The first stone tools, made by Homo habilis, appeared 2.5 million years ago. The kit remained in use, unchanged, until 1.7 million years ago, when it was replaced by a more sophisticated set of implements, the Acheulean industry, made by a more advanced species, Homo ergaster. The time axis, expressed in units of millions of years ago, is not to scale. The lower two bands occupy a time span of 2.25 million years, the upper two bands one of just 0.25 million years. During this latter period, the conservatism of the previous 2 million years was replaced by a much brisker tempo of innovation. The Acheulean gave way to the Middle Stone Age tool kit, made by both the Neanderthals in Europe and the human lineage in Africa. Then from 50,000 years ago, the modern humans who replaced the Neanderthals in Europe started making the highly refined artifacts of the Upper Paleolithic age. These included smaller tools, some designed to be set in wood handles or weapons, as well as decorative and artistic objects.
Ergaster's arms were of human length, not ape length, suggesting it had made a final farewell to the trees and was committed to living at ground level. Its chest cavity had the human barrel shape, not the cone shape of an ape's, the indication of a major change in diet. Apes need enormous guts to digest masses of plant material and their rib cages are cone-shaped because they must be wide enough at the bottom to cover the stomach compartment. The barrel shape of ergaster's chest was positioned above a smaller belly, showing that it was eating a richer diet, consisting of meat and maybe also tubers, the starchy roots that served as storage devices for plants living in dry environments.^
Tubers, a staple of hunter-gatherers, are a likely new food because ergaster had learned for the first time to inhabit dry, hot areas in East Africa where tubers flourish. Aridity, shown by the presence of dust in sea-floor sediments, increased sharply at the time of ergaster's emergence. Ergaster may even have learned to cook the tubers, a significant advance if so because cooking releases nutrients from foods,
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