ABSTRACT
About 530 million years ago, give or take 10 million years, most of today's major groupings of multicellular life appeared in what is called the Cambrian explosion. Was it because hard body parts had recently evolved? Because of increasing oxygen levels? Or because of the availability of high-energy food on the ocean floor?
Chapter 2 considers the causes of the Cambrian explosion as well as the earlier transition from unicellular to multicellular life.
One of the animal groups originating in the explosion were the chordates. These relatively complex animals have a neural tube and a supporting rod running the length of the body, called the notochord. Early chordates had a brain with parts (forebrain, hindbrain, and probably a midbrain), and gave rise to vertebrates, including humans. The ancestral form is thought to be closely modeled by amphioxus, a living species. If so it resembled a small, translucent fish, and possessed a single eye. Thus our brain and senses had begun to evolve, important stepping stones toward cognition.
In addition, Chapter 2 considers how fossils are formed and how dates can be assigned to them. The tree of life is introduced, describing the descent of all of life's divisions from a common ancestor.
