BIPEDALISM THEORIES
apes and man Bipedalism (moving on two legs) is one of the key characteristics that defines hominins and humans. A hominin after all is defined as a creature that stands upright and walks and runs primarily on two legs. It is thought that bipedalism developed in hominins between 4 million and 8 million years ago. No one is sure how or why some apes, who were for the most part arboreal at that time, dropped out of the trees and began walking upright. A popular theory is that it first evolved to free the hands to carry food.
No one really knows why hominins became bipedal. Maybe it because it helped them see threats and food options better. Or perhaps, it helped them reach up into trees for fruit or cool down in the hot African climate. [Source: Bob Yirka, Phys.org. December 13, 2011 =*=]
Some speculate that hominins rose up on their feet to get a better view over high grasses. Others say an upright stance exposed less body to direct sunlight in a hot climate than a body standing on all fours. According to one theory — first proposed in the middle of the 20th century that has largely been dismissed but still has its believers — when the Red Sea pored into northern Rift Valley of Africa 6 million years ago, patches of high ground became islands and vast areas became shallow lagoons. Under these conditions standing up and using one’s hands was the best way to open mollusks and tear apart crustaceans — as capuchin monkeys and crab-eating macaquess do — and walking upright was best suited for roaming the shallows of these lagoons loookung for shellfish, crsutaceans and small fish — food sources that were easy to exploit. [Source: David Attenburough]
Charles Q. Choi wrote in for Live Science: “Our ancestors evolved an upright posture well before our large brains or stone tools even appeared. The question, then: Why stand and walk on two legs when our ape cousins get by on four limbs? Walking as bipeds might actually use less energy than movement on all fours does. Freeing up the arms might also have enabled our ancestors to carry more food. Standing upright might even have helped them control their temperature better by reducing the amount of skin directly exposed to the sun.” [Source: Charles Q. Choi, Live Science, February 22, 2011]
The discovery of an Ardipithecus-like foot in the 2010s from 3.4-million-year-old deposits at Burtele, Ethiopia, further shows that at least two different forms of bipedalism coexisted in the Pliocene.
See Separate Articles: HOMININ BIPEDALISM: COSTS, BENEFITS. WALKING AND RUNNING factsanddetails.com
Walking in the Trees and the Development of Bipedalism
Laura M. MacLatchy, Kieran McNulty and Dan Peppe wrote: Based on the lifestyle of apes alive today, scientists have hypothesized that the very first ones evolved in dense forests, where they successfully fed on fruit, thanks to a few key anatomical innovations. Apes have stable, upright backs. Once the back is vertical, an ape no longer has to walk on the top of small branches like a monkey. Instead, it can grab different branches with its arms and legs, distributing its body mass across multiple supports. Apes can even hang below branches, making them less likely to lose their balance. In this way, they are able to access fruits growing on the edges of tree crowns that otherwise might be available only to smaller species. Laura M. MacLatchy, Kieran McNulty and Dan Peppe wrote: [Source: Laura M. MacLatchy, Professor of Anthropology, University of Michigan, Kieran McNulty, Professor of Anthropology, University of Minnesota, and Dan Peppe, Associate Professor of Geosciences, Baylor University, The Conversation, April 14, 2023]
homo footprints Paleontologist Eric Delso argue that "human ancestors may have moved to the ground more to take advantage of opportunities on the open ground and less because they were forced to." Many scientists believe that bipedalism developed in the trees, where some apes, and perhaps early hominins, walked upright on large branches or stood on their leg to collect fruit overhead. Later they found bipedalism was more than efficient than walking on all fours or leaping from tree to trees when it came time for feeding on trees from the ground and moving from one tree to another in a less dense forests.
Scientists from the College de France believe that bipedalism developed when apes were in the trees. They say that tree branches are ideal place to learn to walk because there are branches that can be used for supports. Robin Compton of Liverpool University told the Observer, “Trees were an ideal nursery for the learning of human walking. they enable an animal to balance itself. They can reach out in any direction, above and below themselves, and find branches, Orangutans do just ths sort of thing.” The clam is backed by evidence of bipedalism in the bones of 6 million-year-old apes found in Kenya and early hominins found in Ethiopia.
