All living things are made up of carbon. This is because

Carbon is the main atom used in the formation of molecules that build cells using the DNA based reproduction system. The carbon atom have affinity to bond to other atoms, and is widely available in Nature and is a stable atom.

Carbon forms four bonds in a tetrahedral arrangement around the carbon. This allows a carbon to bond to carbon or other elements with a great variety of shapes. 99% of the mass of a living organism, such as a human, is made up of just six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus with carbon making up the linking element between all others.

Carbon can from a chain that is linear or branched or form ring structures as well as bonding with other functional groups containing other elements from the periodic table.