Short answers, pulled from the story.
A vertebrate is an animal with a vertebral column and a cranium, where the vertebral column surrounds and protects the spinal cord and the cranium protects the brain. Vertebrates make up the subphylum Vertebrata and include mammals, birds, amphibians, reptiles, and various classes of fish.
There are some 65,000 vertebrate species, making Vertebrata by far the largest ranked grouping in the phylum Chordata. A 2014 tally counted 66,178 described extant vertebrate species, which is less than 5 percent of all described animal species.
The first vertebrates appeared during the Cambrian explosion about 518 million years ago. The earliest known vertebrates belong to the Chengjiang biota and include Haikouichthys, Myllokunmingia, Zhongjianichthys, and probably Yunnanozoon.
Vertebrates are distinguished by a vertebral column, a skull of bone or cartilage, a large brain divided into three or more sections, a muscular heart with multiple chambers, and an inner ear with semicircular canals. They also have sense organs including eyes, ears, and nose, and digestive organs including the intestines, liver, pancreas, and stomach.
Molecular data indicate that hagfish are vertebrates, most closely related to lampreys, with which they form a clade called Cyclostomata. The debate concluded in 2010 when Philippe Janvier accepted that both vertebrates and cyclostomes were monophyletic.
The Living Planet Index recorded a 60 percent decline in vertebrate populations between 1970 and 2014, with freshwater species falling 83 percent. The five main causes are land-use change, overexploitation of natural resources, climate change, pollution, and invasive species.
By the middle of the Devonian, a lineage of bony fishes called the sarcopterygii had both gills and air-breathing lungs and used muscular paired fins to move on land. Those fins evolved into the walking legs of the first tetrapods in the Famennian stage of the Devonian, and these tetrapods established themselves as amphibians in the Carboniferous.