In a June 2007 article in the journal Science, Susannah Thorpe and Roger Holder of the University of Birmingham and Robin Crompton of the University of Liverpool suggested that bipedalism arose much earlier than previously thought among arboreal apes — perhaps as early as between 17 million and 24 million years ago — based on the way wild orangutans navigate their way along fragile tree branches. Thorpe spent a year observing wild orangutans in the forest of Sumatra and saw them walk on two legs on fragile branches to reach fruit, using their arms to keep balance or grasp for fruit while using all four limbs on bigger branches. The finding is significant in that it shows bipedalism might have first evolved as a way to move around in the trees rather than on the ground.
Another theory, known among some scientists as "East Side Story," hypothesize that the sudden expansion and deepening of the Great Rift Valley led to the evolution of mankind. Chimpanzees and gorilla are found almost exclusively on the forested west side of the valley while fossils of ancient upright walking homonids have been almost exclusively on the grassy east side. [Source: John Noble Wilford, New York Times, May 17, 1994]
Bipedalism: Result of Rugged Landscape?
Ardipithecus A new study by archaeologists at the University of York challenges evolutionary theories behind the development of our earliest ancestors from tree dwelling quadrupeds to upright bipeds capable of walking and scrambling. According to the University of York: The researchers say our upright gait may have its origins in the rugged landscape of East and South Africa which was shaped during the Pliocene epoch by volcanoes and shifting tectonic plates. [Source: University of York, phys.org, May 24, 2013]
“Hominins, our early forebears, would have been attracted to the terrain of rocky outcrops and gorges because it offered shelter and opportunities to trap prey. But it also required more upright scrambling and climbing gaits, prompting the emergence of bipedalism. The York research challenges traditional hypotheses which suggest our early forebears were forced out of the trees and onto two feet when climate change reduced tree cover. The study — 'Complex Topography and Human Evolution: the Missing Link' — was developed in conjunction with researchers from the Institut de Physique du Globe in Paris and published in the journal Antiquity.
“Dr Isabelle Winder, from the Department of Archaeology at York and one of the paper's authors, said: "Our research shows that bipedalism may have developed as a response to the terrain, rather than a response to climatically-driven vegetation changes. "The broken, disrupted terrain offered benefits for hominins in terms of security and food, but it also proved a motivation to improve their locomotor skills by climbing, balancing, scrambling and moving swiftly over broken ground - types of movement encouraging a more upright gait."
“The research suggests that the hands and arms of upright hominins were then left free to develop increased manual dexterity and tool use, supporting a further key stage in the evolutionary story. The development of running adaptations to the skeleton and foot may have resulted from later excursions onto the surrounding flat plains in search of prey and new home ranges. Dr Winder said: "The varied terrain may also have contributed to improved cognitive skills such as navigation and communication abilities, accounting for the continued evolution of our brains and social functions such as co-operation and team work. "Our hypothesis offers a new, viable alternative to traditional vegetation or climate change hypotheses. It explains all the key processes in hominin evolution and offers a more convincing scenario than traditional hypotheses."
