The dramatic growth of human ancestors' brains around 2 million years ago is a key topic in the study of human evolution, and several hypotheses have been proposed to explain this phenomenon. Here are the main theories and the one that is often considered the most likely:
Diet and Nutrition(Leslie Aiello and Peter Wheeler, Richard Wrangham): The inclusion of higher-quality foods, particularly after the advent of cooking and increased meat consumption, provided more energy and nutrients that supported brain growth. The "expensive tissue hypothesis" suggests that a diet rich in energy allowed other organs to be smaller, thus freeing up energy for a larger brain.
Social Brain Hypothesis(Robin Dunbar): This theory posits that complex social interactions and the development of larger social groups drove the need for a larger brain. This includes the complexities of communication, remembering individuals, forming alliances, and understanding social dynamics.
Ecological Challenges and Environmental Changes: Adapting to changing environments and ecological challenges may have spurred brain growth. This includes problem-solving, memory, spatial orientation, and understanding and manipulating the environment.
Tool Use and Technology(Kathy Schick and Nicholas Toth) : The development and use of tools may have driven brain expansion. The cognitive demands of making and using tools, which involves planning, dexterity, and understanding cause and effect, could have selected for greater intellectual capabilities.
Language Development(Steven Pinker, Noam Chomsky): The evolution of language capacities may have contributed to brain expansion, as complex language requires significant brain processing power.
Out of these, the most widely accepted and discussed is the combination of diet and social complexities. The dietary change hypothesis, especially the role of cooking and meat consumption, is often considered crucial because it provided the necessary energy and nutrients for supporting a larger brain. At the same time, the social brain hypothesis is also highly regarded, as the cognitive demands of living in complex social structures are thought to have significantly influenced brain development.
However, it's important to note that these hypotheses are not mutually exclusive and likely interact in complex ways. The dramatic enlargement of the brain in human evolution is probably the result of multiple, interrelated factors rather than a single cause.
I feel that the above hypotheses are not clear enough to completely remove the question. I believe that fully bipedalism was the main reason for the rapid increase in brain capacity between about 2 and 1 million years ago.
Fully bipedal walking theory/Eam Taekyoung I believe that fully bipedalism was a major factor in the rapid increase in brain capacity in our human ancestors. Full bipedal walking means walking with the center of gravity of the head very close to the fulcrum where the feet contact the ground. This means walking in a position where the moment caused by the head is almost zero. Moment = weight of the head x distance the head protrudes horizontally from the point of support This fully bipedal walking minimizes the fixed expenditure of energy consumption due to posture. The resulting surplus energy increased brain activity, and increased brain activity led to the evolution of an increase in brain size. The use of fire that began about 1.4 million years ago appears to have acted as another factor in continuing this trend. It appears that complete bipedal walking itself could have been completed through the following process.
1. Extinction of the tail: The appearance of gibbons (living in the rainforest) whose long arms replaced the function of the tail.
2. Beginning of knuckle walking: The appearance of gorillas that knuckle-walk using their finger joints (living in the rainforest)
3. Beginning of bipedal walking: Approximately 21 million years ago, the appearance of Morotopithecus, which lived in the grasslands outside the rainforest by eating leaves with its bumpy molars(Morotopithecus was able to pick leaves by standing upright rather than leaning down, as leaves are everywhere on the branches)
4. Beginning of a mainly bipedal lifestyle: Approximately 3 to 2 million years ago, the appearance of Australopithecus living in open savannas/mixed savanna and rainforest/dry but water-rich river areas/bush forests, etc.
5. Beginning of deterioration of the offensive function of the jaw and teeth: Approximately 2.5 million years ago, the emergence of Homo habilis, which used tools
6. Beginning of fully bipedal life: The emergence of fully bipedal Homo erectus (about 1.9 million years ago) and the use of fire (about 1.4 million years ago) led to a rapid increase in brain capacity.
7. Evolving into a body optimized for running: Approximately 300,000 years ago, the emergence of modern humans (Homo sapiens) with bodies optimized for running.
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