The evolution in the structure of their teeth, odd-toed limbs, obvious mobility of the upper lip, and other aspects, joins the horse to the evolutionary line of odd-toed, hoofed mammals: the Perissodactyls.
The tapirs and rhinoceroses remained adapted to their original style of life, conserving forms suitable for life in tropical forests, but the evolutionary line of the horse mainly adapted to life on dryer land in the much-harsher climatic conditions of the steppes..
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