This book will continue to use commonly accepted names for geological ages. Each chapter, we’ve encountered a shorter unit of time: Eons > Eras > Periods > Epochs. The rest of the book occurs in the present Cenozoic Era of the Phanerozoic Eon, scales that are too long for the timeline below (see Chapters 8 and 9). Chapter 8 left off 30 million years ago in the Paleogene Period. This chapter picks up with the late Paleogene, also known as the Oligocene Epoch. At 23 million years ago, the fossil record shifts to the Neogene Period. The Neogene began with the lengthy Miocene Epoch of 23 – 5 MYA, followed by the shorter Pliocene Epoch (abbreviated “Plio” below). Of all these ages, the Miocene Epoch is the one that most strongly coincides with Chapter 7. As with all geological measurements, these units of time are named according to broad changes in Earth’s overall climate and fossil record; they are not defined by milestones in our ancestral evolution.
Using the more objective logarithmic system, this chapter represents roughly 1/1,000 of the history of the universe. If Chapter 10 were a meter, Chapter 7 would be a barely-visible millimeter. According to the three-dimensional gold model, ten million years represents ten bars of gold.
Chapter 7 is one of the least known time scales. It is underrepresented in school curricula and popular culture, maybe because its icons, fossil apes, are less charismatic than dinosaurs or cavemen. It also happens to be one of the periods of time with the sparsest body of evidence. There is a fossil record for the evolution of the body discussed below, but it is small. We will see, however, that Chapter 7 covers some significant milestones. This was the time when our phylogeny narrowed down to our very own clade, the proto-humans (hominins), with genes that are now uniquely human.
Fossils tell us directly about the evolution of our body’s hard parts. There is more to a skeleton than meets the eye. Bones and teeth provide ample clues about diet, habitat, motion, and even behavior. We can learn more by observing our most closely related species, the great apes. Living species have soft tissue – muscles, brains, and hair – to compare to our own. Ape behavior and social structure can also give us insight into the long-term evolution of human nature.
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