Understanding the Origins of Human Bipedalism: A Deep Dive
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Chapter 1: The Significance of Bipedality
Bipedality, or the ability to walk upright on two legs, is a defining feature of human evolution. While many primates can stand and move for brief periods, humans uniquely rely on this posture for regular locomotion. Fossil evidence indicates that bipedalism may have initiated as far back as six million years ago. However, it was with the emergence of Australopithecus, an early hominin from southern and eastern Africa between four and two million years ago, that our ancestors became established bipeds. Despite this, researchers are still piecing together the reasons behind the development of this trait.
Carol Ward, a paleoanthropologist and anatomist at the University of Missouri, delves into this intriguing question. A specialist in human origins, Ward has dedicated numerous field seasons to various paleontological locations, including Kanapoi and Lomekwi in West Turkana, Kenya, where she and her colleagues uncovered australopithecine fossils. Her recent research employs advanced 3D medical imaging technologies to contrast the anatomy of contemporary primates, including soft tissues and organs, with the skeletal remains of ancient hominids. This innovative approach enables her to draw conclusions about our early ancestors and the ways their bodies adapted to different forms of movement. In a succinct lecture at the American Association for the Advancement of Science (AAAS) annual meeting in Washington, D.C., Ward emphasized that understanding the how and why of human bipedality is crucial for a broader comprehension of human evolution.
The following dialogue with Ward has been refined for clarity and brevity.
Section 1.1: Defining Bipedality
To begin with, could you explain bipedality in simple terms and highlight its importance in human evolution?
Human mobility stands apart from that of other animals on Earth. We traverse the ground upright on two feet, moving in a distinctive manner: one foot after the other, maintaining our body in an upright position through a specific series of movements. This behavior is not observed in any other primate and appears to have been present in some of the earliest members of our evolutionary lineage. This marked a significant adaptive shift from earlier apelike creatures. Thus, it is vital to understand the how and why of our unique walking pattern to grasp the divergence of our lineage from other primate species.
Section 1.2: The Debate: Bipedality vs. Brain Size
Which evolutionary step was more pivotal: the development of larger brains or the adoption of bipedality?
Undoubtedly, one of the most distinctive traits of humans is our remarkable brain. Our cognitive abilities surpass those of other animals, enabling culture and language, which define us as a species today. However, the fossil record indicates that our ancestors began walking upright around six to four million years ago, while significant increases in brain size in early hominins did not occur until after two million years ago. Therefore, for the majority of human evolution, brain size was not a major factor.
Chapter 2: Unraveling the Fossil Record
The evolution of bipedality remains enigmatic. Why is that?
This lack of clarity stems partly from the fossil record. Numerous apelike creatures thrived during the Miocene epoch, between 23 and 5 million years ago, yet none resemble modern humans. Additionally, gaps in the fossil record arise from geological factors, leading to a scarcity of fossils from certain time frames. Then, around four million years ago, we see evidence of committed bipeds, but transitional forms are not well-documented.
Nonetheless, between seven and four million years ago, several primates appear to have developed upright postures, albeit not as specialized as later forms.
Section 2.1: Insights from Early Fossils
Can you elaborate on the fossil evidence from this period?
Certainly! Several ape species existed before the divergence of chimps and humans. Sahelanthropus, from Chad, is estimated to be around six or seven million years old and is known from a single skull. Orrorin, discovered in Kenya, is identified from a thigh bone, a finger bone, and a few teeth, also around six million years old. Another significant species, Ardipithecus, is known from a variety of skeletal remains and likely dates between 5.5 and 4.5 million years ago. While it is related to both australopithecines and humans, it was not a fully committed biped, retaining a grasping big toe that indicates its tree-climbing capabilities.
The video "Evolution of Bipedalism" provides insights into how walking upright has played a crucial role in the development of early hominins.
Section 2.2: Examining Primates’ Torso Structures
In your lecture at the 2019 AAAS annual meeting, you discussed the significance of torso structure in primates and its role in upright walking. How do you investigate this aspect?
Limb bones are generally more robust and fossilize better than torso elements, such as ribs and vertebrae, which tend to be fragile. Additionally, torso bones are not often preserved together, making it challenging to reconstruct the entire structure.
Advancements in CT scanning technology have proven invaluable in this area. We can assess the overall shape of the torso and evaluate longstanding hypotheses regarding the relationship between body structure and locomotion. Over the past decade, my lab has harnessed these technologies to explore muscle structures in 3D, enabling us to test various theories.
Chapter 3: Innovative Technologies in Research
What role does this technology play in your research?
With these advancements, we're no longer limited to examining isolated bones. We can analyze bones in articulation, reflecting their natural arrangement. By applying medical imaging techniques, we can also study soft tissues in living primates to visualize muscle fibers. Understanding bone structure is only useful when we consider how muscles attach, along with associated nerves and circulation, which all contribute to behavior.
If a fossil reveals a unique feature, we can relate it to the animal's overall structure and soft tissues, leading to more informed conclusions.
The video "Three Mysteries of Human Evolution" explores key questions surrounding the development of traits such as bipedality and larger brains in our ancestors.
Section 3.1: Challenging Long-held Theories
How does this new research question established beliefs about human evolution?
By analyzing torso structures alongside new findings about australopithecines, we are discovering they may not be as similar to modern chimps as previously thought. Additionally, examining the ape fossil record reveals that these ancient species were also quite distinct from contemporary chimps. Applying the principles of parsimony, we may find that our ancestors did not evolve from a creature resembling modern chimpanzees or gorillas. Rather, we are beginning to clarify how ancient hominids differed or resembled each other, leading to a more comprehensive understanding of the ancestral traits that facilitated bipedality.
Natural selection can only act on the most recent evolutionary adaptations. To comprehend the reasons behind evolutionary changes, we must first understand the preceding developments.
Tom Garlinghouse is a science journalist based in Santa Cruz, California.