Lecture 13.docx

Lecture 13: The Origin of Humans – and Our Big Brains The Origin of Primates First appeared in the Paleocene epoch about 60-65 MYA Diversify over the next 50 million years Our lineage began to emerge around 6-7 million years ago Diversification of hominins A Tale of Three Humans A third kind of huma...

Lecture 13: The Origin of Humans – and Our Big Brains The Origin of Primates First appeared in the Paleocene epoch about 60-65 MYA Diversify over the next 50 million years Our lineage began to emerge around 6-7 million years ago Diversification of hominins A Tale of Three Humans A third kind of human called, Denisovans Coexisted in Asia with Neanderthals and early modern humans Defined by the DNA from one bone chip and two teeth Closely related to Neanderthals But a big brain was not the defining feature of our earliest ancestors What was? Bipedalism, but why? Free hands to collect food, climb trees, and avoid predation Reduce amount of sun-skin contact Prevent heat stroke through perspiration Reduce heat stress We evolved in a period of major climate changes, leading to changes in the type of habitats available Reflected in tectonic plate movements (continental drift) Reflected in changes in global temperatures The Site of Human Evolution Africa From dense rain forest to savanna That is, from ape friendly habitats (forest) to ape unfriendly habitats (savanna) Bipedalism is an adaptation for life in the savanna Reason: Foraging at mid-day to avoid predation so being upright reduces exposure to sunlight, and this is coupled with hair reduction to aid in perspiration to keep cool. So, what about the brain increase? The Expand of the Hominin Brain: Expansion coincides with the first members of the genus Homo, whose fossils are associated with stone tools and eating meat Rapid increase in brain volume once meat and stone tools were introduced Meat is a higher food quality than vegetation, requiring less work to extract calories and nutrients, and so less tissue devoted to process the food, such as smaller jaws and teeth (and so muscles to power them). Smaller gut allows for bigger brain Even better, do some of the food processing outside the body, making not only meat easier to digest, but also nutritious, but tough to eat vegetables (e.g., tubers, cereals, beans), but how? The Use of Fire Higher quality diet reduces bulk and faster digestion of meat = More complex feeding strategy & smaller gut Larger Brain by quicker energy assimilation Dietary factors account for bursts of brain expansion, but how is that achieved? The Juvenile Ape Hypothesis Humans look a lot more like infant than adult chimpanzees Evo-Devo: Via heterochrony, hominins retained a juvenile brain while maturing the sexual organs (Gould)