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An artist's reconstruction of Australopithecus afarensis. Image by Michael Hagelberg.
Three million years ago a chimpanzee-sized creature stood up on two legs. While she may have looked like a chimpanzee, she was not—she was a member of Australopithecus, an ancient human ancestor. One way we know this is because she walked on two legs just like humans today.She used her two legs to walk between grasslands so that she could reach the trees that had her favorite food—fruit. Some scientists think that early humans began walking on two legs so that they could move between food patches. Early humans might have been driven to walk on two legs as fruit-bearing forests shrunk and grasslands spread when climate changed over the last six million years.
However, this is just one of many ideas of how climate change may have influenced human evolution. How our ancestors responded to climate change falls under the science of ecology.
Standing up in savannas
Modern day savanna in Gambia. Image by Ikiwaner.
The "savanna hypothesis" is one of the oldest ideas on the effects climate change had on early humans. The idea is that the spread of grasslands was important to the evolution of our ancestors in Africa. We know that grasslands spread as the climate became cooler and drier over the last six million years.Some scientists think that our ancestors first stood up on two legs to peek over tall grasses to avoid predators. Others think they evolved to walk on two legs so that they could move across grasslands to reach tree patches on the landscape. These tree patches held the fruits early hominins relied on. Many other ideas like these have been suggested. In fact, almost all major "events" in human evolution (stone tool use, brain size increase, etc.) have at one time been linked to the spread of savanna in Africa. Walking on two legs (bipedalism) is likely the trait most closely tied to spread of the savanna. This idea is still an area of intense research today.
A sketch of Ardipithecus ramidus, adapted to move in trees. Image by Tobias Fluegel.
Ardipithecus ramidus is a 4.4 million year old human ancestor that is known from East Africa. The fossil animals, plants, and ancient soils that are found with A. ramidus suggest that this early human lived in a habitat with a lot of trees, like a forest. This challenges the idea that walking on two legs was driven by spread of the savanna, as one of the earliest bipeds was found in a woody habitat.However, more recently a group of geologists announced a study of ancient East African soils over the last six million years. These scientists found that savannas had been common across East Africa for millions of years. They thought that this supported the idea that savannas could have driven early humans to walk on two legs. Although this is the most recent study, the debate is not settled. Several scientists still think that the earliest bipeds lived in wooded, semiforested habitats.
Variability and flexibility
There are a few alternatives to the savanna hypothesis. One is that changes in temperature or rainfall influenced human evolution. These types of changes are called climate variability.
An example of speciation. Click for more detail.
Other scientists think that great expansions and contractions of lakes broke up and rejoined groups of early humans over time. Lake expansion and contraction could have caused new species to form. Speciation can occur when populations are divided and take on their own evolutionary paths. Such divisions could also have caused the evolution of new adaptations. Hominin species might have had to change their behaviors when they came into contact with one another. These new behaviors may have evolved when species had to compete for food.Changing climates may also drive evolution for other reasons. Humans and our ancestors have to be more flexible in how they interact with an area if their habitat is changing. To be successful in a changing world, you would want to be a generalist rather than a specialist. When a specialist's preferred food, habitat, or any other resource disappears, it is hard for them to survive. Generalists are more flexible and so can more easily change with the environment. For example, stone tool use may have helped hominins exploit a new resource (meat). An ability to hunt for meat could be critical when other resources, like plants, were scarce during periods of global cooling.
James Shreeve
Once upon a time, there was an ape who lived in the middle of a dark forest. It spent most of its days in the trees, munching languidly on fruits and berries. But then one day the ape decided to leave the forest for the savanna nearby. Or perhaps it was the savanna that moved, licking away at the edge of the forest one tree at a time until the fruits and berries all the apes had found so easily weren’t so easy to find anymore. In either case, the venturesome ape found itself out in the open, where the air felt dry and crisp in its lungs.
Life was harder on the savanna: there might be miles between one meal and another, there were seasons of drought to contend with, and large, fierce animals who didn’t mind a little ape for lunch. But the ape did not run back into the forest. Instead it learned to adapt, walking from one place to another on two legs. And it learned to live by its wits.
As the years passed, the ape grew smarter and smarter until it was too smart to be called an ape anymore. It lived anywhere it wanted and gradually made the whole world turn to its own purposes.
Meanwhile, back in the forest, the other apes went on doing the same old thing, lazily munching on leaves and fruit. Which is why they are still just apes, even to this day.
The tale of the ape who stood up on two legs has been told many times over the past century, not in storybooks or nursery rhymes but in anthropology texts and learned scientific journals. The retellings have differed from one another in many respects: the name of the protagonist, for instance, the location in the world where his transformation took place, and the immediate cause of his metamorphosis.
One part of the story, however, has remained remarkably constant: the belief that it was the shift from life in the forest to life in a more open habitat that set the ape apart by forcing it onto two legs.
Bipedalism allowed hominids to see over tall savanna grass, perhaps, or escape predators, or walk more efficiently over long distances. In other scenarios, it freed the hands to make tools for hunting or gathering plants.
