I mentioned a few weeks ago that I would have more to say about primates, brain evolution, and life history. I still plan on exploring that in future posts, but wanted to mention this interesting item that deserves a post of its own.
Von Economo neurons are large, bipolar neurons located in the anterior cingulate cortex and fronto-insula cortex. They are present in orangutans, gorillas, common chimpanzees, bonobos, and humans. They are absent in siamangs, gibbons, old world monkeys, new world monkeys, and prosimmians. Von Economo neurons are larger than other, neighboring, neuron types and unlike, say, layer V pyramidal neurons, are larger in chimps and humans than in orangs and gorillas. There are also more of them in chimps and humans. More importantly, unlike the above mentioned layer V pyramidal cells and layer VI small fusiform cells, the volume of Von Economo neurons is correlated with increased encephalization. In humans the Von Economo neurons occur in clusters and, to add to the interest factor, they also occur in clusters in the bonobo (although there are fewer clusters). Why does this add to the interest factor? Let’s put it in phylogenetic context.
Sometime around 15-20 million years ago orangs split from the rest of the apes. They have the Von Economo neurons, but the neurons are rare. Around 10 million years ago the gorillas split off and in gorillas the neurons are frequent. Then around five million years ago chimps and humans diverged. The neurons are abundant in chimps and abundant and clustered in humans. Somewhere around 3-2 million years ago bonobos and chimps diverged and in bonobos the neurons are abundant and clustered just like in humans. So, what we have is a case of convergent* evolution between humans and bonobos. So what function do the Von Economo neurons serve? Nimchinsky et al suggest that:
…the spindle cells of the anterior cingulate cortex might represent a population of specialized neurons that could integrate inputs with emotional overtones and project to highly specific motor centers controlling vocalization, facial expression, or autonomic function. In addition, the presence of at least some spindle cells in the anterior portion of the insula supports the notion of their role in autonomic control, because this region also is known to be involved in the regulation of visceral, olfactory, and gustatory functions, as well as complex alimentary behaviors (46-49). However, in humans the anterior cingulate cortex also appears to be involved in higher-level processes that are responsible for more than merely sensory input or motor output. Recent functional imaging studies have demonstrated an important role for the dorsal human anterior cingulate cortex in attention, with its degree of activation increasing with task difficulty, and the more ventral portion in the experience of the “unpleasantness” of pain and in the recognition of the emotional content of faces (43, 50, 51). Thus, in humans, at least, the anterior cingulate cortex is involved in complex processes that assist in integration and interpretation of sensory information. In this context, the unique correlation among the resident neurons of layers V and VI of the cellular volume of spindle cells with encephalization in human and great apes lends further support to the possible association of spindle cells with higher cortical functioning.
They go on to focus on the language abilities of the great apes in general, but I think we need to take a closer look at bonobos…
*Update 1: Technically, this is incorrect. This would actually be a case of parallel evolution. Convergent evolution produces analogies between two lineages. Parallel evolution starts with two traits that are already similar (usually due to common ancestry) evolve similar changes. Chimps, bonobos, and human all share abundant Von Economo neurons, bonobos and humans evolved clusters of Von Economo neurons in parallel.
A neuronal morphologic type unique to humans and great apes. Esther A. Nimchinsky, Emmanuel Gilissen, John M. Allman, Daniel P. Perl, Joseph M. Erwin, and Patrick R. Hof. PNAS April 27, 1999 vol. 96 no. 9 5268-5273
Dendritic Architecture of the Von Economo Neurons. K. K. Watson, T. K. Jones, And J. M. Allman. Neuroscience 141 (2006) 1107-1112