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n the treetops, this powerful vision was built for speed—seeing and reacting quickly. On the open grassland, it was the opposite. Safety and finding food relied upon slow, patient observation of the environment, on the ability to pick out details and focus on what they might mean. Our ancestors’ survival depended on the intensity of their attention. The longer and harder they looked, the more they could distinguish between an opportunity and a danger. If they simply scanned the horizon quickly they could see a lot more, but this would overload the mind with information—too many details for such sharp vision. The human visual system is not built for scanning, as a cow’s is, but for depth of focus.