A more recent hypothesis suggests that an erect posture exposes less skin to the sun, keeping body temperature lower in open terrain. Like the painted backdrop to a puppet theater, the savanna can accommodate any number of dramatic scenarios and possible plots. But now that familiar stage set has come crashing down under the weight of a spectacular crop of new hominid fossils from Africa, combined with revelations about the environment of our earliest ancestors...
https://www.discovermagazine.com/the-sciences/sunset-on-the-savanna
Barnabas H. Daru, Bezeng S. Bezeng, Tristan Charles-Dominique, Gareth P. Hempson, Ronny M. Kabongo, Olivier Maurin, A. Muthama Muasya, Michelle van der Bank & William J. Bond
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. Though controversial, the savanna hypothesis continues to motivate research into the palaeo-environments of Africa. Reconstruction of these ancient environments has depended heavily on carbon isotopic analysis of fossil bones and palaeosols. The sparsity of the fossil record, however, imposes a limit to the strength of inference that can be drawn from such data. Time-calibrated phylogenies offer an additional tool for dating the spread of savanna habitat. Here, using the evolutionary ages of African savanna trees, we suggest an initial tropical or subtropical expansion of savanna between 10 and 15 Ma, which then extended to higher latitudes, reaching southern Africa ca. 3 Ma. Our phylogenetic estimates of the origin and latitudinal spread of savannas broadly correspond with isotopic age estimates and encompass the entire hominin fossil record. Our results are consistent with the savanna hypothesis of early hominin evolution and reignite the debate on the drivers of savanna expansion. Our analysis demonstrates the utility of phylogenetic proxies for dating major ecological transitions in geological time, especially in regions where fossils are rare or absent or occur in discontinuous sediments.
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. The savanna hypothesis of human evolution suggests that the transition from a predominately arboreal lifestyle in forest to one in open habitats favoured an upright posture and selected for bipedalism along with a shift in diet that necessitated travel over greater distances across the landscape1. The early support for the savanna hypothesis waned, in part, due to confusion as to the definition of prehistoric savannas—as open grassland or as a grassland-tree mosaic—nonetheless, it continues to influence thinking about the selective landscape that shaped human evolution, and generate large interest in the palaeo-environments of Africa where our ancestors emerged2,3,4,5,6. However, while our understanding of hominin evolution is continually updated by new fossil finds, the palaeontological reconstruction of the ancient African environment has been greatly limited by the sparse record of fossil bones and palaeosols that capture the signature of these past ecosystems7.
The origin and spread of savannas is thought to be closely linked to the attributes of the C4 grasses that dominate the herbaceous layer8,9. An influential hypothesis for the spread of the savanna biome suggests that C4 grasses likely first evolved when atmospheric CO2 decreased below a threshold of 500 ppm, which was thought to have occurred in the Late Miocene, appearing first at the equator with warm growing season temperatures and progressively moving to higher latitudes with cooler climates10,11. Low CO2 concentrations and high temperatures during the growing season would have favoured the C4 photosynthetic pathway, a CO2 concentrating mechanism, allowing C4 grasses to thrive relative to C3 plants11. Subsequent studies suggested that pCO2 fell below the threshold favouring C4 photosynthesis in the Oligocene, leading to rejection of the physiological model for a late Miocene origin of savannas12,13,14. However, new proxies support a steep decline in pCO2 from ~ 7 Ma15,16 which again raises the question of whether the timing of savanna origins along a latitudinal gradient is rooted in C3 versus C4 photosynthetic physiology.
Much of the evidence for a shift from forests to savannas comes from analyses of carbon isotopes in fossil soils and fossil bones. For example, it is possible to estimate tree cover in savannas from the ratio of C13 to C12 in current and fossil soil carbon2. Similarly, the dietary mix of fossil hominins can be traced from carbon isotope analysis17. Other proxies for reconstructing ancient habitats include pollen, alkanes, and rates of dust deposition into lakes or marine cores3,5,6,18. Together, these proxies are providing increasingly detailed reconstructions of environmental conditions and their variability over the past few million years. However, sites with suitable temporal continuity are few and largely restricted to East Africa. Fossil sites elsewhere in Africa are patchily distributed in space and time, and therefore provide only snapshots of environmental conditions in the past3,19.
Dated phylogenetic trees reconstructed using molecular sequence data and calibrated from the fossil record, offer an alternative source of information, and insights deeper in time. For example, the phylogenetic structure of modern species assemblages can reveal insights into historical biogeography, while the timing of lineage diversification and evolutionary radiations may be linked to shifts in dominant habitat types and climates20,21. New ecological opportunity brought about by dispersal to a new environment, the acquisition of a key innovation, or extinction of a competitor, might trigger adaptive radiations. The appearance of new clades sharing a particular adaptive trait can therefore capture important ecological shifts that might not be detectable in the fossil record22,23.
