InformedHealth.org How does our sense of balance work?
To walk upright successfully needs a fully functioning balance organ in the inner ear, as anyone suffering from Ménière's disease will testify, so the study of the origins of bipedalism in the remote ancestors of humans needs to take into account changes in the inner ear that would facilitate it.
Humans and our closest relatives, the great apes and the simians, display a range of locomotion but only humans are normally fully bipedal, although chimpanzees can use bipedal locomotion when carrying a load for example.
The monkeys normally run along branches on all fours, balanced on top of them and jumping from branch to branch; the apes hang beneath the branches in locomotion known as brachiation, but humans are ungainly in trees and prefer bipedal locomotion on the ground. The question is, when did this ability evolve in our ancestry?
We can be sure our hominin ancestors the Australopithecines, were fully bipedal because we have a record of their footprints in volcanic ash at Laetoli, and their lower limbs and feet were almost indistinguishable from those of Homo sapiens. 'Lucy' (Au afarensis) was probably mostly bipedal but may have taken to trees for safety and possibly to sleep on constructed platforms like chimpanzees do. The evidence of injuries to her fossilised skeletal remains suggests she may have died by falling out of a tree.
To investigate this stage in our evolution a group of researchers, led by Professor Xijun Ni, which included Yinan Zhang, a doctoral student, both of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (IVPP), and Terry Harrison, a New York University anthropologist, used 3-dimensional CT scanning to examine the inner ear of a 6-million-year-old fossil ape, Lufengpithecus, unearthed in China’s Yunnan Province in the early 1980s, and compared it to the inner ear of other living and fossil apes and humans from Asia, Europe, and Africa.
The formation the fossil was found in has been previously dated magnetobiostratigraphically to about 6 million years. This technique depends on the record of periodic changes in Earth's polarity trapped in magnetic particles in sedimentary rocks and by recording the microfossils such as pollen associated with these changes:
What is magnetobiostratigraphic dating and how is it used by palaeontologists? Magnetobiostratigraphy is a dating method used by paleontologists and geologists to determine the age of rocks and fossils based on changes in Earth's magnetic field and the fossilized remains of organisms. This technique relies on the principles of both magnetostratigraphy and biostratigraphy.
- Magnetostratigraphy:
- Earth's magnetic field has undergone changes over geological time. As molten rock cools and solidifies, magnetic minerals within the rocks align themselves with the prevailing magnetic field. This creates a record of the Earth's magnetic history in the rock layers.
- Biostratigraphy:
- Biostratigraphy involves the study of the distribution of fossils within rock layers. Different species of organisms existed at different times in Earth's history, and the appearance or disappearance of specific fossilized species can be used to correlate and date rock layers.
How Magnetobiostratigraphy Works:Magnetobiostratigraphy is particularly valuable for dating marine sediments and is often used in conjunction with other dating methods to refine the age estimates. It provides a valuable tool for understanding the timing of geological events and the evolution of life on Earth.
- Sampling:
- Researchers collect samples of sedimentary rocks or volcanic rocks containing fossils. These rocks must preserve a record of the Earth's magnetic field at the time they formed.
- Magnetic Analysis:
- The researchers analyze the magnetic orientation of minerals within the rock. This involves measuring the direction and strength of the magnetic field recorded in the rock.
- Biostratigraphic Analysis:
- Concurrently, the fossils within the rock are studied to identify the species present. Biostratigraphic markers, such as the first appearance or last appearance of certain fossil species, are noted.
- Correlation:
- By comparing the magnetic data with the known magnetic history of Earth (polarity chronology), researchers can correlate the magnetic changes recorded in the rock layers with specific time intervals.
- Dating:
- The combined analysis of magnetic and biostratigraphic data allows scientists to assign an age to the rock layers. This age determination is based on the known sequence of magnetic polarity changes and the fossil assemblages present.
The team's research is published in the Cell Press journal, The Innovations and is explained in a press release from New York University:
The semicircular canals, located in the skull between our brains and the external ear, are critical to providing our sense of balance and position when we move, and they provide a fundamental component of our locomotion that most people are probably unaware of. The size and shape of the semicircular canals correlate with how mammals, including apes and humans, move around their environment. Using modern imaging technologies, we were able to visualize the internal structure of fossil skulls and study the anatomical details of the semicircular canals to reveal how extinct mammals moved.
