 According to Vrba , the impala evolved from an alcelaphine ancestor . She noted that while this ancestor has diverged at least 18 times into various morphologically different forms , the impala has continued in its basic form for at least five million years . Several fossil species have been discovered , including A. datoadeni from the Pliocene of Ethiopia . The oldest fossil discovered suggests its ancient ancestors were slightly smaller than the modern form , but otherwise similar in all aspects to the latter . This implies that the impala has efficiently adapted to its environment since prehistoric times . Its gregarious nature , variety in diet , positive population trend , defence against ticks and symbiotic relationship with the tick @-@ feeding oxpeckers could have played a role in preventing major changes in morphology and behaviour . 
