I'm almost sure your answer is going to be "Because the sun's mass creates a gravitational force that pulls earth towards it."
correct me if I am wrong. 
 
        
                    
             
        
        
        
<u>Answer:</u>
Fossils are the sources to learn the changes on the earth and how much it changed in these years. Its provides us with information about the existence of plant and animals in the past. 
Actually, the fossils we found till the date are the ancestors of the plants and animals that live today.  Moreover, some fossils of animals are easily identifiable since they exist till the date.  
But there are fossils of animals that exist no longer in the earth. By studying and analyzing the fossils, one can tell how life on the earth has changed. 
Also, we can learn about the ancestors of the plants and animals and how their long life existed on earth. 
In other words, one can learn where and how those organisms were lived. So that we can know about our ancient environments.
 
        
             
        
        
        
Yes both occur in mitocondria.
        
             
        
        
        
Antibodies
<span>Antibodies are a kind of protein that gets secreted by B lymphocytes of the immune system, and each antibody is going to be specific to one epitope. Once the antibody finds its specific target, such as the surface of a pathogenic bacteria, other cells in the immune system will perform a certain function depending on the kind of antibody.</span>
        
             
        
        
        
Because we would die without oxygen