Final answer:
The research of Philip Reno suggests that reduced sexual dimorphism in early hominins is indicative of cooperative social structures, likely reflecting monogamous systems or pair bonding as an adaptation for enhanced offspring survival and mutual parental investment.
Step-by-step explanation:
Sexual Dimorphism and Evidence for Hominin Social Structures
Based on the research of Philip Reno, reduced levels of sexual dimorphism observed in early hominins point towards a cooperative social structure. This can be seen as evidence for social systems that contrast with those of highly sexually dimorphic species, which usually exhibit strong male dominance and competition for mates. Reduced sexual dimorphism may indicate the existence of strategies such as monogamy, pair bonding, or perhaps multifamily groupings where competition is less pronounced, and cooperation plays a more vital role.
The clearest implication of reduced sexual dimorphism is related to the monogamous pairing of individuals, which implies mutual and sustained cooperation between mating partners. Given the complexities highlighted in studies of primate societies, where cooperation can often supersede competition, it is likely that early hominins adopted a social structure favouring monogamy or pair bonding, which are systems inherently cooperative in nature. These structures would potentially provide advantages in terms of offspring survival and parental investment, aligning with the male-assistance hypothesis where males aid in rearing young to ensure reproductive success.
Therefore, based on Reno's findings and supported by studies of primate and early human behavior, answer a. monogamy is the most likely evidence for cooperation indicated by reduced sexual dimorphism in early hominins.