There is a large variation in the number of neurons needed to form a brain. Animals like sponges don’t have any neurons, so they don’t have a brain at all. The tiny roundworm C. elegans has only 302 neurons, but this is enough for complex behaviors like learning, mating and looking for food. The organization of these neurons are identical in every worm, which is useful for scientists who study them. The number of neurons grows a lot from there—the fruit fly Drosophila melanogaster has about 100,000 neurons, and the human brain has around 85 billion neurons! The overall structure of the brain (like lobes) is the same in most people, but unlike C. elegans, the connections between neurons develop in a completely different way for each individual. This allows us to better adapt to new situations. Interestingly, the African elephant has three times as many neurons as a human, but brain size doesn’t always dictate intelligence-- there are other factors that may affect intelligence, like the number of connections between neurons, brain size as a percentage of body size, the number of supporting cells in the brain, or the complexity of the synapses.