There are multiple reasons. 1) there are similar fossils found on different continents. 2) the continents shapes show that they could easily fit together. 3) there are similar rocks, minerals,and land forms on different continents. 4) there's evidence that there was once lush vegetation in areas with extreme cold.
Depends on the context. They can be considered one type of nucleic acid or more generally a polymer or biological macromolecule.
A cell is the basic unit of life
A tissue is a group of cells that come together to perform a given function
An organ is a group of similar tissues that come together to perform a particular function