In his view, the progenitor assumed a two-legged posture to free the hands for fashioning tools and performing other activities that in turn nourished the development of an increasingly refined intelligence. By the turn of this century, however, most anthropologists believed that the critical move to the grasslands had occurred in Asia. Though the bones of a primitive hominid--which later came to be called Homo erectus--had been discovered on the Southeast Asian island of Java, the change of venue had more to do with cultural values and racist reasoning than with hard evidence.
Africa was the dark continent, where progress was slowed by heat, disease, and biotic excess. In such a place, it was thought, the mind would vegetate--witness the regressive races that inhabited the place in modern times. The plains of central Asia, on the other hand, seemed just the sort of daunting habitat that would call out the best in an enterprising ape.
In that environment, wrote the American paleontologist Henry Fairfield Osborn, the struggle for existence was severe and evoked all the inventive and resourceful faculties of man. In hindsight, the contrasts made between dark and light, forest and plain, slovenly ape and resourceful man seem crudely moralistic. But at the time the idea was highly influential. Among those impressed was a young anatomist in South Africa named Raymond Dart.
In Dart announced the discovery, near the town of Taung, of what he believed to be the skull of a juvenile man-ape, which he called Australopithecus africanus. Forests might provide apes with an easy and sluggish solution to the problems of existence, wrote Dart, but for the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of intellect--a more open veld country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species.
The question was less whether the savanna played a part in the origin of bipedalism-- that was obvious--than how. Dart originally proposed that Australopithecus had taken to two legs to avoid predators for sudden and swift bipedal movement, to elude capture , but later he reversed this scenario and imagined his killer ape the eater rather than the eaten, forsaking the trees for the more attractive fleshy foods that lay in the vast savannas of the southern plains.
Later studies suggested that the environments in eastern and southern Africa where early hominids lived were not the vast, unchanging plains Dart imagined. Instead they appeared to be variable, often characterized by seasonally semiarid terrain, a plain studded with scraggly trees and patches of denser woodland. But no matter: This savanna mosaic was still drier and more open than the thick forest that harbors the African apes today. It just made good sense, moreover, that our ancestors would have come down to the ground and assumed their bipedal stance in a habitat where there were not as many trees to climb around in.
The satisfying darkness-into-light theme of early hominid development held up, albeit with a little less wattage. For decades the popularity of the savanna hypothesis rested on the twin supports of its moral resonance and general plausibility: our origin should have happened this way, and it would make awfully good sense if it did.
In East Africa, geography seemed to reinforce the sheer rightness of the hypothesis. Most of the earliest hominid fossils have come from the eastern branch of the great African Rift Valley; researchers believed that when these hominids were alive, the region was much like the dry open grasslands that dominate it today.
The lusher, more forested western branch, meanwhile, is home to those lazy chimps and gorillas--but to no hominid fossils. Still, despite such suggestive correspondence, until 20 years ago something was missing from the hypothesis: some hard data to link environment to human evolution. Then paleontologist Elisabeth Vrba of Yale began offering what was the strongest evidence that the drying up of African environments helped shape early human evolution. In studying the bones of antelope and other bovids from hominid sites in South Africa, Vrba noticed a dramatic change occurring between 2.
Many species that were adapted to wooded environments, she saw, suddenly disappeared from the fossil record, while those suited to grassy regions appeared and multiplied. This turnover pulse of extinctions and origins coincides with a sudden global cooling, which may have triggered the spread of savannas and the fragmentation of forests.
Other investigators, meanwhile, were documenting an earlier turnover pulse around 5 million years ago. For humans, both dates are full of significance. This earlier pulse corresponds to the date when our lineage is thought to have diverged from that of the apes and become bipedal. The inference was clear: our early ancestors were savanna born and savanna bred. All the evidence, as I see it, Vrba wrote in , indicates that the lineage of upright primates known as australopithecines, the first hominids, was one of the founding groups of the great African savanna biota.
With empirical evidence drawn from two different sources, the turnover pulse is a great improvement on the traditional savanna hypothesis which in retrospect looks not so much like a hypothesis as a really keen idea.
