Abstract
Within the last 2 million years changes in hominin and early human brain size have become more frequent along with the advancement of cognitive abilities. Many researchers believe that encephalization, or “the evolutionary increase in the relative size of the brain” and cognitive development in early to mid-Pleistocene Homo may be closely related; arguing that as the brain became larger, more cognitive abilities developed. Despite believing this, researchers are unsure if the changes in brain size and newly developed cognitive abilities are a result of surrounding environmental pressures, or if they were evolutionarily inevitable. It can be theorized that these changes are a form of environmental adaptations that allowed early to mid-Pleistocene hominins to survive in their new environments. However, it can also be argued that as hominins experienced changes in body mass and size, encephalization, would have occurred in proportion to these physical changes. Consequently, it is unknown whether these two factors contributed jointly or separately to changes in brain size and the increase of cognitive abilities. This paper will discuss both factors in relation to encephalization and the progression of cognitive abilities amongst early to mid-Pleistoene hominins, and early humans, and will review various environmental conditions that may have been drivers for encephalization and cognitive evolution.
Introduction
Historically, brain size evolution or encephalization can be identified among early to Mid-Pleistocene hominins can be identified through changes in fossil cranial capacity. Early hominin Australopithecines were known to have a cranial capacity that was a bit larger than apes (Shultz, Nelson, and Dunbar 2130). And The first sign of major brain size evolution was evident within the Homo clade, allowing researchers to hypothesize trends of changes in brain size for Homo (Rightmire 110). For about four million to three million years there were no major changes in average brain size, but around about 2 mya to 1.8 mya anthropologists began to see an evolution of brain size or encephalization. Evidence of a major increase in brain size was discovered in the specimen Homo erectus, displaying an increased cranial capacity compared to its predecessors. Encephalization was not a quick process, but instead was experienced over periods of accelerated brain size increase and periods of stasis (McHenry 161-162). The sudden increase in brain size after three million years has been questioned by researchers, who have tried to determine the reasoning behind this evolutionary choice. As encephalization progressed, hominins gained an increase in cognitive abilities, that lead to the development of new social, innovational, spatial, linguistic practices. The process of encephalization was very expensive for early to mid-Pleistocene hominins, requiring more energy for brain growth and usage than other organs. Some researchers hypothesize that there may have been a long-term evolutionary benefit that outweighed the costs of expending a large amount of physical energy on brain development. The evolutionary mechanisms that triggered an increase in brain size are unknown, but there are two arguments that have been heavily debated. The first argument suggests that encephalization was influenced by a series of environmental factors that acted as drivers for encephalization, while the counterargument claims that encephalization was change proportionately to an increase body mass and size. Encephalization, or “the evolutionary increase in the relative size of the brain”, in relation to the advancement of cognitive abilities, can be attributed various environmental drivers, and physical changes in body mass and size among early to mid- Pleistocene hominins as they evolved and migrated from Africa to Asia and Europe.
TIMELINE AND PROCESS OF ENCEPHALIZATION
To understand the timeline and process of encephalization, a brief background on hominin brain evolution must be reviewed. In the early Homo species, which includes Homo habilis and Homo ergaster, one begins to see the beginnings of Homo encephalization, increasing from 497 cm3 to 600-700 cm3 (McMillen 2014). Subsequently, in the later Homo species, changes in brain size can be observed within Homo erectus and Homo sapiens. As previously stated, the biggest change in encephalization occurred about 1.8 mya with the appearance of Homo erectus within the fossil record. Hominin brain size increased to about 900 to 1000 cm3 from the previous size of 600 to 700 cm3 (Stiedter, 2005). With the major expansion in brain size came an increase in body mass, and it is believed that body mass relatively increased with brain size. The body mass had to increase, so that the body could support a larger brain that required more energy. Homo erectus had evolved to store larger amounts of fat to meet the metabolic requirements needed for encephalization (Leonard et al., 2003). The First Modern Homo sapiens increased to an overall size of 1200 to 1400 cm3, which is resembled the size of modern human brain.
