ABSTRACT
Among the exciting developments in Chapter 3 are new hard parts, crawling the land, early fur, and asteroid-borne disaster. The hard parts were teeth and bone, emerging a little less than 500 million years ago during vertebrate evolution. They allowed effective predation while protecting the nervous system. Tying into that were the eyes, possessing for the first time a small area of detailed central vision. That would lead to cognitive advances including improved object recognition.
Crawling the land occurred by 360 million years ago, preceded by a 25-million-year transition in which lobe fins became limbs and gills gave way to lungs.
Early fur formed a "halo" around fossilized skeletons as far back as 165 million years ago. But mammalian characteristics did not evolve in one giant leap. They were acquired gradually: Warm-bloodedness by 250 million years ago; a large brain relative to body size by 195 million years ago; and in our ancestors the eutherians, a placenta by 160-170 million years ago.
Then one day, 66 million years ago, a six-mile-diameter asteroid struck the Yucatán peninsula, blasting out an enormous quantity of partially melted rock and dispersing light-blocking material into the atmosphere. A staggering 81 degree Fahrenheit drop in average global temperature followed. All large land animals became extinct. The future belonged to mammals, and to mammalian cognition.