If so, then similar turnovers in animal species should have appeared in the fossil record whenever global change occurred. Over the last 15 years, Andrew Hill and John Kingston, both also at Yale, have been looking for signs of those dramatic shifts at some sites in the Tugen Hills of Kenya. In the heart of the fossil-rich eastern branch of the Rift Valley, the Tugen Hills offer a look at a succession of geologic layers from 16 million years ago to a mere , years ago-- studded with fragmentary remains of ancient apes and hominids.
To gauge the past climate of the Tugen Hills, the researchers have looked at the signatures of the ancient soils preserved in rock. Different plants incorporate different ratios of isotopes of carbon in their tissues, and when those plants die and decompose, that distinctive ratio remains in the soil.
Thus grasslands and forests leave distinguishing isotopic marks. Instead of signs that the environment was opening up, they found that there was a little bit of grass all the time, with no dramatic changes, and no evidence that early hominids there ever encountered an open grassland.
It would have been lovely if it had also been true. Laura Bishop of Liverpool University in England, for instance, has been studying pig fossils from several East African sites.
Some of those fossil animals, she has found, had limbs that were adapted not for open habitats but for heavy woods. Over the past 5 million years, he has found, the African climate has cycled back and forth between dry and wet climates, but the pattern became dramatic only 2.
What may finally kill the savanna hypothesis--or save it--are the hominids themselves. More than anything else, walking on two feet is what makes a hominid a hominid. Bone structure Our bones are organised to help us walk on two legs. Preserved footprints At Laetoli in Tanzania, just south of Olduvai Gorge, a set of Australopithecus afarensis footprints of two individuals walking along side by side, has been dated to 3.
Fossil footprints of Australopithecus afarensis Source: Flickr. How did bipedalism begin? The advantages The host of advantages bipedalism brought meant that all future hominid species would carry this trait.
Bipedalism allowed hominids to free their arms, allowing the use of tools. Did you know? Many dinosaurs were bipedal! How our joints help us to walk. Ball-and-socket joint The ball-and-socket joint is the most mobile type of joint, allowing us to swing our arms and legs in many different directions. Hinge joint Hinge joints, such as the knee and elbow, act as a lever that enables our arms and legs to flex bend and straighten.
Pivot joint In a pivot joint, one bone twists against another. Semi-movable joint Semi-movable joints, such as those found in the spine, only allow partial movement. Several of these theories, therefore, would be incorporated into the savannah-based theory. A few studies have demonstrated that an intimidating visual display and heat loss were possible advantages of bipedalism, but the logic would not remain in an opposite direction.
The clear advantage of bipedalism was the possibility for ancient hominin species to use their hands. With the evolution of bipedalism, this special advantage was evolved only by the primates. This advantage offered a benefit that overcame the fatal disadvantage of slow speed. Several species could not have utilized their hands for effective provisioning or tool use, even if they had become bipedal through evolution. However, the prehensile hands and feet of primates evolved from the mobile hands of the semi-arboreal tree shrews that lived approximately million years ago and enabled provisioning in ape-like ancestors Schmidt and Lanz Schmidt H and Lanz, U.
Surgical anatomy of the hand. Stuttgart: Thieme. Similar to humans, modern-day chimpanzees have a limited ability to use their limbs and even sticks to obtain termites in a manner similar to human fishing. The last of prominent bipedalism theories, the provisioning model, was proposed by Owen Lovejoy, who suggested a modified version of Darwin's explanation. What would have been so advantageous about using two hands? Lovejoy proposed that walking on two legs was a main adaptation for pair-bonding to succeed because carrying with two hands was effective for food transport Lovejoy Lovejoy CO.
Lovejoy's theory also proposed that sexual dimorphism suggested that food gathering would improve the infant survival rate. Males were responsible for provisioning the females, whereas females protected their offspring Lovejoy Lovejoy CO. Hominid Origins-The Role of Bipedalism. Females would mate exclusively with the provisioning male, and other males would no longer need to fight with each other over the females.
Therefore, the males' jagged, blade-like canine teeth diminished over time. Several studies have demonstrated that chimpanzees could carry twice as many nuts in a bipedal position than when walking on all fours Carvalho et al.
Anthropological evidence also supported this theory. The downsizing of male canine teeth, decrease in antagonistic behavior and body size dimorphism corroborated Lovejoy's theory. As mentioned previously, there could be multiple answers to bipedalism, and there were two aspects of bipedal evolution: 1 that ancient hominins were already partially bipedal, and 2 that hominins evolved full bipedalism. Although the postural feeding theory provided an explanation for the first aspect, the savanna-based theory could provide an answer to why hominins became increasingly bipedal over time.
