Human faces became flatter primarily due to dietary changes and the adoption of tool use in early Homo species.
As the environment transitioned to drier, less wooded landscapes over the last 2 million years, the diets of our ancestors shifted. This environmental change led to the following evolutionary changes:
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Dietary Shift: Early humans began consuming a wider variety of foods, including tougher items like meat and roots.
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Tool Use: The development and utilization of tools for breaking down food and cutting meat reduced the need for strong jaws and large teeth. This is a crucial point; tools essentially took over some of the work that our jaws and teeth previously performed.
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Jaw and Teeth Reduction: With less demanding diets and the use of tools, the jaws and teeth of early Homo species gradually became smaller and less robust.
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Facial Retraction: The reduction in jaw size led to a flattening of the face. Strong, protruding jaws are necessary for powerful chewing; without that need, the face receded.
In essence, the selective pressures favoring powerful jaws diminished as technology advanced and diets softened. The result was a more delicate and flatter facial structure in modern humans compared to our earlier hominin ancestors.
