Final answer:
Birds evolved from reptiles, specifically from the group known as archosaurs within the diapsid reptiles. This group includes the theropod dinosaurs, which are directly related to the ancestors of modern birds. Reptiles were the first to lay amniotic eggs, an evolutionary trait that enabled them to live away from water sources.
Step-by-step explanation:
Birds' Evolutionary Ancestry
It is widely accepted among scientists that birds evolved from reptiles. More specifically, birds are highly modified diapsids and fall under the archosaur group. This evolutionary lineage also includes the formidable theropod dinosaurs. Hence, answering the student's query, birds evolved from what other type of vertebrate? The answer is reptiles, option 1). The first reptiles emerged from an amphibian ancestor at least 300 million years ago and had the significant evolutionary adaptation of laying amniotic eggs, which allowed them to reproduce without returning to water.
The evolutionary path of vertebrates started with lobe-finned fish developing into amphibians around 365 million years ago. Later, reptiles arose from amphibians, eventually leading to the ancestors of modern birds and mammals. The first amniotic eggs were laid by these early reptiles, marking an evolutionary turning point by freeing future generations from the need for aquatic environments for reproduction.
It is essential to note that, while we categorize birds separately for discussion, birds are actually a subset of reptiles when we look at their evolutionary history. Birds are a result of the divergence of the diapsid reptiles into lepidosaurs and archosaurs, with birds evolving from the latter. This means that when we refer to birds' ancestry as reptilian, we are recognizing their descent from theropod dinosaurs, a group of bipedal carnivores that thrived during the Mesozoic Era.