<P> Multicellular organisms, especially long - living animals, face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development . Changes in tissue morphology can be observed during this process . Cancer in animals (metazoans) has often been described as a loss of multicellularity . There is a discussion about the possibility of existence of cancer in other multicellular organisms or even in protozoa . For example, plant galls have been characterized as tumors but some authors argue that plants do not develop cancer . </P> <P> In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved . However, Weismannist development is relatively rare (e.g. vertebrates, arthropods, Volvox), as great part of species have the capacity for somatic embryogenesis (e.g. land plants, most algae, many invertebrates). </P> <P> One hypothesis for the origin of multicellularity is that a group of function - specific cells aggregated into a slug - like mass called a grex, which moved as a multicellular unit . This is essentially what slime molds do . Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte . A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism is observable in Drosophila). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues . This is what plant and animal embryos do as well as colonial choanoflagellates . </P> <P> Because the first multicellular organisms were simple, soft organisms lacking bone, shell or other hard body parts, they are not well preserved in the fossil record . One exception may be the demosponge, which may have left a chemical signature in ancient rocks . The earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon (Gabonionta). The Doushantuo Formation has yielded 600 million year old microfossils with evidence of multicellular traits . </P>

Would you expect a cell of a multicellular organism to be more complex