Tempo of evolution is defined by short rapid changes, followed by long periods of stasis. These temporal changes may have been a gradual and continuous process, or a series of short bursts, with periods of delay between them. Due to the tempo of brain evolution, selective pressures may have been driven by “long term low-level directional selection”, however not much is known on the tempo of brain evolution. However, there is an issue with clearly defining the temporal timeline of encephalization (Shultz ,2132). Susanne Shultz argues that if encephalization was a “single, gradual process”, then it should be an increasing trend with brain size increasing each year. And, if brain size was triggered by a “series of punctuated events”, there should be period of fast growth then none, and if encephalization is a result of speciation events, then it should have been recorded as flat lines between several speciation events (2132). Tracking encephalization amongst early to mid-Pleistocene hominins began with Shultz et. al, researching temporal encephalization changes across all hominins (Shultz et. al, 2133). All hominins were broken down into four super species, including Australopithecus spp. and Homo habilis; H. erectus, which included both H. erectus and H.ergaster; H. sapiens, including H. sapiens, H. heidelbergensis and Homo neanderthalensis. The last group Shultz chose to classify and break down was H. sapiens into anatomically modern humans. Within these four super species, temporal changes were categorized as step changes, instead of previously suggested single, gradual processes (2133). The first step changes can be attributed to the appearance of H. habilis and H. erectus sensu lato, who both appeared around 1.9 mya and 1.8 Mya, and the second step changes coincides with anatomically modern humans at about 195 kya. The biggest and most drastic step change was seen with Early H. erectus sensu lato, but it may have been attributed to a change in body size (2133). Although, there was much debate on temporal fluctuations of encephalization, Shultz et. al’s research provides evidence that the tempo of encephalization was instead a series of established step changes that have two major changes in brain size, eventually with two more major changes in brain size with a period of stasis in between.
BODY MASS & ENCEPHALIZATION
To determine the relationship between body mass and encephalization in Pleistocene Homo, one must look at the fossil remains of a range in body mass fluctuations over a series of individuals ranging from early Homo to late archaic H. sapiens. The body mass of Pleistocene Homo as averaged at about 10% larger than living humans (Ruff, Trinkaus, Holliday 173; 1997). According to Ruff, the major increase in encephalization within Homo occurred in the Middle Pleistocene 600 -150 kya and was immediately followed by a long period of stasis.
PRIME MOVERS FOR ENCEPHALIZATON
Majority of hominin species originated from East Africa and migrated into Eurasia after about two million years, and around 1.8 and 1.9 Mya Homo sensu stricto appeared with a new species called Paranthropus genera, marking the largest amount of hominin diversity at the time (Shultz, Maslin 1). During the same period, Homo erectus sensu lato experienced an increase in brain size, developing a brain that was 80% larger than the gracile Australopithecines and 40% larger than Homo habilis at the time. Homo erectus’ brain size averaged at about 900 cc, and late Homo erectus evolving to 1,100 cc (Rightmire 223-224; Donald 113; ). It was unknown as to why these major changes in encephalization had occurred after about 2 million years, so researchers began to hypothesize a series of environmental pressures that may have forced Homo erectus sensu lato to develop larger brain sizes (1). The environmental drivers are all interrelated, and caused what could be considered a series of triggers leading to changes in habitat, diet, social and linguistic, etc. for early Homo.
One of the first major environmental drivers that could have forced increases in encephalization is climatic fluctuations. Climatic fluctuations began in the East Africa with a theory called the savannah hypothesis, was increased aridity due to constant cooling and drying, causing the expansion of the savannah overtime (1; 2131). The variability selection hypothesis, states that in environments that experience unpredictable patterns of climate changes, often selects for “behavioral and ecological flexibility (1). Overtime, this forced hominins to move into novel habitats and alter their use of resources because these fluctuations created a dynamic and inconsistent habitat use (2131). Climatic pulses, such as the East African Rift system, introduced a series of selective environmental pulses, according to Shultz, drove a speciation and hominin future dispersal events. When trying to identify missing pieces within the fossil record that could provide evidence of brain expansion, The East African Rift system’s Rift lakes can be a climate indictor that can identify climatic fluctuations (Shultz). Hominin brain expansion coincided with deep water lakes, while following expansions coincided with extreme periods. The aridity of this environment can provide fossil evidence for speciation occurrences within the East African climate.
Due to the unpredictable climate, adaptive behavior was favored, which was flexibility and innovation in habitat, resource or space usage. Overtime a correlation between environmental variance and hominin brain size overtime became obvious (Shultz 2132). Consequently, hominins had to develop the cognitive ability to have a broad, flexible diet within a mosaic habitat and to develop tools to math their constantly changing environment, to maximize their chances of survival. The changes in savannah climate forced many hominins to move to wide open habitats, and increases their risk of predation (2132). Species such as, Homo erectus and Homo ergaster, were forced into open habitats as they lost many of the vegetation heavy refuges. Due to their small, predation was an issue for Australopithecines, stature, often being attacked by carnivores and raptors. It can be argues that as an adaptation their new environment and the risk of predation, H. erectus and H. ergaster developed a larger brain size and the cognitive ability to create defense strategies or escape plans (2132). Often predators preferred smaller brained prey, due to their ability to not be able to learn from constant predator attacks. A larger brain capacity would allow hominins to learn from previous predator attacks and prepare themselves accordingly. As a result of this selective pressure, hominins would have developed the ability to think and plan ahead, which is not an ability others share. Predation could possibly be a major factor in brain size development.
