In regards to the skeletal reconstruction of Lucy, the 3.2 million-year-old Australopithecus, Jerry Coyne observed: “Lucy had a very apelike head with a chimp-sized braincase… One could not ask for a better transitional form between humans and ancient apes than Lucy. From the neck up, she’s apelike; in the middle, she’s a mixture, and from the waist down, she’s almost a modern human. And she tells us a critical fact about our evolution: our upright posture evolved long before our big brain.” (Coyne, 2009)
Lucy was the toast of 20th-century anthropology, a world-famous specimen who pushed the clock back on our bipedal ancestry. As noted, however, our understanding of human evolution is always subject to new discoveries.
In 1994 paleoanthropologist Tim White announced that his team had discovered the fossils of a new species in the Afar Rift region of Ethiopia. He named the new species Ardipithecus ramidus. He and his large team continued to toil in the Aramis desert for two decades of digging, scraping, and assembling new fossils before publishing their comprehensive findings in 2009. The results: they discovered over 100 bones from 36 individuals of the new species of hominin including the partial skeleton and crushed skull of a female they named Ardi.
Why is this significant? Ardi was about 4 feet tall, 100 lbs. with a chimp-sized braincase but smaller canines than a chimp’s much larger canines used for fighting and intimidating. She walked on two legs. And she was 4.4 million years old.
This pushes back the timeline on bipedal hominin evolution by 1.2 million years over Lucy. Moreover, the 47 scientists who contributed to the trove of scholarly research papers on the project concluded that Ardi didn’t live in a grassy savanna but in a lush, wooded environment. This changes the whole idea of why our ancestors started walking on two legs: the hypothesis that due to climate change (which began shrinking forests in certain African regions) our ancient ancestors had to walk erect to see over the tall grasses of the savanna and avoid predators. But since Ardi’s species lived in woodlands, there must be other evolutionary advantages to bipedalism.
Two major ideas were advanced. First, walking on two legs frees up the arms and hands to carry food and other objects. Second, knuckle-walking chimpanzees use 75% more energy locomoting than two-legged humans. Thus, walking on two legs is more energy efficient and also allows for greater versatility. (to be continued in the next post)