoth African elephants, from birdbrains to bat brains, to the brains of cetaceans and those of an array of our primate cousins, the majesty of different anatomical answers to questions are unveiled. As broad as is the range of species portrayed, so too is the diversity of scientific and technical approaches used by the investigators. From classic gross neuroanatomical dissection and histology, to neurochemistry, neurophysiology, molecular biology, and the latest advances in magnetic resonance and diffusion-tensor imaging, the arsenal of approaches employed in the current world of brain diversity studies is unfurled. Perhaps what emerges most from this issue, however, is the meticulous care and precision the researchers use in charting homologs for brain regions between or among species, and how these areas may be represented, and how they may have changed evolutionarily. From the diligent tracing of the facial motor nucleus across mammals to new and extraordinarily insightful observations on areas of language homologs in chimpanzees and orangutans, the specific neighborhoods of the brain are explored in detail.

The guest editors have sought to provide in this special issue the extraordinary diversity vertebrate brains have taken in the course of their respective species evolution. They have succeeded not only in offering a clear view of this, but also an excellent glimpse into the creativity of the neurobiologists who ask the questions. These heirs of Broca have served his legacy well. He would be very pleased.