Transition From Trees to the Ground Was Gradual
The last common ancestor of humans and chimpanzees are believed to have had shoulders similar to those of modern African apes, a finding supports the theory that early humans moved away from life in trees gradually. Charles Q. Choi wrote in Live Science: “The human lineage diverged from that of chimpanzees, humanity's closest living relative, about 6 million or 7 million years ago. Knowing the characteristics of the last common ancestor of humans and chimps would shed light on how the anatomy and behavior of both lineages evolved over time, "but fossils from that time are rare," said lead author of the new study Nathan Young, an evolutionary biologist at the University of California, San Francisco. [Source: Charles Q. Choi, Live Science, September 8, 2015 +]
“There are currently at least two competing scenarios for what the last common ancestor might have looked like. One suggests that similarities seen in modern African apes, such as in chimps and gorillas, were inherited from the last common ancestor, meaning that modern African apes may reflect what the last common ancestor was like. "A lot of people use chimpanzees as a model for the last common ancestor," Young told Live Science. +\
“The other scenario suggests these similarities instead evolved independently in modern African apes, and that the last common ancestor may have possessed more-primitive traits than those seen in modern African apes. For instance, instead of knuckle-walking on the ground like chimps and gorillas do, the last common ancestor may have swung and hung from tree branches like orangutans, which are Asian apes. Humans aren't the only species that have evolved and changed over time — chimpanzees and gorillas have evolved and changed over time, too, so looking at their modern forms for insights into what the last common ancestor was like could be misleading in a lot of ways," Young said. +\
“The ancestral state of the shoulder is key to understanding human evolution, because the shoulder is linked to many important shifts in behavior in the human lineage. Shoulder evolution could help show when early human ancestors began using tools more, spent reduced time in trees and learned to throw weapons. However, the human shoulder possesses a unique combination of features that makes it difficult to reconstruct the body part's history. For instance, while humans are most closely related to knuckle-walking chimps, in some respects the human shoulder is more similar in shape to that of tree-dwelling orangutans. +\
“To see what the shoulder of the last common ancestor might have looked like, researchers generated 3D shoulder models from museum specimens of modern humans, chimps, bonobos, gorillas, orangutans, gibbons and monkeys. The scientists compared these data with 3D models that other scientists previously generated of ancient, extinct relatives of modern humans, such as Australopithecus afarensis, Australopithecus sediba, Homo ergaster and Neanderthals. +\ Australopithecines such as Australopithecus afarensis and Australopithecus sediba are the leading candidates for direct ancestors of humans. "Recent data from the australopithecines helped us now test different models of human evolution," Young said. +\
“The scientists found the strongest model showed the human shoulder gradually evolving from an African apelike form to its modern state. "We found australopithecines were perfect intermediate forms between African apes and modern humans," Young said. This finding suggests the human lineage experienced a long, gradual shift out of the trees and increased reliance on tools as it became more terrestrial, he said. "These results pretty much confirm that the simplest explanation for how the human shoulder evolved is the most likely one," Young said. The scientists detailed their findings online Sept. 7, 2015 in the journal Proceedings of the National Academy of Sciences.” +\
chimp and human thighbones
Climate Theory, Changes from Forests to Savannah, and Bipedalism
Laura M. MacLatchy, Kieran McNulty and Dan Peppe wrote: Human evolution is tightly connected to the environment and landscape of Africa, where our ancestors first emerged. For a long time, researchers have linked the expansion of grasslands in Africa to the evolution of numerous human traits, including walking on two legs, using tools and hunting. According to the traditional scientific narrative, Africa was once a verdant idyll of vast forests stretching from coast to coast. In these lush habitats, around 21 million years ago, the earliest ancestors of apes and humans first evolved traits — including upright posture — that distinguished them from their monkey cousins.
But then, the story went, global climates cooled and dried, and forests began to shrink. By about 10 million years ago, grasses and shrubs that were better able to tolerate the increasingly dry conditions started to take over eastern Africa, replacing forests. The earliest hominins, our distant ancestors, ventured out of the forest remnants that had been home onto the grass-covered savanna. The idea was that this new ecosystem pushed a radical change for our lineage: We became bipedal. [Source: Laura M. MacLatchy, Professor of Anthropology, University of Michigan, Kieran McNulty, Professor of Anthropology, University of Minnesota, and Dan Peppe, Associate Professor of Geosciences, Baylor University, The Conversation, April 14, 2023]
Norman Owen-Smith wrote: Starting during the late Miocene, around 10 million years ago, a plume of molten magma, hot liquid material from deep inside the Earth, pushed eastern parts of Africa upward. This led to rifting of the Earth’s crust, volcanic eruptions and soils enriched in mineral nutrients from the lava and ash. Grassy savannas spread and animals adapted increasingly to graze this vegetation component. Apes from that time were forced to spend less time up in trees and more time walking upright on two legs. [Source: Norman Owen-Smith, Emeritus Research Professor of African Ecology, University of the Witwatersrand, The Conversation, January 26, 2023]
Progressive reductions in rainfall, restricting plant growth and worsening dry season aridity, forced the early ape-men, (Australopithecines), to change their diet. They went from eating mainly fruits from forest trees to consuming underground bulbs and tubers found between the widely spaced trees. These were tough to extract and chew. This led to the emergence through evolution of the genus Paranthropus (colloquially “nutcracker man”), characterised by huge jaws and teeth. By about a million years ago they were gone. Apparently, the effort of extracting and processing these well-defended plant parts became too formidable.