♣ Late Pliocene archaeological record (evidence)
♣ Lecture climatic fluctuations (FORAMS)
BIOLOGICAL/ECOLOGICAL IMPLICATIONS
There were changes in Earth temperature from about 5.0 to 3.2 mya , and after a few years, the Earth entered a cooling phase (Naya, Naya, Lessa 2015). Another climatic hypothesis is the theory that due to decreases in Earth temperature lead to an increase in thermoregulatory costs (Naya, Naya, Lessa 2015). These fluctuations in temperature forced the brain to allocate energy to the brain to maintain a balanced temperature for the early human brain and body. The decrease in Earth temperature served as an evolutionary driver for the evolution of an “expanded, heat-generating brain”. It is implied from this hypothesis that the decrease in Earth temperature was a driver for brain enlargement, or it could have possible been a negative selective pressure, with the brain returning to its normal state, causing a change in costs of maintenance. Naya proposes two scenarios, one that focuses on a correlation between Earth temperature and brain costs, and the second, temperature causes other prime movers or drivers to influence encephalization. In order for the brain to maintain a safe temperature, during rises or decreases in temperature, the brain experiences fluctuations in energy usage. With the encephalization or expansion of brain size came other physical adaptations, such as “replacing body hair with a layer of fat that permits an efficient heat loss by sweating, and the retention of hair on the head, which protects the surface of the body from solar radiation at midday and reduces amount of heat that meets the head.
CULTURAL/COGNTIVE IMPLICATIONS
An increase in encephalization with the combination of environmental pressures leads to the innovation of new forms of technologies and subsistence patterns. This is can be observed in the practice of hunting and tool-making among Australopithecus and H. erectus. Hunting and tool making can be hypothesized as an indirect cognitive ability developed as a result of encephalization. It widely known that hunting was a selective pressure obtained during human evolution that allowed early humans to properly use their resources on their changing landscape. Dean Falk states that H. erectus could carry out hunting parties with groups of “tool-bearing males”, who operated away from their homebase (Falk 103; 1980). G.S. Krantz who is mentioned by Falk, theorized “persistence hunting” as the prime mover of brain evolution for Australopithecus and H.erectus (Krantz 1968). For early hominins to organize a successful hunting group, it requires hunters to be able to retain a form of long term memory because hunters would need to keep the hunting task on their mind for at least several days (Falk 1980). Increase in brain size or previous encephalization can assist in improved memory, leading to more successful hunting trips and more food, and eventually better chances of survival. As time progresses, large brain sizes are favored over smaller brains. Accordingly, both hominins could create new hunting strategies overtime allowing them to successfully utilize their landscape and resources.
Tool manufacturing increased immediately with the expansion of Homo erectus’ brain. Some of the Early H.erectus’ tools are associated with Oldowan-type stone tools, but the majority along with a sophisticated toolkit that is later on identified with the Acheulian culture (Donald 113). The Acheulian tools can be classified into two or three categories with varying degrees of detail and specialization. Also, One of the later Homo erectus sites, in Choukoutien, China where various tools were found, archaeologists also found various forms of continuous fire usage that occurred about 400 kya (Donald 113-114). It can be hypothesized that due to Homo erectus’ increased encephalization they were able to be well-organized groups of individuals, who had the cognitive ability to cook, manufacture tools, hunt, transmit skills, and have a small-scale form of culture.
Another indirect result of encephalization or increases in brain size, and environmental pressures is the development of sociality. Aiello and Dunbar, argue for a strong correlation between group size and brain size, using anthropoid primates as a comparison. To decrease their risk of being attacked by predators in their new open environment, most primates will increase their group size because smaller groups tend to suffer from large amounts of predators (Shultz 2132). This hypothesis was used to estimate the average social group size for hominin species, however the correlation between brain size and group size are not strictly correlated. Increasing their ability to live safely in large group sizes, early hominins had to develop the cognitive capacity to be able to coordinate themselves within a social environment. Hominins had to develop the ability to empathize, resolve conflict, cooperate, negotiate, implement perspective taking and theory of mind (Shultz 2133). Living within a social environment one can argue that the demands of language needs may have drove hominin brain evolution. Hominin brains needed create a form of communication, so they could effectively communicate within a large social group. Consequently, this may have led to the creation of a complex language.
Even though, there is much debate over which factors have selectively chosen for encephalization or increases in brain size in early to mid-Pleistocene hominins, it can be attributed to a series of factors that are both environmental and evolutionarily biological. It is the combination of these factors that have led to various derived selective behaviors, such a tool making, language creation, hunting, etc. that have proved that encephalization is not just a result of two separate occurrences, but instead an adaptation to changing environments and surroundings. Encephalization, or “the evolutionary increase in the relative size of the brain”, in relation to the advancement of cognitive abilities, can be attributed various environmental drivers, and physical changes in body mass and size among early to mid- Pleistocene hominins.