How can you explain the degree of similarity and difference between objects represented

How can you explain the degree of similarity and difference between objects represented in the form of fossils and modern organisms?

The degree of similarity between objects represented in the form of fossils and modern organisms can be explained by two reasons: 1) all organisms originated from one common ancestor, and each modern organism has “progenitors”; 2) the similarities are due to a change in the environment, the habitat of this or that organism: if the habitat has not changed for a long time.
The degree of difference is due to changes in the environment, as many organisms have the ability to adapt to conditions during evolution.