In a line of human ancestors that lived more than 3 million years ago, adult males were only around 15 percent larger than adult females, a new study finds.
Such a moderate sex difference in Australopithecus afarensis suggests that males in the ancient species formed coalitions with each other and often established monogamous relationships with females just as do modern human males and those of other species with nearly equal-size sexes, say Philip L. Reno of Kent (Ohio) State University and his coworkers. A. afarensis is best known for the partial skeleton called Lucy found nearly 30 years ago in Ethiopia.
Prior research indicated that A. afarensis males were substantially larger than females, as is the case for male gorillas and orangutans, which can be 50 percent larger than females. Such species typically feature a lot of fighting among males and frequent switching of sexual partners.
The size gap between genders closed for Lucy’s kind when Reno’s team used new statistical methods to estimate body proportions and identify sexes using fossils from more than 20 A. afarensis individuals. Skeletal analyses of people, chimpanzees, and gorillas indicated that the modest size difference between A. afarensis sexes matched that between the human sexes, the scientists report in an upcoming Proceedings of the National Academy of Sciences.
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