How can we tell that two species shared a common ancestor, and are therefore related?

Homologous structures provide evidence for common ancestry, while analogous structures show that similar selective pressures can produce similar adaptations (beneficial features). Similarities and differences among biological molecules

Explanation:

There are multiple ways you can tell

you can look at the bone structure of an organism and see if it's homolougous

Or use a cladogram to tell what species share a common ancestor ( hope this helps)

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products