One of the first criticisms levelled against Darwin's theory of evolution through natural selection was that many simple organisms have been observed to remain largely unchanged over long periods of geological time.
The Foraminfera represent an important group of amoeboid protozoa that live in shells that have been preserved in sedimentary rocks since the Cambrian and indeed are a major constituent of chalk and limestone rocks. They are often cited as simple organisms that have not evolved. This citation however, was the result of a fundamental misunderstanding of the nature of natural selection, which does not have to be in some direction or other. Foraminifera have remained unicells from their first appearance, but they have evolved.
In particular, large foraminiferans appear to have evolved over and over again. The large living foraminifera appear to owe their size to symbiotic relationships with intracellular photosynthetic protists (unicellular algae or dinoflagellates) once again illustrating the importance of symbiosis in evolution.
The adaptive significance of most foram shell shapes is not understood and so possible reasons for convergent evolution in forams of different geological eras are hard to find!
