Cephalization put brains and sensors up front early
A head gathers brain, eyes, ears, nose, and mouth into one anterior unit for sensing and feeding. Bilaterian animals concentrated nervous tissue forward in a trend called cephalization, tied to heads appearing around 555 million years ago in animal history.
Heads are the product of cephalization, a long evolutionary drift in which bilaterally symmetric animals gathered nervous tissue at their front end and then crowded sense organs and feeding gear around it. That trend in the Bilateria is dated to some 555 million years ago. Very simple animals can manage without a head, but many two-sided creatures of every size have one.
In humans the head is a unit of skull, hyoid bone and neck vertebrae. The skull has a braincase of eight bones around the cranial cavity and a facial skeleton of fourteen, including the lower jaw. Sculptors work from a simpler frame of cranium, jawbone and cheekbone; faces carry roughly the same number of muscles, but their shapes vary with use, growth and habitual expression. Leonardo da Vinci studied the proportions of head and eyes on a drawn grid, and the illustrator Jack Hamm taught heads as an egg shape refined in six steps. Philosophers argue over what fills the skull: the identist John Searle holds that the brain is all that is in there, while Henry Bennet-Clark pictured billions of tiny agents with no single boss.
Arthropods solved the problem differently. In trilobites the cephalon is built from fused segments. Insect heads combine eyes, antennae and mouthparts that vary so much between species that they serve as identification keys: compound eyes, sometimes with two or three simple ocelli; paired, segmented antennae between the eyes, thread-like or clubbed; and a labrum, mandibles, maxillae with palps and a labium, occasionally plus a hypopharynx.
How the vertebrate head arose remains debated. Hyman argued in 1979 that it formed by fusing a fixed number of front segments, while the new head hypothesis of the 1980s credited neural crest and cranial placodes. In 2014 a short-lived larval tissue in lancelets proved nearly identical to the crest-derived cartilage of vertebrate skulls, hinting at a plausible route.
Source: Head
