Final answer:
Scientists look at the size, shape, and structure of teeth and jaws, as well as the presence of certain features like a sagittal crest to determine ancestry. The DNA of whales is closer to that of horses than sharks, despite the aquatic environment of the latter. In humans, deciduous teeth emerge in infancy, permanent teeth during childhood to adolescence, and wisdom teeth in late adolescence to early adulthood.
Step-by-step explanation:
When determining ancestry based on the jaws and teeth, scientists examine various structural characteristics. For instance, the size and shape of teeth, the presence of a sagittal crest indicating strong jaw muscles, and the prognathism of the jaw are telling of different dietary adaptations and evolutionary paths. As seen in different hominin species, such as the robust and gracile australopithecines, these features help paleoanthropologists classify species and infer their lineage.
When comparing DNA sequences to determine common ancestry among animals, it is worth noting that a whale would have more DNA sequences in common with a horse than with a shark. Despite the shark and whale sharing a marine environment, the whale is actually more closely related to terrestrial mammals, having evolved from even-toed ungulates. Phylogenetic trees can represent these relationships by showing derived and lost traits due to evolution.
As for the human lifespan, deciduous teeth typically emerge during infancy, permanent teeth replace them beginning in childhood into adolescence, and wisdom teeth generally emerge in late adolescence to early adulthood. These dental stages can provide insights into the age and development of human ancestors, as seen in the Homo heidelbergensis specimen known as Mauer.