Lines of evidence: The science of evolution :
Homologies: cellular/molecular evidence
All living things are fundamentally alike. At the cellular and molecular
level living things are remarkably similar to each other. These fundamental
similarities are most easily explained by evolutionary theory: life shares
a common ancestor.
The cellular level
All organisms are made of cells, which consist of membranes filled with water containing genetic material, proteins, lipids, carbohydrates, salts and other substances. The cells of most living things use sugar for fuel while producing proteins as building blocks and messengers. Notice the similarity between the typical animal and plant cells pictured below only three structures are unique to one or the other.
The molecular level
Different species share genetic homologies as well as anatomical ones. Roundworms, for example, share 25% of their genes with humans. These genes are slightly different in each species, but their striking similarites nevertheless reveal their common ancestry. In fact, the DNA code itself is a homology that links all life on Earth to a common ancestor. DNA and RNA possess a simple four-base code that provides
the recipe for all living things. In some cases, if we were to transfer
material from the cell of one living thing to the cell of another,
the recipient would follow the new instructions as if they were its own.
These characteristics of life demonstrate the fundamental sameness
of all living things on Earth and serve as the basis of today's efforts
at genetic engineering.