66 million years ago, a 6-9 mile (10-15 km) wide asteroid hit the Earth, and famously wiped out the non-flying dinosaurs and many other animal groups. The angle at which it hit, plus the location, a shallow seabed next to the Yucatan peninsula full of sulfur-containing gypsum, made it even more deadly. The impact spewed vast quantities of vaporized seabed into the upper atmosphere, blocking the sun. Some hot vapor condensed, falling back as fiery debris, triggering forest and brush fires across scattered regions of the world. If the asteroid had been larger, it could have ended most multicellular life. The location where it struck was random chance. If it had landed in the middle of the Pacific ocean, there would have been little lasting significance.

Most plant and animal species did not die immediately. The sulfur compounds and other fine particles thrown into the upper atmosphere reduced sunlight to only 10% of normal within 24 hours and lasted years after the impact. Worldwide temperatures plunged to below freezing for years. The upper atmosphere would only lose about 10% of those fine particles each year, causing sunlight and warmth to return at a crawl.

Fifty percent of all plant species went extinct. The ones that survived came back from their seeds, roots, and spores. Of those that survived, many of the flowering plant species already had an unusual genetic adaptation. They were polyploidy, that is, having three or more paired sets of their genome. This made them more adaptable, and able survive a far greater range of environmental conditions than conifers, ferns, or mosses. Some lichen and fungi species also thrived, living off massive amounts of dead plant material.

As living plants expired in the cold and gloom, plant-eating dinosaurs died of starvation. Meat-eating dinosaurs also succumbed as their prey perished. Pterosaurs and other animal groups died as well, unable to obtain enough food in a collapsing food chain caused by lack of sunlight.

While birds survived the immediate impact, over 90% of bird lineages went extinct. Evidence indicates that body size, behavior, and beaks mattered. Only the smallest-bodied, ground-nesting bird species having beaks that could process seeds were able to locate enough food to avoid starvation in the first few years. They also had larger keels on their breastbones, with powerful flight muscles attached, that allowed them to take off rapidly in the event of a predator attack, and fly long distances. In addition, they grew from egg to adult quickly, in a few months to less than one year, not dependent on extensive parental involvement at a time when lifespans could be short.

The territory where birds lived mattered too. In forested regions, fires destroyed habitat, nests, and food, and produced thick choking smoke. Open areas near water, where ground nesting was common, were relatively safer areas for the immediate ancestors of today's ostriches, chickens, ducks, and possibly other species.

Animals using powered flight, like birds, bats, and pterosaurs, must be warm-blooded to fly at all. Chemical reactions providing enough energy for continuous powered flight need body temperatures of at least 102°F (39°C). Most birds have body temperatures of 104-108°F (40-42°C), with smaller birds having the highest temperatures.