Final answer:
All 4-legged vertebrates, or tetrapods, such as amphibians, reptiles, birds, and mammals, descended from ancient fish ancestors that developed adaptations for life on land, including the amniotic egg. Mammals, characterized by hair, evolved this feature after the amniotic egg. The evolution of tetrapods occurred around 400 million years ago.
Step-by-step explanation:
The organism from which all 4-legged vertebrates descended is linked to ancient fish that ventured onto land around 400 million years ago. These early tetrapods made significant evolutionary changes that enabled them to adapt to terrestrial living.
Specifically, the development of the amniotic egg was a crucial turning point in the evolution of vertebrates. This adaptation allowed the embryos of these organisms to develop in a protected, nutrient-rich environment, which was essential for successful life on land.
Consequently, all tetrapods, including amphibians, reptiles, birds, and mammals, share a common ancestor that had produced an amniotic egg. Additionally, among these diverse groups, mammals are characterized by having hair, an evolutionary trait that likely appeared after the development of the amniotic egg.
Furthermore, the emergence of tetrapods with fully-formed limbs evolved directly from the fins of their fish ancestors. The vertebrates can be traced back through this phylogenetic reference to gnathostomes, creatures that had a jawed mouth.
Such anatomical innovations were likely related to whole genome duplications that expanded the possible anatomical features and variations within the vertebrate lineage.