A brain as a mass of nerve tissue located in the head of an animal with a spinal cord, and it controls automatic and voluntary motion and interprets information through the senses. Brains are found in both vertebrate and invertebrate animals but not in plants, fungi, protista, archaeabacteria or eubacteria.
Humans have the largest brains relative to body size of all vertebrates. This is significant because the average ratio of brain mass to body mass predicts the average intelligence of a species. An elephant's brain weighs about 10 pounds but is less than one-tenth of 1 percent of its body mass, while a human brain weighs around 3 pounds and comprises 2 percent of body mass.
Science is hindered in the study of brain evolution because the brain is made of soft tissue that decomposes quickly after death and leaves few traces behind. However, in vertebrates, fossilized skulls give scientists some indication of the requirements necessary to contain and protect the brain of an organism. Among invertebrates, scientists infer information about brain evolution by studying contemporary species.
One recent discovery about brain evolution is that even single-celled organisms can think in a rudimentary way. While bacteria lack neurons, they share a component of brain structure known as ion channels, which are smaller than neurons. Just as neurons transfer electrical impulses to each other, ion channels allow electrically charged molecules to flow in and out of cells. These transfers act as message carriers in both neurons and ion channels. Long before human brains evolved, ion channels helped bacteria navigate their environments.