I was reading a review paper that was frustrating because I wanted to know more—it’s on the evolution of complex brains, and briefly summarizes some of the current confusion about what, exactly, is involved in building a brain with complex problem solving ability. It’s not as simple as “size matters”—we have to jigger the formulae a fair bit to take into account brain:body size ratios, for instance, to get humans to come out on top, and maybe bulk is an inaccurate proxy for more significant matters, such as the number of synapses and nerve conduction velocities.
There’s also a growing amount of literature that takes genomic approaches, searching for sequences that show the signatures of selection, and plucking those out for analysis. There have been some provocative results from that kind of work, finding some candidate genes like ASPM, but another of the lessons of that kind of work seems to be that evolution has been working harder on our testis-specific genes than on our brains.
The encouraging part of the paper is that the authors advocate expanding our search for the correlates of intelligence with another group of organisms with a reputation for big brains, but brains that have evolved independently of vertebrates’. You know what I’m talking about: cephalopods!
The Cephalopoda are an ancient group of mollusks originating in the late Cambrian. Ancestors of modern coleoid cephalopods (octopus and squid) diverged from the externally-shelled nautiloids in the Ordovician, with approximately 600 million years of separate evolution between the cephalopod and the vertebrate lineages. The evolution of modern coleoids has been strongly influenced by competition and predatory pressures from fish, to a degree that the behavior of squid and octopus are more akin to that of fast-moving aquatic vertebrates than to other mollusks. Squid and octopuses are agile and active animals with sophisticated sensory and motor capabilities. Their central nervous systems are much larger than those of other mollusks, with the main ganglia fused into a brain that surrounds the esophagus with additional lateral optic lobes. The number of neurons in an adult cephalopod brain can reach 200 million, approximately four orders of magnitude higher than the 20-30,000 neurons found in model mollusks such as Aplysia or Lymnaea. Cephalopods exhibit sophisticated behaviors a number of studies have presented evidence for diverse modes of learning and memory in Octopus and cuttlefish models. This learning capacity is reflected in a sophisticated circuitry of neural networks in the cephalopod nervous system. Moreover, electrophysiological studies have revealed vertebrate-like properties in the cephalopod brain, such as compound field potentials and long-term potentiation. Thus cephalopods exhibit all the attributes of complex nervous systems on the anatomical, cellular, functional and behavioral levels.
Unfortunately, the purpose of the paper is to highlight an unfortunate deficiency in our modern research program: there is no cephalopod genome project. The closest thing to it is an effort to sequence the genome of another mollusc, Aplysia, which is a very good thing—Aplysia is a famous and indispensable subject of much research in learning and memory—but it’s no squid. The authors are advocating additional work on another animal, one with a more elaborate brain.
A parallel effort on a well-studied octopus or squid should provide insights on the evolutionary processes that allowed development of the sophisticated cephalopod nervous system. For example, have cephalopods undergone accelerated evolution in specific nervous system genes, as has been suggested for primates? Have specific gene families undergone expansion in the cephalopod lineage and are these expressed in the nervous system? Are there clear parallels in accelerated evolution, gene family expansion, and other evolutionary processes between cephalopods and vertebrates? Answers to these and related questions will provide useful perspectives for evaluation of the processes thought to be involved in the evolution of the vertebrate brain.
I’m all for it—let’s see a Euprymna genome project!
Jaaro H, Fainzilber M (2006) Building complex brains—missing pieces in an evolutionary puzzle. Brain Behav Evol 68(3):191-195.