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No other animal moves like us. That is very strange. Even among other two-legged species, no one walks with a straight back and a gait that, technically, is just a form of controlled fall. Our bipedism not only distinguishes us, postulates paleoanthropologist Jeremy DeSilva; it is what makes us human.
There is no shortage of books that propose this or that characteristic – the use of tools or self-awareness, for example – as the very definition of humanity. But much of our supposed singularity does not resist this tradition. In the first steps, DeSilva takes a slightly different approach. Our way of walking, he argues, has provoked a number of consequences that inform our peculiar evolutionary history.
DeSilva begins his tour of the annals of bipedalism with other erect organisms. Tyrannosaurus and ancient relatives of crocodiles trot out to show how they moved on two legs, thanks to the long counterweight tails (SN: 6/12/20). DeSilva stumbles a bit here, as if arguing that "bipedalism has not been a successful locomotion for many dinosaur lineages." A whole group, the theropods, walked on two legs and still follow him with their avian costumes. But the comparison to the dinosaurs is still worth it. Without a tail, our way of walking is even stranger. "Let's face it," DeSilva writes, "humans are strangers."
Each subsequent chapter is safer when DeSilva guides readers through what we know about how our ancestors were bipedal. It is a windy popular science at its best, intertwining anecdotes from the field and the laboratory with scientific findings and the occasional reference to pop culture. DeSilva receives extra credit for appointing experts who have often overlooked making key discoveries.
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Instead of presenting a march of progress towards increasing bipedal perfection, DeSilva highlights how our ancestors had varied ways of walking upright, such as the somewhat wheeled march of Australopithecus sediba (SN: 25/07/13) . Our way of walking now, he argues, has been an evolutionary pathway among many possibilities.
But walking upright has opened up unique evolutionary pathways, DeSilva points out. Freed from locomotion, our arms and hands could be harder to create and manipulate tools. Our ancestors also developed a cup-shaped pelvis to comfortably cradle our viscera. But this arrangement complicated childbirth, especially when human babies began to have larger heads that needed to go through a narrow birth canal created by this anatomical change. DeSilva concludes that these compensations, including the debilitating twisted ankles and broken bones, may require our ancestors to care for each other. While it may be a step too far in speculation, it nevertheless makes a compelling case overall. “Our bipedal locomotion has been a gateway to many of the unique traits that make us human,” he writes, an evolutionary event that formed the context of how we came to be.
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