How Science Works - Revising Our View of Why Humans Evolved an Upright Stature

How Science Works

Revising Our View of Why Humans Evolved an Upright Stature

Artistic rendering of the open woodland habitat reconstruction at Moroto II with Morotopithecus bishopi vertically climbing with infant on back and juvenile below.

Image credit: Corbin Rainbolt

Apes may have evolved upright stature for leaves, not fruit, in open woodland habitats | University of Michigan News

3D view of a Motopithecus bishopi vertebra

Here we have another example of scientists challenging the accepted consensus which, unlike my last example which concerned a fundamental model of cosmology, this time concerns conflicting ideas of hominid evolution.

Not, as creationists like to delude themselves, by rejecting the whole concept of evolution in favour of their magical superstition, but the precise mechanism by which we, or rather our ancestors, evolved an upright stature and a bipedal gait. That this is an evolved feature is beyond doubt.

The scientific consensus is that human ancestors evolved an upright posture as a result of a combination of factors, including the need to adapt to an increasingly savannah-like environment, as well as the need to free up the hands for tool use and other activities.

One theory suggests that the shift to bipedalism allowed early hominins to better see over tall grasses and detect predators, while also freeing up their hands for tool use and other tasks. Another theory proposes that upright posture evolved as a means of thermoregulation in hot environments, allowing early hominins to better dissipate heat through a smaller surface area exposed to the sun.

There is also evidence to suggest that the development of bipedalism was a gradual process, with multiple adaptations and changes in anatomy occurring over time. For example, the pelvis and lower limb bones of early hominins gradually became more elongated and robust to support bipedalism.

References:

1. Lovejoy CO. The origin of man. Science. 1981 Jul 24;213(4506):341-50. doi: 10.1126/science.211.4480.341. PMID: 17748254.
2. Richmond BG, Jungers WL. Orrorin tugenensis femoral morphology and the evolution of hominin bipedalism. Science. 2008 Mar 21;319(5870):1662-5. doi: 10.1126/science.1154197. PMID: 18356526.
3. Aiello LC, Wheeler P. The expensive-tissue hypothesis: the brain and the digestive system in human and primate evolution. Curr Anthropol. 1995;36(2):199-221. doi: 10.1086/204350.
4. Carrier DR. The energetic paradox of human running and hominid evolution. Curr Anthropol. 1984;25(4):483-95. doi: 10.1086/203165.

ChatGPT. (2023, April 15). What is the current scientific consensus regarding why human ancestors evolved an upright stature? With references, please. [Response to a question]. Retrieved from https://github.com/AbhinavS99/gpt3-sandbox/blob/main/nbs/02_OpenAIGPT3/ChatGPT.ipynb

However, a minority view is that it evolved because it made it easier for our ancestors to collect fruit in forests. It is this minority view that researchers from the University of Michigan are challenging. They believe they have found evidence that it evolved in open woodland in which the diet consisted of leaves.

The University of Michigan news release explains the finding and its significance for anthropologists and the human evolutionary story:

Searching for small fossils at the fossil site Bukwa II on the flanks of Mount Elgon, eastern Uganda. Paleoenvironmental evidence associated with the fossil sites on Bukwa support the early evolution of grassy woodland habitats ~20 million years ago.

Image credit: J. Kingston

Anthropologists have long thought that our ape ancestors evolved an upright torso in order to pick fruit in forests, but new research from the University of Michigan suggests a life in open woodlands and a diet that included leaves drove apes’ upright stature.

The findings shed light on ape origins and push back the origin of grassy woodlands from between 7 million and 10 million years ago to 21 million years ago in equatorial Africa, during the Early Miocene.

Fruit grows on the spindly peripheries of trees. To reach it, large apes need to distribute their weight on branches stemming from the trunk, then reach out with their hands toward their prize. This is much easier if an ape is upright because it can more easily grab onto different branches with its hands and feet. If its back is horizontal, then its hands and feet are generally underneath the body, making it much harder to move outward to the smaller branches of a tree—especially if the ape is large bodied.

This is how modern day apes reach fruit, and, it’s been theorized, that’s why apes evolved to be upright, according to U-M researchers Laura MacLatchy and John Kingston.

But new research centered around a 21-million-year-old fossil ape called Morotopithecus and led by MacLatchy suggests this might not be the case. Instead, researchers think early apes ate leaves and lived in a seasonal woodland with a broken canopy and open, grassy areas. The researchers suggest this landscape, instead of fruit in closed canopy forests, drove apes’ upright stature.

