Introduction to Gene Expression Edit
All cells today have some common properties. Their genetic instructions are stored in DNA molecules. The DNA can be thought as 2 candy bars. These 2 bars will be separated, each bar transcribed to one RNA. In that bar, amino acids (can be thought as candies), will be separated again, rearranged into meaningful orders. This is called translation. When it is done, the bar (which arranged candies) is now our protein. There are 20 amino acids. However, their order in proteins varies from species to species. That means all cells have same set of amino acids, but they translate it differently. It is analogous to Latin symbols, each language has its own properties.
The Great Ancestor Edit
Because basic properties of all cells are so identical, people think they must have been derived from one ancestor. Scientists have estimated that ancestor living around 3.5 to 3.8 billion years ago.
Cells reproduce themselves by splitting their genetic instructions into 2 parts, each one pasted to a daughter cell. That explains why 2 daughter cells look like their mother. But this splitting (mutation) is not perfect. It corrupts at some stage, somewhere. That again explains why 2 daughter cells are not completely identical to their mother. If mutation leads to bad results (the descendants die easier, etc.) the daughter cells will be eliminated (because they die soon). If mutation leads to good results, its descendants will survive. If mutation is neither good nor bad, the nature will not like it very much, but it will survive anyway. From time to time, species emerged and species extinct. The good survive, the bad die. All diverge from one ancestor.
It is helpful to note that 2 daughter cells are chemically alike, in other words, they are one species. It is not necessary that their size is the same, as yeasts' daughter cells are asymmetric.