Australopithecus afarensis, humanity's ancient relative famously represented by the fossils of the 3.2 million year old Lucy, was a polygynous species, findings of a new study suggest.

Ancient Footprints In Laetoli, Tanzania

Researchers have found a new set of footprints left behind by a early human ancestors that walked on Africa more than 3.6 million years ago. The newly found footprints in Laetoli, a popular archeological site in Tanzania, add to a set of tracks discovered in 1978. 

William Jungers, a paleoanthropologist from the Stony Brook University said that the two sets of footprints could have been made on the same day millions of years earlier.

Size Differences Between Male And Female Australopithecus

The footprints show that the now extinct species have had major differences when it comes to the sizes of the sexes suggesting that the species may have been polygynous, a pattern of mating wherein the males have multiple female partners.

Earlier studies have suggested that polygyny causes intense competition among males to monopolize all females and thus favored the evolution of bigger males that can better deal with rivals.

The newly found footprints belonged to two individuals one of whom was likely male estimated to be about 5 feet 5 inches tall and 98.5 lbs. The other is believed to be a female who was about 4 feet 10 inches tall and 87 lbs.

The estimated size of the male makes the owner the largest Australopithecus afarensis to have so far been identified. The female is also taller than the female specimen that were earlier found in Laetoli.

Polygynous Species

By looking at the new prints and the ones discovered in 1978, the researchers said that several early hominids that walked with their two feet likely moved as a group through the area. Study researchers said that the group may have consisted of one male, two or three female and one or two juveniles.

The footprints hint of the social structure of the group. Citing the significant difference between the male and the females, the researchers said that the species may have socially mirrored gorillas known to have multiple females sharing one male mate. The male in the group and possibly other males of the species may have had more than one mate.

"Gorillas are polygynous species with strong sexual dimorphism due to intense male-male competition, whereas chimpanzees are promiscuous with definitively smaller sexual dimorphism," Marco Cherin, from Università di Perugia in Italy, and colleagues wrote in their study.

"It is reasonable to assume that complex relationships among body size, sexual dimorphism, mating system (and/or reproductive strategy) and social structure/behaviour also applied to extinct hominins."

The findings was published in the journal eLife on Dec. 14.