Their results are published in Science and are bolstered by a companion paper examining these paleo grassy woodland habitats, published in the same issue of the journal.

The expectation was: We have this ape with an upright back. It must be living in forests and it must be eating fruit. But as more and more bits of information became available, the first surprising thing we found was that the ape was eating leaves. The second surprise was that it was living in woodlands.

These open environments have been invoked to explain human origins, and it was thought that you started to get these more open, seasonal environments between 10 and 7 million years ago. Such an environmental shift is thought to have been selected for terrestrial bipedalism—our ancestors started striding around on the ground because the trees were further apart.

Now that we’ve shown that such environments were present at least 10 million years before bipedalism evolved, we need to really rethink human origins, too.

Professor Laura MacLatchy, first author
Department of Anthropology
University of Michigan, Ann Arbor, MI, USA.

The two papers grew out of a U.S. National Science Foundation-funded collaboration of international paleontologists, collectively known as the Research on Eastern African Catarrhine and Hominoid Evolution project or REACHE, each of whom focus on different aspects of early ape paleoenvironments. The study led by MacLatchy focuses on a 21-million-year-old site called the Moroto site in eastern Uganda.

There, the group, which included U-M researchers William Sanders and Miranda Cosman, examined fossils found in a single stratigraphic layer, including fossils of the oldest, clearly documented ape, Morotopithecus. Also within this layer were fossils of other mammals, ancient soils called paleosols, and tiny silica particles from plants called phytoliths. The researchers used these lines of evidence to recreate the ancient environment of Morotopithecus.

MacLatchy and Kingston discovered that the plants living in this landscape were what’s called “water stressed,” meaning they lived through seasonal periods of rain and of aridity. This also means that at least part of the year, apes had to rely on something other than fruit to survive. Together, these findings indicate that Morotopithecus lived in an open woodland punctuated by broken canopy forests composed of trees and shrubs.

The first clue that these ancient apes were eating leaves was in the apes’ molars. The molars were very “cresty”: they were craggy, with peaks and valleys. Molars like this are used for tearing fibrous leaves apart, while molars used for eating fruit are typically more rounded, MacLatchy said.

Morotopithecus bishopi vertebra (backbone), specimen UMP 67-28. The transverse processes (bony projections on the side of the vertebra) have a dorsal place of origin at the base of the neural arch, a feature that allows for a stable lower back in apes.

Image credit: L. MacLatchy and J. Kingston

Morotopithecus bishopi partial face and maxilla (upper jaw), specimen UMP 62-11. This specimen is the “holotype,” the single fossil specimen whose features define the name of the species.

Image credit: L. MacLatchy and J. Kingston

Morotopithecus bishopi femur (thigh bone), specimen UMP MORII 94’80. The femur is short like those of modern apes and is indicative of vertical climbing abilities.

Image credit: L. MacLatchy

The researchers also examined the apes’ dental enamel, as well as the dental enamel of other mammals found in the same stratigraphic layer. They found that isotopic ratios—the abundance of two isotopes of the same element—in their dental enamel showed that the apes and other mammals had been eating water stressed C3 plants that are more common in open woodland or grassy woodland environments today. C3 plants are primarily woody shrubs and trees while C4 plants are arid-adapted grasses.

Putting together the locomotion, the diet and the environment, we basically discovered a new model for ape origins. In anthropology, we care a lot about ape evolution because humans are closely related to apes and features like lower back stability represent an arboreal adaptation that may have ultimately given rise to bipedality in humans.

Professor Larau MacLatchy/

Previously, researchers believed equatorial Africa during the Early Miocene was thickly carpeted with forest, and that open seasonal woodlands and grasslands evolved only between 7 million and 10 million years ago.

Dan Peppe (Baylor University) and John Kingston (University of Michigan) trench fossil soils for paleoenvironmental indicators at the fossil site of Moroto II in eastern Uganda. This is the stratigraphic level of the site Moroto II which yielded both fossils of Morotopithecus and the paleoenvironmental data indicating the presence of abundant C4 grasses in an open woodland habitat.

Image credit: L. MacLatchy

But the second paper uses a set of environmental proxies to reconstruct the vegetation structure from nine fossil ape sites across Africa, including the Moroto site, during the Early Miocene. These proxies revealed that C4 grasses were “everywhere” at the fossil ape sites during that time period, said Kingston, a biological anthropologist and associate professor in the U-M Department of Anthropology.

