What is a “Theory”? Scientific theories explain facts and laws, have predictive power, and so can be tested. Most people rate facts and laws as more important than theories, thinking of theories as mere "guesses”. But for scientists, theories are the highest level of understanding. They are not just stepping-stones to more knowledge, but the goal of science. Examples of explanatory theories include gravity and evolution.

How Does Natural Selection Work? Natural selection is the process by which species evolve over time. Individuals inherit traits, or features, from their parents because these traits often help them survive and breed. Possession of traits that do not facilitate survival and breeding can lead to the extinction of a species.

For example, saddleback tortoises have long necks and can reach food at high elevations. On islands that lack food close to the ground, animals with this trait have a better chance of surviving and reproducing than their short-necked cousins. Over time, the long-necked tortoises are naturally selected: they survive at higher rates than their short-necked cousins. This is an example of how a species can evolve! Natural Selection at Work!

Big Question: How do scientists think natural selection affected human evolution?

How did this happen?

Discuss: How would larger brains allow humans to survive better than competing species?

Bigger brains (especially in the frontal lobe) led to more advanced cognitive functions (thinking!) * Jaws became smaller due to the use of hands to eat food

Thinking led to the Physical and Cultural Evolution of Human Beings Stone tools for cutting, cooking, & killing Control of fire: cooking, settlement of colder areas Enhanced nutrition - eat plants and meat Division of labor by gender: –Women raise children, gather plants –Men hunt Tools!

Evolutionary advantages allowed for the migration of early humans out of Africa and into Europe, the Middle East, and Asia.

Evolution and migration of Homo Sapiens Displacement of competing species through natural selection

Displacement in Action Homo Sapiens and Neanderthals co-existed for a time. What happened? Neanderthal Homo Sapien

Three Theories Neanderthals disappeared because : -1. Ancient form of genocide (homo sapiens killed them) -2. Driven to extinction (homo sapiens were better at hunting, and adapting to new conditions) -3. Interbreeding (so they are still among us…)

Summary Question In your own words, how did humans become a dominant species?

Natural selection Evolution of both advantageous Physical and Cultural traits Increasing adaptability