Moroz now counts nine to 12 independent evolutionary origins of the nervous system… ‘There is more than one way to make a neuron, more than one way to make a brain,’ says Moroz. In each of these evolutionary branches, a different subset of genes, proteins and molecules was blindly chosen, through random gene duplication and mutation, to take part in building a nervous system.
Early cells probably inhabited aquatic environments, such as hot springs or brine pools, that contained a mixture of dissolved minerals including some, like calcium, that threatened life. (Important biological molecules such as DNA, RNA and ATP are known to coalesce into refractory goo when exposed to calcium – similar to the scum that forms in bathtubs.) So biologists surmise that early life must have evolved ways to keep all but the lowest levels of calcium outside its cells. This protective machinery might include proteins that pump calcium atoms out of a cell, and an alarm system that goes off when calcium levels rise. Evolution later harnessed this exquisite responsiveness to calcium to signal within and between cells – to control the beating of cilia and flagella that microbes use to move, or to control the contraction of muscle cells or trigger the electric firing of neurons in organisms such as ours. By the time nervous systems began to emerge, roughly half a billion years ago, many of the critical building blocks were already set.
– From Aliens in our midst by Douglas Fox.