Aineo wrote: Well, Evilusitonist if all life did not evolve from a one celled organism where did the first multi-cell organism come from?
Two separate issues here. The first is whether or not the Cambrian explosion marks the transition from one-celled organisms to multi-cellular organisms. It most certainly does not as multi-cellular organisms were present prior to the Cambrian explosion. Second, how did multi-cellular life arise is a good and interesting question. At present, there is no single answer, but there are tantalizing clues. The amoeba
dictyostelium is a single-celled organism that undergoes a unique change during environmental stress. Essentially, the organism becomes a multi-cellular fruiting body. Here is a small excerpt on this type of organism from a paper by A. Fortunato et al., 2004.
The social amoeba, Dictyostelium discoideum, is an organism that can form multicellular chimeras through aggregation (Strassmann et al. 2000). It is a haploid amoeba frequently found in the soil of deciduous forests in the temperate zone (Bonner 1967; Raper 1984; Landolt &
Stephenson 1990). Molecular biologists have extensively studied D. discoideum and employ it as a model organism,for example, in the study of cell signalling and differentiation (Kessin 2001). The life cycle of D. discoideum is characterized by a vegetative phase during which unicellular
amoebae feed on bacteria and reproduce asexually. When starving, the amoebae initiate aggregation, with the emission of cAMP regulating this process (Konijn et al. 1967;Barkley 1969). From these aggregations, there emergesa multicellular pseudoplasmodium or ‘slug’ capable of movement. During the multicellular phase the amoebae differentiate allowing the pseudoplasmodium to form a fruiting body or sorocarp consisting of two principle cell types: the spore and the stalk cells (Bonner 1967; Raper
1984; Williams 1997; Kessin 2001). The spores are found in a sorus, a globular structure at the apex of a thin stalk that attaches to the substrate by a disc of cells (Raper 1984). Formation of the fruiting body probably guarantees better survival and dispersion of the spores but requires the
death of the cells that differentiate into the stalk and the basal disc. Altogether, approximately 20% of the cells that initially comprise the pseudoplasmodium die in the for- mation of the stalk and the basal disc (Raper 1984). The dramatic differences in the destinies of the cells that come together in forming a pseudoplasmodium offer the opportunity to study conflict between the cells that aggregate, particularly if amoebae of different genotypes join to form pseudoplasmodia (Buss 1982; Armstrong 1984; DeAngelo et al. 1990; Matsuda & Harada 1990; Gadagkar & Bonner
1994; Strassmann et al. 2000; Hudson et al. 2002; Foster et al.
2002; Queller et al. in press). In this case the pseudoplasmodium represents not only a multicellular organism, but also a society of unicellular organisms. As in a colony of social insects in which the workers do not reproduce but rather labour on behalf of their colony, the stalk cells of D. discoideum altruistically sacrifice their own reproduction in order to aid the success of the cells that become the spores.
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