It would seem reasonable to use phylogenetic methods to date the origin of savannas by dating the origin of C4 grasses from C3 ancestors, yet phylogenetic analyses of C4grasses24 have placed their evolutionary origin in the Oligocene, ca. 30 Ma ago, and more than 20 Ma before isotopic evidence supporting the spread of savannas10,19. Diverse proxies indicate a steep decline in CO2 in the early Oligocene12, consistent with the physiological arguments for the advantages of the C4 CO2 concentration mechanism. However, environmental conditions favouring selection for C4photosynthesis were not the same as those favouring the assembly and spread of C4grassy biomes25: the evolution of the C4 pathway was a necessary but not sufficient prerequisite. The C4 pathway appears to have been a preadaptation to the more seasonal, open environments which were to emerge later. The phylogeny of the grasses that define the biome does not, therefore, allow us to date the key ecological transition from closed forests to open grassy ecosystems.
Rather than focus on the age of taxa that define a habitat, the dating of lineages that have diversified within that habitat might better capture information on its origins. For example, epiphytes or lianas, rather than trees, can be used as markers of closed forest: trees create the closed canopy, but epiphytes and lianas that are restricted to forest, provide a much better indicator of the presence and extent of forest habitat26. Predicated on the reasonable assumption that speciation rates correlate with available area27, lineage diversification within a habitat may provide an indicator of its geographical extent. Thus, by dating the radiation of clades tightly associated with a particular biome, we may infer its likely age28. Knowledge on the ecology of radiating clades can additionally provide insights into the environmental context that favoured important ecological and evolutionary transitions. For example, in the Cerrado, a South American C4 savanna, the evolution of fire-resistance arose multiple times independently within the last 4 to 10 Ma, and suggests a relatively recent, fiery, origin of Cerrado vegetation29. Here we use the evolutionary relationships between savanna and forest trees to explore the origins of African savanna.
Savannah hypothesis
The savannah hypothesis (or savanna hypothesis) is a hypothesis that human bipedalismevolved as a direct result of human ancestors' transition from an arboreal lifestyle to one on the savannas. According to this hypothesis, millions of years ago hominins left the woodlands that had previously been their natural habitat, and adapted to their new habitat by walking upright.
The idea that a climate-driven retraction of tropical forests forced early hominini into bipedalism has been around for a long time, often implicitly. Some early authors saw savannahs as open grasslands, while others saw a mosaic of environments from woodlands to grasslands. The hypothesis has seen rising criticism since at least the late 1960s.[1]:98 The open grasslands version is mostly dismissed, while the mosaic version still has relatively wide support, although the transition from forest to savanna probably was more gradual than previously thought.
History
The fundamental ideas behind it date back to Lamarck, Darwin and Wallace.[2][3][4] Also Gustav Steinmann saw reducing rain forest due to climate change as important driver for bipedalism.[5] Osborn thought man probably originated from the forests and flood-plains of southern Asia.[6] Hilzheimer stated it was open landscapes that stimulated development.[7]
The hypothesis first came to prominence however with the discovery of Australopithecus africanus by Raymond Dart in 1924. In an article on the discovery, published in the journal Nature, Dart wrote:
Weinert stated apes are very reluctant to leave the safety of the trees, and the ancestors of modern man did not leave the trees, but the trees left them.[9] Grabau echoed this by saying Instead of the apes leaving the trees, the trees left the apes.[10]
Not everyone agreed with this hypothesis, such as Weidenreich, but he did conclude it was a widely spread belief.[11]
The work of Robert Ardrey helped popularize the ideas that Dart had developed with a wide audience.
In the decades following Dart's discovery, more hominid fossils were found in Eastern and Southern Africa, leading researches to conclude that these were savanna dwellers as well. Much of the academic discussion at the time took for granted that the transition to the savannas was responsible for the emergence of bipedalism, and focused instead on determining particular mechanisms by which this happened.[12]…
… Not everyone was willing to write off the savannah hypothesis. A poor definition of what a savannah actually is, contributed to this. Critics of the hypothesis often saw the savannah as open grasslands with sporadic tree growth. However, savannas can have a high tree density and can also be humid. The big difference between savannas and forests is the lack of grasses in the latter. Thure E. Cerling developed a method to determine the forest cover of ancient landscapes, thus no longer requiring a definition of what a savannah is. By distinguishing between the C3 plants of the tropical forests and the mix of trees and C4 grassesof the savannah, they investigated the stable carbon isotope of paleosols from some sites in East Africa. They described landscapes varying from forest, woodland/bushland/shrubland, wooded grasslands to grasslands. They concluded that the early hominini lived in a more open environment than Australopithecus, rendering the savannah hypothesis still a plausible possibility.[32]
Following on from Cerling, Manuel Domínguez-Rodrigo stated that the usual division of landscapes into grassy, wooded and wooded is of little use, because it tells nothing about the evolutionary pressure on mammals. For example, the selection pressure of grass fields in tropical forests is incomparable to the grasslands of savannas. Tropical forests also have many different species of trees, while savannas only have a few species, which hardly carry any fruit. Another factor is that of scale. Paleontologists often only investigate the site itself, an area of several hundred to thousands of meters. These habitats are referred to as biomes, but the latter include many hundreds of kilometres. According to Domínguez-Rodrigo, the savannah hypothesis can still give a good explanation, although the transition of environment has probably been less abrupt than some earlier authors thought.[33] …
https://en.wikipedia.org/wiki/Savannah_hypothesis
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