This paper looks at all these sites, pulls all this data together, and says, ‘Look, no matter how you evaluate the data, there’s no way you can escape the fact that all these proxies are converging on the same place—namely, that these environments are open, and they’re open with C4 grasses.

For the first time, we’re showing that these grasses are widespread, and it’s this general context of open seasonal woodland ecosystems that were integral in shaping the evolution of different mammalian lineages, including and especially in our case, how different ape lineages evolved.

There’s mountains and volcanoes, there’s cliffs and escarpments and valleys related to the rifting. The landscape is just physically highly variable, and that, no doubt, is related to the vegetation heterogeneity.

John D. Kinston, co-author
Department of Anthropology
University of Michigan, Ann Arbor, MI, USA.

The nine sites are scattered across eastern equatorial Africa, enough to develop a “regional picture” of what the sites’ landscapes looked like in the Early Miocene, Kingston said. During this time, the East African Rift was forming. Earth was pulling apart. As a result, the entire region was uplifted, causing huge variation in topography, and therefore, regional climate and vegetation.

The findings have transformed what we thought we knew about early apes, and the origin for where, when and why they navigate through the trees and on the ground in multiple different ways.

For the first time, by combining diverse lines of evidence, this collaborative research team tied specific aspects of early ape anatomy to nuanced environmental changes in their habitat in eastern Africa, now revealed as more open and less forested than previously thought. The effort outlines a new framework for future studies regarding ape evolutionary origins.

Robin Bernstein
Program director for biological anthropology
National Science Foundation.

To reconstruct the paleoenvironment at each location, the researchers used carbon isotope analyses of ancient soil organic matter, plant wax biomarkers and phytoliths found at each site. The carbon isotope analyses revealed that a wide range of plants lived in the grasslands, ranging from those that comprise closed canopy to wooded grasslands.

The wax biomarkers—left over from the waxy material that protects leaves—also indicate a large variety of shrubs and trees as well as grasses. Phytoliths—microscopic biosilica bodies that give plants their structure as well as a defense against being eaten—can tell the researchers the proportion of C4 grasses at a given site and provide further evidence for abundant C4 grasses.

After using these proxies to rebuild the paleoenvironments at these nine sites, the researchers found that C4 grasses were abundant across eastern equatorial Africa, and were a key part of the landscape’s heterogeneous habitats. Their data also pushes back the oldest evidence of C4 grass-dominated habitats in Africa and globally by more than 10 million years.

More detail is given in the scientists' abstract to their papers in Nature
89

Structured Abstract

INTRODUCTION

Inherent in traditional views of ape origins is the idea that, like living apes, early large-bodied apes lived in tropical forests. In response to constraints related to locomoting in forest canopies, it has been proposed that early apes evolved their quintessential upright torsos and acrobatic climbing and suspensory abilities, enhancing their locomotor versatility, to distribute their weight among small supports and thus reach ripe fruit in the terminal branches. This feeding and locomotor transition from a quadruped with a horizontal torso is thought to have occurred in the Middle Miocene due to an increasingly seasonal climate and feeding competition from evolving monkeys. Although ecological and behavioral comparisons among living apes and monkeys provide evidence for versions of terminal branch forest frugivory hypotheses, corroboration from the early ape fossil record has been lacking, as have detailed reconstructions of the habitats where the first apes evolved.

RATIONALE

The Early Miocene fossil site of Moroto II in Uganda provides a unique opportunity to test the predictions of terminal branch forest frugivory hypotheses. Moroto II documents the oldest [21 million years ago (Ma)] well-established paleontological record of ape teeth and postcranial bones from a single locality and preserves paleoecological proxies to reconstruct the environment. The following lines of evidence from Moroto II were analyzed: (i) the functional anatomy of femora and a vertebra attributed to the ape Morotopithecus; (ii) dental traits, including molar shape and isotopic profiles of Morotopithecus enamel; (iii) isotopic dietary paleoecology of associated fossil mammals; (iv) biogeochemical signals from paleosols (ancient soils) that reflect local relative proportions of C3 (trees and shrubs) and C4 (tropical grasses and sedges that can endure water stress) vegetation as well as rainfall; and (v) assemblages of phytoliths, microscopic plant-derived silica bodies that reflect past plant communities.

