Simplify the following text for a Grade 10 native English-speaking person.
Original: Algae constitute a paraphyletic and polyphyletic group , as they do not include all the descendants of the last universal ancestor nor do they all descend from a common algal ancestor , although their plastids seem to have a single origin . Diatoms are also examples of algae .
Simplified: However , algae are definitely not a monophyletic group , as they do not all descend from a common algal ancestor .