Final answer:
The species H. ergaster was the first to exhibit significantly reduced sexual dimorphism with males being roughly 20% larger than females.
Step-by-step explanation:
The species H. ergaster was the first to show a trend toward reduced sexual dimorphism. In H. ergaster, males were only about 20 percent larger than females. This reduction in size difference between the sexes hints at less male-male competition for mates compared to prior hominins, such as australopithecines, and possibly indicates a shift towards a more monogamous mating system with greater parental investment in offspring.