Abrupt Climate Changes at the Time Hominins First Evolved?
Some scientists theorize that hominins developed as separate species during periods of climate change, theorizing an abrupt climate change brought about the development of the upright walking style and a larger brain. The reasoning goes something like this: the cooler and drier climate that occurred in eastern Africa after the creation of the Panama isthmus transformed rain forests into savannahs, where food sources are more scattered and farther away. The changes forced proto-hominins out of the trees in the rain forests and onto savannah grasslands, where they needed a larger brain to develop more complicated food gathering tasks, locate scarce food and remember when they were in season.
The apes that lived in the new environment found that walking upright and moving about on two legs was a much better way for getting around on the ground than knuckle-walking style that chimpanzees and gorillas now employ. Walking upright was the most energy efficient way to cover distances and reach scattered food sources. It also freed the hands to gather a broad range of foods. The erect posture kept the body cool by exposing more skin to breezes and less skin to the sun. Later, a larger brain helped hominins to develop strategies to hunt large animals and use tools which in turn requiring more intellectual reasoning and larger brain, speeding along the evolution process.
Before 3.5 million years ago, North and South America were not connected and waters from the Atlantic and Pacific mixed and lowered salinity levels in the Atlantic, which meant that lighter water from the tropics was carried all the way to the Arctic Ocean. When the isthmus of Panama was later created, water from the Atlantic and the Pacific no longer mixed, which increased the level of salinity in the North Atlantic Current, causing it to sink before it reached the Arctic, causing an icecap to form there. The changes also caused the northern diversion of the equatorial Atlantic Ocean current and the intensification of the Gulf Stream, which resulted in more snowfall in the north and the built up of glaciers. These changes led to an ice age. Between 2.8 and 2.5 million years ago glaciers began creeping from the Arctic Ice cap down over much of the northern hemisphere and the climate in Africa became noticeable colder and drier. Evidence of rainfall and climate changes in the period is based on analysis of dust particles in rock strata and pollen deposited in a coastal ocean-floor sediments.
Support and Criticism of Climate Theory of Bipedalism
Backing up this so-called climate theory of bipedalism are primate studies. Fruit eating primates, for example, generally have a larger brain than leaf-eating ones because it requires more sophisticated reasoning, scientists suggest, to find food which is only found in some areas at certain times of the year. The brain of the leaf-spider monkey, for instance, is only half the size of the fruit-eating howler monkey, even though occupy roughly the same terrain. [Source: Eugene Linden, National Geographic, March 1992]
Arctic ice-cores and the fossil record thus far seem to indicate that global climatic changes took place about the same time as major developments in human evolution. A cold, dry spell about 2.8 million years ago, for instance, associated with appearance of grasslands in Africa occurred at about the same time as the first homonids. Another cold period a million years coincides with extinction of the genus Australopithecus. During these same time periods forests antelopes were replaced by giant buffalo and other grazers.
There are lot of problems with the climatic theory and bipedalism. Some studies indicate the savannah grasslands in and around the Great Rift Valley, where almost all of remains of our earliest ancestors have been found, have remained pretty much unchanged for the last 15 million years. Bone samples, dated Between 2.8 and 2.5 million years ago, from Lake Turkana provide no evidence of abnormally rapid evolutionary activity at this time. This has led scientists to argue that "human evolution was much more a response to a prolonged series of climate fluctuations rather than any single shift."
Perhaps most damning of all is the time of climate changes is evidence that has come forth with fairly recent hominin discoveries. Discoveries of new Australopithecus and Ardipithecus species seem to indicate the timing of climate theory is off. These species began walking upright between 5 million and 3 million years ago long before the climate changes associated with the creation of the Panama isthmus,
Other evidence seems to indicate that the first hominins to walk upright did so in the forest rather than the savannah. Evidence found with remains of “ Australopithecus ramidus” , for example, seem to indicate it lived in a wooded environment rather than savannah grasslands.
Ardipithecus Ramidus Debunks Climate Theory
Scientists believe that “Ardipithecus kadabba” and “Ardipithecus ramidus” , who lived around three and four million years ago, lived in the forest because: 1) their teeth indicate they ate woodland foods: and 2) their remains were found among fossils of forest dwelling plants and monkeys.
