What evidence exist that suggest human activity is related to global warming?
Data collected on Earth's temperature show that our atmosphere is becoming warmer. At the same time Scientist have collected data on the amount of Carbon Dioxide that exist in the atmospehere. There is a postive correlation that shows when CO2 levels increase so does the temperature of the Earth. While CO2 is naturally released Scientist have show that in the last 100 years the atmospehric CO2 levels have risen dramatically and in-line with the use of fossil fuels by humans. SO with the data collected, one can suggest that the use of fossil fuels has increased the amount of CO2 in the atmosphere which in return has contirbuted to the rise of Earth's global temperature.
Why does Earth looked different now then it did millions of years ago?
The process of plate tectonics has changed the positions of the land masses and also driven voclanic activity. Both of wich have change the Earth's appearance. The addition of living organisms to Earth helped form an atmosphere (oxygen, carbin dioxide, etc) that allows for weather. With rain, wind, and sun we get erosion and sediments (Grand Canyon, Mississippi delta). These processes have also altered the visual appearance of the Earth.
How does the human body release the stored energy in food?
Many people think that we eat food, digest that food, and then use the digested food for direct energy. However, while the food is stored (potential) energy, are body cannot utilize it in that form. Instead we break the food down and use the sugar in the food to create ATP through Cellular Respiration. It is the ATP that are body uses to carry out life functions.