Short answers, pulled from the story.
The first vertebrates emerged from the Cambrian explosion approximately 518 million years ago. These early ancestors included species such as Haikouichthys and Myllokunmingia and lived within the Chengjiang biota. They possessed a vertebral column and a notochord that allowed for more complex movement.
The first jawed vertebrates likely appeared during the late Ordovician period around 445 million years ago. They quickly became the dominant force in the Devonian period, which is often referred to as the Age of Fishes. The jaw itself evolved from the first pair of gill arches to allow vertebrates to become active predators.
The transition from water to land occurred towards the end of the Devonian period and culminated in the Carboniferous. The first tetrapods evolved from lobe-finned fishes and possessed limbs with bones and joints to support their body weight against gravity. This process involved the modification of gills into lungs and the development of walking legs from fins.
The Mesozoic era spanned from the Triassic to the Cretaceous periods and was defined by the dominance of dinosaurs. At the onset of the Mesozoic, the largest mass extinction in Earth's history devastated all larger vertebrate groups. Dinosaurs rose to become the dominant terrestrial vertebrates while marine reptiles evolved in the seas.
Jean-Baptiste Lamarck defined vertebrates as a distinct phylum in 1801. He grouped them into four classes: fish, reptiles, birds, and mammals. Modern phylogenetic analysis now relies on a classification system based on known evolutionary history to better understand relationships between diverse groups.
Vertebrate populations experienced a decline of 60% between 1970 and 2014 according to the Living Planet Index. Freshwater species experienced an 83% drop and tropical populations in South and Central America suffered a 89% reduction. The five main causes of this biodiversity loss are land-use change, overexploitation of natural resources, climate change, pollution, and the impact of invasive species.