If it is indeed true that “Ardipithecus” lived in woodlands and is bipedal it means he likely developed the ability to walk in woodlands which would debunk the “savannah hypotheses”—that man first walked up right to survive in a grassland habitat — and the “climate theory” — that changes in climate which caused woodlands to change to savannahs caused early hominin to walk upright.
Lovejoy has theorized that Ardipithecus came out trees for sex. Based on the fact that the canines of Ardipithecus males are small and similar to females — unlike male apes which have large canines and use them mainly in fights with other males — Ardipithecus won over females by coming down of the trees to collect high-protein, high-fat food given to the females offspring in return for sex and bipedalism developed as a way to carry back food.
Scientists have found some evidence (from soil samples and analysis of teeth) that “Ardipithecus” lived in a savannah environment not a wooded one. Most scientists agree that a lot of analysis and research still needs to be done to make any authoritative claims about Ardi and her kin.
See Separate Article: EARLIEST HOMININS AND OLDEST HUMAN ANCESTORS: SAHELANTHROPUS, ORRORIN AND ARDIPITHECUS factsanddetails.com
Did Apes Really Adapt to Forests Changing Into Savannahs?
Laura M. MacLatchy, Kieran McNulty and Dan Peppe studied an ape named Morotopithecus that lived in Uganda 21 million years ago and is the oldest ape for which scientists have found fossils from the cranium, teeth and other parts of the skeleton. They found the body was adapted for standing up right in trees but the teeth were adapted to eating savannah grasses rather than fruit in trees. The scientists wrote: Such inconsistencies between our evidence and the traditional narrative of ape origins led us to question other assumptions: Did Morotopithecus live in a forested habitat at all? To figure out Morotopithecus’ habitat, we studied the chemistry of fossil soils — called paleosols — and the microscopic remains of plants they contain in order to reconstruct the ancient climate and vegetation at Moroto. [Source: Laura M. MacLatchy, Professor of Anthropology, University of Michigan, Kieran McNulty, Professor of Anthropology, University of Minnesota, and Dan Peppe, Associate Professor of Geosciences, Baylor University, The Conversation, April 14, 2023]
Trees and most shrubs and nontropical grasses are classified as C plants, based on the type of photosynthesis they perform. Tropical grasses, which rely on a different photosynthetic system, are known as C plants. Importantly, C plants and C plants differ in the proportions of the various carbon isotopes they take inches That means carbon isotope ratios preserved in the paleosols can tell us the composition of the ancient vegetation. We measured three distinct carbon isotope signatures, each providing a different perspective on the plant community, found evidence of abundant C plant biomass — tropical grasses — consistent with a savannah environment. But scientists hadn’t thought the levels of C biomass we measured at Moroto had evolved in Africa until 10 million years ago. Our data suggests it happened twice as far back in time, 21 million years ago. This evidence dramatically contradicts the traditional view of ape origins — that apes evolved upright torsos to reach fruit in forest canopies. Instead, Morotopithecus, the earliest known ape with upright locomotion, consumed leaves and inhabited an open woodland with grassy areas.
We then used the same approach to reconstruct habitats at eight other fossil sites in Kenya and Uganda, ranging in age from around 16 million to 21 million years old and found the same result. Our isotopic proxies at each fossil site contributed two significant revelations. First, vegetation types ranged from closed canopy forests to open wooded grasslands. And second, every site had a mixture of C and C vegetation, with some locations having a high proportion of C grass biomass. Phytoliths from the same paleosols again corroborated that abundant C grasses were present at multiple sites.
The realization that such a variety of environments, especially open habitats with C grasses, was present at the dawn of the apes forces a reassessment not just of the evolution of apes but of humans and other African mammals. Regarding human origins, our study adds to a growing body of evidence that our divergence from apes — in anatomy, ecology, behavior — cannot be simply explained by the appearance of grassland habitats.
Image Sources: Wikimedia commons, except Kurdish family on all fours from Science Nordic
Text Sources: National Geographic, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Nature, Scientific American. Live Science, Discover magazine, Discovery News, Times of London, Natural History magazine, Archaeology magazine, The New Yorker, Time, Newsweek, BBC, The Guardian, Reuters, AP, AFP and various books and other publications.
Last updated April 2024