1. Natural selection
... the forested river delta area in India were eating Homo erectus without the protective, external spine. So the allele for spikes increased in frequency. A mountain range developed over thousands of years, and the two populations of Homo erectus gradually separate from each other. The Homo erectus living on the north side of the ... mountain range keep their spines to protect them from the Sunderban tigers. And the Homo erectus on the south side of the mountain range gradually lose their spines. When the mountains cut off rain to the south, the rain ... forest gradually disappears in th...
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