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The species of Lucy was polygynous, new footprints show

New footprints of early bipedal hominins discovered at Laetoli, Tanzania, indicate marked body size variation among our 3.65 million-years-old ancestors and suggest a new insight into their social behaviour. This discovery is being published on the open-access journal eLife. Giovanni Boschian, professor at the University of Pisa, is one of the author of the study, together with Marco Cherin, University of Perugia, Giorgio Manzi, Sapienza University of Rome, Jacopo Moggi-Cecchi, University of Florence, and Fidelis T. Masao, University of Dar es Salaam.

The species of Lucy was polygynous, new footprints show
Reconstruction of the Laetoli palaeolandscape 
[Credit: Università di Pisa]
Fossil bones and teeth tell us a lot about various aspects of human evolution, but footprints are a different story. Footprints are rare: they can be impressed in the ground, preserved over time and eventually discovered millions of years later only because of unique circumstances. Like a spotlight on a prehistoric scene, fossil tracks provide data about the locomotion biomechanics and body size of the extinct creatures and reveal the diversity among individuals, explaining even their reproductive strategies.

The species of Lucy was polygynous, new footprints show
Four hominin tracks photographed at sunset in test-pit L8 at the Laetoli Site 
[Credit: Università di Pisa]
The new trackway was found at Laetoli, in the Ngorongoro Conservation Area of northern Tanzania, in the same area where the legendary Mary Leakey and her team of researchers discovered in the late 1970s a trackway of more than 3.6 million years ago, commonly attributed to Australopithecus afarensis (the species of the renowned Lucy). Surrounded by dozens of footprints of other mammals and birds and by raindrop impressions, this new trackway was left by two bipedal individuals walking on the same palaeosurface, at the same time and direction and at a similar moderate speed as those documented in the late 1970s.

The species of Lucy was polygynous, new footprints show
The Italian-Tanzanian research group working at Laetoli 
[Credit: Università di Pisa]
This novel evidence, taken as a whole with the previous one, portrays several bipedal early hominins moving as a group through the landscape, after a volcanic eruption and a subsequent rainfall. Of course, this is a fascinating image, but there is more. The footprints of one of the new individuals are astonishingly larger than anyone else’s in the group, suggesting he was a very large male of the species. This exceptional body size makes him the largest Australopithecus afarensis specimen identified so far.

The species of Lucy was polygynous, new footprints show
Preliminary digging and cleaning operations at the Laetoli Site 
[Credit: Università di Pisa]
A conclusion is that the Laetoli individuals were one male, two or three females and one or two juvenile individuals. This suggests that the traditional representation of the 1970s trackway with a couple of Australopithecus, romantically walking arm in arm and followed by their kid, can be misleading.

The species of Lucy was polygynous, new footprints show
Minimum and maximum estimated statures of selected fossil hominins 
[Credit: Università di Pisa]
Conversely, both the new composition of the group and the impressive body size difference suggested by the inclusive Laetoli footprint-set point to a considerable sexual dimorphism in Australopithecus afarensis. In turn, this view supports social organization and reproductive strategies closer to the polygynous gorillas than to other moderately dimorphic species, like the promiscuous chimpanzees and bonobos or most of the extant and, possibly, the extinct humans.

Source: Università di Pisa [December 14, 2016]

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1 comment :

  1. Lucy was not our direct ancestor. We already knew that A.afarensis was about as dimorphic as extant gorillas are, and this new fascinating study confirms it. In fact, there's nothing uniquely human in A.afarensis, e.g. its more humanlike feet are also seen in prenatal chimps & gorillas, whose flat feet become more handlike as they approach birth (C.Coon). Its low pelvis is also seen in humans & monkeys, but great apes evolved elongated iliac bones probably not so long ago in parallel. Lucy's curved hand-bones indicate frequent vertical climbing. Its flat feet are maladaptive in fast running, but are seen in wading or swimming animals. Australopiths are typically found in swamp forests & wetlands (K.Reed): this explains the curious combination of vertical spines, curved hand-bones & flat feet: they waded & walked bipedally in shallow water & at the waterside (this study) and climbed arms overhead in the branches above the swamps, much like extant bonobos & lowland gorillas (still) do, but much more frequently (the Pliocene was wetter & hotter, and had probably more wetlands & swamp forests). Dental, microwear, isotopic, gnathic & other evidence suggests australopiths had a lot of papyrus sedges & other wetland plants & hard-shelled invertebrates in their diet. For a good impression of Lucy's lifestyle IMO we have to google illustrations at "bonobo wading" or "gorilla bai".


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