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Increase in hominin brain size Task: Use the P-R-O strategy to construct a theory-based explanation for a puzzling problem. Refer to the handout you received in lecture to complete this assignment. The handout is also available on Canvas under Lecture Materials. Puzzling Problem (P): Homo sapiens has the largest brain size (relative to body size) of any mammal. This increase happened quite recently, in the genus Homo. The graphe shows the brain sizes of Sahelanthropus, an ape that lived close to the time that hominin and chimpanzee lineages split, Australopithecus, and four species of Homo: Homo habilis, Homo erectus, Homo neanderthalensis, and Homo sapiens. During the evolutionary history of hominins, there was a behavioral change: hominins began to cook their food. There were also structural changes: a decrease in jaw muscle size and strength and an increase in brain size. Reasoning steps (R) Guiding questions: 1. Which came first, cooking or a decrease in jaw muscle size and strength? Justify your answer by considering how the conditions for natural selection favoring a decrease in jaw muscle size and strength would have been met in an early population in the genus Homo. 2. How did both cooking and a decrease in jaw muscle strength change the conditions for natural selection, leading to the evolution of increased brain size in the genus Homo? Outcome (0) 3. Based on this case study, explain one of the following. • A leading cause of fire deaths in young children is accidental fires set by the child. Hint: Consider why children are so fascinated by fire. Why might this behavior have been adaptive in our ancestors? Most mammals are terrified of fire. Two exceptions are dogs and cats, who often curl up in front of fireplaces. Hint: Humans have domesticated many animals, but dogs and cats are the most likely to be pets and to share our homes. Consider the role that selection played in the process of domestication • Unlike earlier hominins. Homo erectus made nests and slept on the ground, instead of in trees. Hint: Why did early hominins build nests and sleep in trees instead of on the ground? What dangers do they avoid? What allowed Homo erectus to sleep on the ground instead of in trees, despite these dangers?

Answer 1

Answer:

1.The one which came first is a difficulty or a hindrance in food uptake. Plant fibres eaten by apes and chimps are very hard to digest and needs a lot of force and energy to chew them all up. Which is why our ancestors must have been provided with strong jaw muscles. However when for the first time food was cooked ,it became so much easier to digest and there was no need of a lot of energy and force in chewing. As nature favored ones with great nutrition uptake, weak jaw muscles got favoured (which also needed less energy). So cooking came first as an innovation for better life which leaded to decreased size and strength of jaw muscles.

2. Larger brains with complex structures and highly active neurons need a lot of energy per day. However those of our ancestors which eat raw food invested much of their energy in chewing and digesting the hard plant fibres. However for the ones who cooked their food , there was a lot of availability of energy which was invested on brain development . So they gradually became the ones with great cranial capacity.

3. The fact that children are so fascinated by the fire is linked to the evolutionary instinct. It's one of the stored memory in our ancestral gene. Fire for the first time in the history enabled us to cook our food and hence the development of our brain. It allowed us to be protected from most of the animals which was a major factor of survival. Human beings are the only ones who knew the technique to master fire and probably that became the reason of their evolutionary success eventually. So that's so intense that it is associated in our genomes as an instinct to use it even as toddlers.

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