RESULTS

A short, strong femur biomechanically favorable to vertical climbing and a vertebra indicating a dorsostable lower back confirm that ape fossils from Moroto II shared locomotor traits with living apes. Both Morotopithecus and a smaller ape from the site have elongated molars with well-developed crests for shearing leaves. Carbon isotopic signatures of the enamel of these apes and of other fossil mammals indicate that some mammals consistently fed on water-stressed C3 plants, and possibly also C4 vegetation, in a woodland setting. Carbon isotope values of pedogenic carbonates, paleosol organic matter, and plant waxes all point to substantial C4 grass biomass on the landscape. Analysis of paleosols also indicates subhumid, strongly seasonal rainfall, and phytolith assemblages include forms from both arid-adapted C4 grasses and forest-indicator plants.

CONCLUSION

The ancient co-occurrence of dental specializations for leaf eating, rather than ripe fruit consumption, along with ape-like locomotor abilities counters the predictions of the terminal branch forest frugivory hypotheses. The combined paleoecological evidence situates Morotopithecus in a woodland with a broken canopy and substantial grass understory including C4 species. These findings call for a new paradigm for the evolutionary origins of early apes. We propose that seasonal, wooded environments may have exerted previously unrecognized selective pressures in the evolution of arboreal apes. For example, some apes may have needed to access leaves in the higher canopy in times of low fruit availability and to be adept at ascending and descending from trees that lacked a continuous canopy.

Hominoid habitat comparisons. Shown are reconstructions of a traditionally conceived hominoid habitat (A) and the 21 Ma Moroto II, Uganda, habitat (B).

Abstract

Living hominoids are distinguished by upright torsos and versatile locomotion. It is hypothesized that these features evolved for feeding on fruit from terminal branches in forests. To investigate the evolutionary context of hominoid adaptive origins, we analyzed multiple paleoenvironmental proxies in conjunction with hominoid fossils from the Moroto II site in Uganda. The data indicate seasonally dry woodlands with the earliest evidence of abundant C4 grasses in Africa based on a confirmed age of 21 million years ago (Ma). We demonstrate that the leaf-eating hominoid Morotopithecus consumed water-stressed vegetation, and postcrania from the site indicate ape-like locomotor adaptations. These findings suggest that the origin of hominoid locomotor versatility is associated with foraging on leaves in heterogeneous, open woodlands rather than forests.

Laura M. MacLatchy et al.
The evolution of hominoid locomotor versatility: Evidence from Moroto, a 21 Ma site in Uganda.
Science 380, eabq2835(2023). DOI:10.1126/science.abq2835

Copyright © 2023 The Authors
Published by American Association for the Advancement of Science. Reprinted with kind permission under license #5530210937919

The second paper referred to:

Abstract

The assembly of Africa’s iconic C4 grassland ecosystems is central to evolutionary interpretations of many mammal lineages, including hominins. C4 grasses are thought to have become ecologically dominant in Africa only after 10 million years ago (Ma). However, paleobotanical records older than 10 Ma are sparse, limiting assessment of the timing and nature of C4 biomass expansion. This study uses a multiproxy design to document vegetation structure from nine Early Miocene mammal site complexes across eastern Africa. Results demonstrate that between ~21 and 16 Ma, C4 grasses were locally abundant, contributing to heterogeneous habitats ranging from forests to wooded grasslands. These data push back the oldest evidence of C4 grass–dominated habitats in Africa—and globally—by more than 10 million years, calling for revised paleoecological interpretations of mammalian evolution.

Daniel J. Peppe et al.
Oldest evidence of abundant C4 grasses and habitat heterogeneity in eastern Africa.
Science 380, 173-177(2023). DOI:10.1126/science.abq2834

Copyright © 2023 The Authors
Published by American Association for the Advancement of Science. Reprinted with kind permission under license #5530210695147

Creationists probably find it strange the way scientists disagree without rancour and accusations flying around, and how the orthodox view is challengeable and subject to constant reassessment and revision, rather than being handed down on the authority of cult leaders.

But the resul is that science converges on a single answer with each revision, whereas religions tend to diverge into mutually hostile sects based on a few simplistic notions that were prevalent in the fearful infancy of our species at a time before we had even invented the wheel, and thought the world was flat and ran on magic and, like today's toddlers, thought teleologically because they knew no better.

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