Evidences 1

Below are diagrams of the Vertebrate set of branches of the "Tree of Life" to which palaentologists add data on an almost daily basis.

But what evidence do scientists in this field have that: 

a) these animals are actually related?  and

b) they came in branching lines from a common ancestor?

Above: https://evolution.berkeley.edu/wp-content/uploads/2021/10/patterns_intro_english.png   Below: https://www.oeb.harvard.edu/news/how-mammals-got-their-stride

a) Homologous structures:

Background Information:

An organism's body structure, such as how its bones are arranged, is its basic body plan.

Fish, amphibians, reptiles, birds and mammals all have a similar body structure: an internal skeleton with a backbone. Scientists classify all five groups of animals together as vertebrates.

The theory of evolution explains that these groups inherited these similarities in structure from a shared early vertebrate ancestor.

Similar structures that related species have inherited from a common ancestor are called homologous structures. Information from homologous structures is used by scientists as evidence for evolution and to determine if organisms share a common ancestor.

Pentadactyl limbs in different vertebrates are homologous structures because they share a common underlying bone arrangement (one humerus, two forearm bones, wrist bones, hand bones, and five digits) but are adapted for different functions. Their similarity indicates a shared common ancestor. Examples include the wings of a bat, the flippers of a whale, and the hands of a human, all derived from a similar ancestral limb plan.