All living things have in common their genetic building blocks, which are DNA. Though each organism's DNA differs in what specific genes they carry, which account for the diversity of life, all known living things use these molecules to develop and reproduce.
DNA, or deoxyribonucleic acid, is the microscopic structure that carries the genetic information that defines all aspects of every living thing's physical structure from mental capacity to appearance. Complex organisms have a greater number and diversity of genes than simple organisms, but they can share nearly identical DNA structures because complex living things retain genes from their basic evolutionary ancestors, which govern the more primal and subconscious aspects of their existence.
DNA was first discovered by Friedrich Miescher, but at the time, his instruments were too simple to decipher the actual structure of DNA. It wasn't until James Watson and Francis Crick did their research that they came up with the concept of a double helix, which is now the accepted structure of DNA.
Through the discoveries of Watson and Crick, scientists were able to develop a whole new field of science in genetics. They were able to create models of DNA and to use these models to see how different proteins create all the building blocks of life. Now, this knowledge can be used to understand how life occurs, what genes account for what aspects of our physicality, and what genes contribute to health and disease.