• Carbon (C – element 6 on the periodic table) serves essentially as the “backbone” structural and chemical element for all life on Earth, which is why Earth-based life is often said to be “carbon-based”.
• A myriad of essential, complex molecules for Earth-based life are formed by carbon joined with other elements, including oxygen, hydrogen and nitrogen – as well as joined, and “chained”,  with other carbon atoms.
• There are many likely or potential reasons for carbon’s unique role for life on Earth.  One is simply that carbon is abundant (on Earth as well as in the universe generally).  Another is that carbon has a high number of valence electrons (four), which are essentially electrons available to participate in chemical bonding.  Still another is its light weight and size (it is the lightest element with this many valence electrons), which makes it easier for enzymes to manipulate carbon-based molecules.  Furthermore, the energy required to make or break a carbon bond happens to be at a good level for building molecules that are stable, yet reactive.  Additionally and importantly, carbon bonds readily with itself, allowing for long, complex chains with interconnecting carbon bonds.  Moreover, the angle with which carbon bonds with another carbon allows for the formation of an enormous variety of three-dimensional chemical structures.
• While carbon seems optimally suited to its role, (very) hypothetically, other elements could potentially serve as the elemental “backbone” of an alien form of life.

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