Release a drop of red food coloring into a glass filled with water. Watch the drop slowly spread until it imbues the water with a rosy tint. Then, add a drop of blue coloring and observe the boundaries of purple expand. According to Vinayak Eswaran, this process, known as diffusion, reflects how, over the past 200,000 years, people evolved to have the relatively thin bones, small jaws, and other distinctive aspects of their current physical form.
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