Yinan Zhang, lead author
Doctoral student
Institute of Vertebrate Paleontology and Paleoanthropology
Chinese Academy of Sciences (IVPP).Most studies of the evolution of ape locomotion had focused on comparisons of the bones of the limbs, shoulders, pelvis, and spine and the way they are associated with the different types of locomotor behaviors seen in living apes and humans. However, the diversity of locomotor behaviors in living apes and the incompleteness of the fossil record have hampered the development of a clear picture of human bipedalism’s origins. The skulls of Lufengpithecus—originally discovered in China’s Yunnan Province in the early 1980s—have given scientists the opportunity to address, in new ways, unanswered questions about the evolution of locomotion. However, the heavy compression and distortion of the skulls obscured the bony ear region and led previous researchers to believe that the delicate semicircular canals were not preserved. To better explore this region, Zhang, Ni and Harrison, along with other researchers at IVPP and the Yunnan Institute of Cultural Relics and Archaeology (YICRA), used three dimensional scanning technologies to illuminate these portions of the skulls to create a virtual reconstruction of the inner ear’s bony canals. They then compared these scans to those collected from other living and fossil apes and humans from Asia, Europe, and Africa.Our study points to a three-step evolution of human bipedalism. First, the earliest apes moved in the trees in a style that was most similar to aspects of the way that gibbons in Asia do today. Second, the last common ancestor of apes and humans was similar in its locomotor repertoire to Lufengpithecus, using a combination of climbing and clambering, forelimb suspension, arboreal bipedalism, and terrestrial quadrupedalism. It is from this broad ancestral locomotor repertoire that human bipedalism evolved.
Terry Harrison, co-author
Anthropologist
New York University, NY, USA.
Reconstruction of the locomotor behavior and paleoenvironment of Lufengpithecus.
Illustration by Xiaocong Guo; image courtesy of Xijun Ni,
Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences.By studying the rate of evolutionary change in the bony labyrinth, the international team proposed that climate change may have been an important environmental catalyst in promoting the locomotor diversification of apes and humans.Our analyses show that early apes shared a locomotor repertoire that was ancestral to human bipedalism. It appears that the inner ear provides a unique record of the evolutionary history of ape locomotion that offers an invaluable alternative to the study of the postcranial skeleton. Most fossil apes and their inferred ancestors are intermediate in locomotor mode between gibbons and African apes. Later, the human lineage diverged from the great apes with the acquisition of bipedalism, as seen in Australopithecus, an early human relative from Africa.
Professor Xijun Ni, senior author
Project leader
Institute of Vertebrate Paleontology and Paleoanthropology
Chinese Academy of Sciences (IVPP).Cooler global temperatures, associated with the build up of glacial ice sheets in the northern hemisphere approximately 3.2 million years ago, correspond with an uptick in the rate of change of the bony labyrinth and this may signal a rapid increase in the pace of ape and human locomotor evolution
Terry Harrison
In their open access paper just published in the Cell Press journal The Innovation the scientists say:
So, we have evidence of the evolution of the inner ear as a record of the evolution of a bipedal gait in humans, and the evidence that the scientists who investigated it had no doubts about the suitability of the Theory of Evolution for explaining the observations. We also have the evidence of the existence of hominins 6 million years before creationists think the Universe was magicked up out of nothing with some magic words.Public summaryGraphic abstract Various lines of evidence have been used to infer the origin of human bipedalism, but the paucity of hominoid postcranial fossils and the diversity of inferred locomotor modes have tended to confound the reconstruction of ancestral morphotypes. Examination of the bony labyrinth morphology of the inner ear of extinct and living hominoids provides independent evidence for inferring the evolution of hominoid locomotor patterns. New computed tomography data and morphometric analyses of the Late Miocene ape Lufengpithecus indicate that it and other stem great apes possess labyrinths similar to one another and show that hominoids initially evolved from a positional repertoire that included orthogrady, below-branch forelimb suspension and progression, above-branch bipedalism, climbing, clambering, and leaping (hylobatid-like) to one that comprised above-branch quadrupedalism, below-branch forelimb suspension, vertical climbing, limited leaping, terrestrial quadrupedal running and walking, possibly with knuckle walking, and short bouts of bipedalism (chimpanzee-like). The bony labyrinth morphology of Lufengpithecus indicates that it probably conforms more closely to the last common ancestors of crown hominoids and hominids in its locomotor behavior than do other Miocene hominoids. Human bipedalism evolved from this common archetypal Lufengpithecus-like locomotor repertoire. The low evolutionary rate of semicircular canal morphology suggests that Lufengpithecus experienced a relative stasis in locomotor behavior, probably due to the uplift of the Tibetan Plateau, which created a stable environment in the Miocene of southwestern China.
