6. Which of the following would suggest that two species shared a common ancestor?

"Each carbon atom has only three covalent bonds instead of the required four. "

Explanation:

In the covalent bond the chemical bonding takes place by distribution of electrons among the atoms. The electrons that has been paired are also termed as the bonding pairs, they are also very much constant and balanced of attractive and repulsive forces among atoms.  This covalent bond helps in other unstable atom in getting stable, by archiving electron from other atoms which will help them in forming a complete electron shell.