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In page Longitudinal fissure:
"It is thought that a majority of existing animals, including Homo sapiens, have evolved from a common wormlike ancestor that lived around 600 million years ago, called the urbilaterian. A bilaterian animal is one that has symmetrical left and right body halves. While it is still debated whether this species had a complex brain or not, development of similar species support the hypothesis that it had at least a simple anterior collection of nerve cells, called a cephalon.[1] Furthermore, studies have shown that this cephalon was bilateral, consisting of two or more connected sub-collections that are separated by the mid-sagittal plane,[2] suggesting the first example of such a division.
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