Carbon is central to life partly because its outer shell holds four electrons, allowing it to make:

1. Group 14 elements, of which carbon is the most important to living systems, have four electrons in their outer shell. 2. Four electrons is exactly half of a full octet. 3. That allows carbon to make several covalent bonds with other atoms. 4. Being able to bond in four directions is what lets carbon build long, branched skeletons. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_