- Shapes of the balance organs within the inner ear shared between Lufengpithecus nd other Miocene apes support that they shared a common pattern of locomotion.
- The ability to habitually walk and run upright on two feet in human probably evolved from this Lufengpithecus-like locomotion.
- Global cooling around 3.2 million years ago may have triggered an acceleration in the evolution of human bipedal locomotion.
Introduction
Determining the evolutionary pathway to human bipedalism from the quadrupedal arboreal locomotor modes of apes has been a critical question examined from a variety of perspectives and using a diversity of datasets, but it is one that is currently unresolved. It has been suggested that chimpanzee- and gorilla-like knuckle walking, orangutan-like brachiation and clambering, or hylobatid-like arboreal bipedalism may represent the ancestral locomotor pattern that gave rise to human bipedality.1,2,3,4,5,6 However, no definitive ancestral locomotor morphotypes have been inferred for the last common ancestors (LCAs) of hominoids, hominids, or hominins. Studies of the postcranial fossils of Miocene hominids, including the early hominins Ardipithecus and Sahelanthropus, indicate possible positional behaviors that are not found among extant apes.2,7,8,9,10,11,12,13 The bony labyrinth of the inner ear of vertebrates houses the peripheral vestibular system comprised of three fluid-filled semicircular canals that are functionally tied to sense of balance, spatial orientation, posture, and body movements. This, in turn, is linked to modes of locomotion among living and extinct taxa.14,15,16,17,18,19 Comparative morphology and experimental evidence from across Vertebrata support the hypothesis that vestibular shape reflects aspects of positional behavior and that the brain and cranium are likely shaped in part by natural selection on the semicircular canals for locomotor function.15 Given the lack of consensus in the determination of ancestral locomotor repertoires derived from the fossil record and analyses of postcranial anatomy, a detailed examination of the bony labyrinth of extant and extinct apes allows for the formulation of testable hypotheses regarding the evolution of human and ape locomotor behavior. Here, we report on the bony labyrinth morphology of the Asian Miocene ape Lufengpithecus. Our morphometric analyses provide evidence to support a close similarity between Lufengpithecus nd European Miocene dryopiths (i.e., Rudapithecus and Hispanopithecus) and extant African apes.20,21 The results shed new light on reconstructing the ancestral locomotor mode of hominids and the origin of human bipedalism.
The Late Miocene great ape Lufengpithecus (∼7–8 Ma) from southern China was initially thought to be closely related to Ramapithecus, a purported early human ancestor.22 This viewpoint has since been abandoned, but the phylogenetic relationship between Lufengpithecus nd other hominoids remains unsettled. The molars of Lufengpithecus have relatively thick enamel, peripheralized cusps, expansive occlusal basins, and dense and complex enamel crenulations.23,24 These features are considered to be similar to those of orangutans.25,26 Some cranial features, such as broad interorbital distance, stepped nasoalveolar clivus to the nasal passage floor, square orbits, and presence of a frontal sinus, indicate that Lufengpithecus may have a closer phylogenetic relationship with African apes.27 Various authors have suggested that Lufengpithecus is a member of the Sivapithecus-orangutan clade, the sister taxon of extant great apes and humans, or a stem hominid.23,27,28,29,30,31 The largest collection of Lufengpithecus (L. lufengensis) specimens is from Shihuiba in Lufeng County, Yunnan Province. During the excavations of 1975–1983, four relatively complete but crushed crania of male and female specimens were recovered.32 We applied high-resolution computed tomography (CT) scanning to these crania, and this revealed that three petrosals were preserved (supplemental information).
Figure 4 Polymorphospace of the bony labyrinths and conceptual graph of hominoid locomotor evolution (A) Canonical plot of quadratic discriminant analysis. Green ellipses are 95% confidence area of extant primates and humans. Symbols for fossil apes are shown in (B).
(B) The hominoids developed two main generalized locomotor modes. Human bipedalism was derived from a common Lufengpithecus-like locomotor mode. Orange, blue, and violet lines represent three alternative phylogenetic hypotheses.Zhang, Yinan; Ni, Xijun; Li, Qiang; Stidham, Thomas; Lu, Dan; Gao, Feng; Zhang, Chi; Harrison, Terry
Lufengpithecus inner ear provides evidence of a common locomotor repertoire ancestral to human bipedalism
The Innovation doi: 10.1016/j.xinn.2024.100580
Copyright: © 2024 The authors.
Published by Cell Press. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
And so, another bad week for creationism continues unabated, and the evidence against creationism that creationists need to ignore continues to pile up.
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