|On the origin of the nervous system|
Miller, G. (2009). On the origin of the nervous system. Science (Wash.) 325(5936): 24-26
In: Science (Washington). American Association for the Advancement of Science: Washington DC. ISSN 0036-8075, more
Peering back through the ages for a glimpse of the first nervous systems is no easy trick. In the seventh essay in Science's series in honor of the Year of Darwin, Greg Miller discusses some tantalizing clues that scientists have recently gained about the evolutionary origins of nervous systems. They've found that some of the key molecular building blocks of neurons predate even the first multicellular organisms. By looking down the tree of life, they are concluding that assembling these components into a cell a modern neuroscientist would recognize as a neuron probably happened very early in animal evolution, more than 600 million years ago. Most scientists agree that circuits of interconnected neurons probably arose soon thereafter, first as diffuse webs and later as a centralized brain and nerves. But the resolution on this picture is fuzzy. The order in which early branches split off the animal tree of life is controversial, and different arrangements imply different story lines for the origins and early evolution of nervous systems. Scientists also disagree on which animals were the first to have a centralized nervous system and how many times neurons and nervous systems evolved independently.