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Scientists finally uncovered why human shoulders and elbows first evolved

[Sept. 7, 2023: Staff Writer, The Brighter Side of News]

Apes and early humans, bearing heavier bodies compared to their arboreal counterparts, faced a challenge when descending from trees. (CREDIT: Creative Commons)

The mere act of reaching for a high shelf or tossing a ball at the park may have deeper evolutionary roots than once thought. In the pages of the Royal Society Open Science journal, Dartmouth researchers present a riveting revelation: our ancestors might've developed rotating shoulders and extending elbows as a crucial braking system when descending trees.

Apes and early humans, bearing heavier bodies compared to their arboreal counterparts, faced a challenge when descending from trees. The pull of gravity on their substantial mass made safe descents tricky. Over time, free-moving shoulders and flexible elbows might have become the natural answer to this problem.


As these early humans transitioned from dense forests to the expansive savannas, these versatile appendages proved indispensable. They not only facilitated food gathering but also equipped humans with the dexterity required for tool use, hunting, and defense.

Researchers adopted sports-analysis and statistical software to study videos of chimpanzees and small mangabey monkeys clambering about their natural habitats.


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"Chimps and mangabeys both climb with their limbs held close, but descending, chimps extend their arms, reminiscent of someone climbing down a ladder," observed Luke Fannin, the study's lead author and a graduate student at Dartmouth's Ecology, Evolution, Environment, and Society program.

He added, "It's among the first pieces of evidence highlighting the importance of 'downclimbing' in the evolutionary journey of apes and early humans."


Challenging Established Ideas

Jeremy DeSilva, study co-author and chair of anthropology at Dartmouth, remarked, "We've concentrated on how apes climb up trees. But we've overlooked how they get down."

Human shoulders and elbows first evolved as brakes for climbing apes – study (CREDIT: Danny Lawson/PA)

DeSilva elucidates the evolutionary trajectory: "Apes, evolving 20 million years ago, would ascend trees for nourishment and then descend to move to the next source. As larger animals, falls posed life-threatening risks, making safe descent crucial."


Australopithecus, an early human ancestor, benefited from this legacy of flexible anatomy, skillfully climbing up for nocturnal refuge and descending by day. "By the time Homo erectus harnessed fire for protection, our anatomy had further refined. Apes might struggle with an accurate throw, but the evolution in early human anatomy rendered them adept at spearing," DeSilva pointed out.

Dartmouth researchers report that apes and early humans evolved more flexible shoulders and elbows than monkeys (above) to safely get out of trees. For early humans, these versatile appendages would have been essential for gathering food and deploying tools for hunting and defense. (CREDIT: Luke Fannin, Dartmouth)

Drawing a parallel with modern times, he added, "Today, when an NFL quarterback throws a football, he unwittingly pays homage to our ape lineage."


A Close Anatomical Resemblance

Despite the sometimes ungainly descent of chimps, their limbs, optimized for safe grounding, bear striking similarities to modern humans. Fannin stated, "Our evolutionary past, marked by challenges in descending, has indelibly stamped our current skeletal structure."

Chimp and mangabey skeletal structures, studied in collaboration with Harvard and The Ohio State University, further highlighted these differences. While chimps have a shallow ball-and-socket shoulder granting vast movement range, mangabeys resemble quadrupedal animals, with more limited flexibility.

Nathaniel Dominy, study co-author, noted, "Descending is more challenging than ascending. Our research brings formal measurements into this debate, demonstrating the crucial differences between monkeys and chimps, potentially explaining the evolutionary need for enhanced flexibility in apes."

Mary Joy, another co-author, while analyzing descent videos, commented on the chimps' descent being "a controlled fall." Drawing parallels with her trail-running experiences, she pointed out, "Speed and momentum in descending could be a more energy-efficient approach for heavier primates."


Joy went on to philosophize about human movement as a culmination of evolutionary trade-offs, wondering about the evolutionary benefits if this range of motion were to be reduced. "What's the evolutionary advantage of reduced mobility? I can't discern any," she mused.

Bouts of vertical climbing in chimpanzees (a,b) and sooty mangabeys (c,d). Maximum angles of shoulder flexion and elbow extension were greater during downclimbs (b,d) compared to upclimbs (a,c), and the magnitudes of these differences were greatest among chimpanzees. (CREDIT: Royal Society Open Science)

In summary, the dynamics of descending from trees, an activity so easily overlooked, has been seminal in shaping our evolutionary journey. As we stretch, reach, and throw, we unknowingly echo the survival instincts of our ancient forebears, marking a testament to evolution's enduring legacy.


The paper, "Downclimbing and the evolution of ape forelimb morphologies," was published by Royal Society Open Science.

For more science news stories check out our New Discoveries section at The Brighter Side of News.


Note: Materials provided above by The Brighter Side of News. Content may be edited for style and length.


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