Lovejoy's provisioning model lied between these two theories. Hunt's theory, which suggested that bipedalism involved reaching for food and balancing on the branches, would logically fall before using the hands for provision.
The early hominins spent time on the trees, but the species eventually evolved to walk like modern humans on the ground. The evolutionary momentum that was driven by balancing and reaching on the trees should have affected the early hominins. The provisioning model demonstrated how hominins became more bipedal over the time not only by food gathering but also by provisioning infants monogamy.
Nonetheless, the provisioning model did not have sufficient evidence explaining why hominins would have begun to walk like modern humans and lost all adaptations to arboreal life. To this gap was where the savannah-based theory contributed its explanations.
When largely bipedal hominins started to settle on the ground, the savannah based-theory would be the explanation for their full bipedalism. The savannah based-theory included various other models that already assumed that hominins started to live a terrestrial life, such as sentinel behavior, threat, running endurance and thermoregulatory models. The general order of the theories was the following: postural-feeding, provisioning, and savannah-based theories.
However, there were no straight lines between these theories, and it was possible that the three forces worked together at one point. First, there were arboreal hominids that possessed ambiguous traits of bipedalism.
These were gradually replaced by two lines of species: one consisting of Pan species and the other comprising hominins. Hominin-like species and modern chimp-like species gradually evolved to undergo specific adaptations to live on the ground and on the trees, and the new hominins presented a survival advantage over their common ancestors. The ambiguous traits were eliminated through the choice and selection.
However, when the split between the two species became clear, the hominins and chimpanzees would not have competed for resources. The stated biological relationship between the chimps and humans was similar to the remarkable relationship between the okapi and the giraffe.
Similar to the unique adaptation of bipedal locomotion that was only observed in Homo species, the giraffe's long neck was also an evolutionary product exclusive to this species Badlangana et al. Observations on the giraffe central nervous system related to the corticospinal tract, motor cortex and spinal cord: What difference does a long neck make? The okapi and the giraffe are currently the only living members of the Giraffidae family. Although the short-necked okapi's outer appearance resembles a zebra, the okapi is the closest surviving species to the giraffe.
Apparently, Darwinian natural selection has led the ancestral giraffes with long necks to reproduce and pass on their genes because they had a competitive advantage that enabled them to reach higher branches. Consequently, the giraffe ancestors fed on acacia leaves and spread through the savannah where tall trees grew.
In contrast, the long-neck adaptations became futile for the okapi species, which, ultimately, inhabited canopy forests and fed on the buds, grasses, ferns, fruits, or fungi Hart and Hart Hart JA, Hart TB.
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Reprints and Permissions. Savanna tree evolutionary ages inform the reconstruction of the paleoenvironment of our hominin ancestors. Sci Rep 10, Download citation. Received : 03 March Accepted : 31 March Published : 24 July Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.
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Advanced search. Skip to main content Thank you for visiting nature. Download PDF. Abstract Ideas on hominin evolution have long invoked the emergence from forests into open habitats as generating selection for traits such as bipedalism and dietary shifts.
Introduction The emergence of savannas and other tropical grassy biomes has been a topic of intense research interest, not least because it coincides with early hominin evolution.
Results Using a well-sampled, time-calibrated phylogenetic tree of the woody flora, we examine the timing of evolutionary splits between savanna trees in Africa as a marker for the age of the savanna biome. Figure 1. Full size image. Figure 2. Discussion Using evolutionary divergence times between savanna trees, we infer a latitudinal gradient in the age of the savanna biome in Africa. Methods Phylogenetic and spatial data We focused on the woody plants of sub-Saharan Africa, and sample all the major lineages of southern African trees, following recent taxonomic rearrangements notably, the quintessentially African genus Acacia is now recognised as polyphyletic and African Acacia are included in our dataset as two separate genera, Senegalia and Vachellia , following ref.
Statistical analysis It is not straightforward to assess the directionality of an evolutionary split between forest and savanna trees across the forest-savanna boundary without information on the ancestral state; therefore, we identified early lineage diversification events that likely occurred within the savanna biome by considering only splits in which both sister lineages were savanna trees and extracted their divergence times.
References 1. Google Scholar 2.
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