Fossil protists have been found in rocks dated at more than 1.8 billion years old. The transition from single cell life to multicell life is one of the most fascinating questions of modern biology. This step is exemplified by the colonial alga Volvox . All animal life is now thought to have evolved from a simple colonial organism that llived over 700 million years ago. We have also posted a downloadable annotated list of web links on this topic .
Protists represent some of the most diverse branches in the tree of life.
The evolutionary branch leading to Closterium , the bright green alga cell seen in our gallery, may have split from the yellow pigmented diatoms as far back as two billion years ago. The line leading to ciliated protists such as Blepharisma or Stentor probably branched even earlier. Although amoebas share the characteristics that their cell membrane is flexible, there is even a great diversity among this protist group. Evidence from molecular biology studies shows that some amoebas are much more closely related to Euglena than they are to the classic textbook amoeba, Amoeba proteus . A close study of Paramecium with a student microscope allows for the observation of phagocytosis, digestion, water balance, ciliary action, cell excretion and a variety of responses to stimuli.