What were the ancestors of reptiles (lizards, snakes) that live on present Earth?
These and other questions concerning the origin of large groups of living things were difficult to answer until, in 1859, the Natural Selection Theory of Biological Evolution, proposed by Charles Darwin and Alfred Russel Wallace. With the understanding of "how" biological evolution occurs, biologists came to suggest hypotheses to explain the possible kinship relationship between the various groups of living beings.
Tree-shaped diagrams - made up of comparative anatomy and embryology data, as well as information derived from the study of fossils - showed the hypothetical origin of groups from supposed ancestors. These so-called "family trees" or "phylogenetics" (from the Greek, phylon = race, tribe + genesis = source, origin, beginning) symbolized the evolutionary history of the groups that were compared, and suggested a probable time of origin for each of them. As an example see the figure below.
The scheme represents a probable "evolutionary history" of vertebrates. Note that the current groups - at the top of the scheme - as well as the likely ancestors are represented. Note that the lamprey group (considered "jawless fish") is very old (over 500 million years old). Already about 150 million years old, probably from a group of ancestral dinosaurs. Note further that the relationship between birds and reptiles is greater than that between mammals and reptiles, and that the three groups originated from a common ancestor.
Nowadays, with more information about taxonomic groups, computers have been used to generate phylogenetic trees and cladograms to establish the innumerable relationships between living beings.