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Basically, it's all chemistry. Genes produce proteins, right? Well, some of them are not as active in the presence of a given chemical. Then, other genes are suppressed by the presence of proteins produced by genes which activate in response to environmental chemicals, or heat, or whatever. You can have whole networks of things like that. I seem to recall, for example, that the human menstrual cycle is controlled by the production and then suppression of a bunch of hormones - estrogen is just the most noticeable.
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If you study the development of an embryo, it certainly gives some important insight into how the simple cellular organisms may have worked together to form larger, multicellular organisms with specialized cells.
The embryo starts out as a single cell with all the information it needs to create an entire organism. For some time, the non-differentiated cells replicate, but eventually they begin to form specific shapes, and then begin to specialize into different organs and body parts.
It's quite complex, but also fascinating, and while it doesn't completely map out the evolutionary road from single celled organisms to complex animals, it gives you a lot to think about.
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quote:Originally posted by IanO: Wait a minute. So the fact that a cell contains the genes for a number of functions means that it was capable of all of them at some point? Now, I suppose I can see that a cell, for example, can become a nerve cell, muscle cell, blood cell, etc. And that only certain genes are expressed so that each cell handles only certain functions. But I still don't see how these cells naturally began to work together so that some cells could become muscle, other bones, other nerves, all according to the genome, and they harmoniously act as a single organism (to a certain extent), to become a rat, a fish, or a human. How does each cell know what to become and where it should be? How did the fully centralized organism's genome come to be, describing a large multicellular, multi-cell type organism? I'm trying to wrap my head around this.
What you describe is one of the most active (and exciting) areas of developmental genetics right now. It's an incredibly complex system, with transcription factors activating other transciption factors, feedback loops that feed in on themselves, and genes that actually plot out the body plan in a Cartesian coordinate system. To focus on one specific example, the Hox gene family patterns the anterior-posterior axis of the body plan (that is, it distinguishes head from tail end). This is an absolutely critical part of the development of bilateral symmetry. Based on genetic data from just about every animal phylum, it appears that all Hox genes share an evolutionary origin: a single axial patterning gene still present in some modern-day anemone species!
Gene duplications and divergence over time resulted in a cluster of Hox genes, each of which was responsible for patterning a specific segment of the body plan. This is what you observe in insects like Drosophila: a single cluster of genes which patterns the entire anterior-posterior axis. In higher-order animals, include all vertebrates, further gene duplications, along with duplications of chromosomes and the entire genome, have resulted in multiple sets of Hox clusters, each of which plays an overlapping role in axial patterning.
What's really amazing about all this is how clear the data is in supporting the shared evolutionary origin of the Hox genes. The locations of each Hox gene within a given cluster reflect its phenotypic role: the genes on the 5' end of the cluster pattern the anterior end, and the genes on the 3' end are responsible for posterior patterning. This chromosomal sequence is conserved in all known Hox clusters, suggesting that all Hox clusters (including the multiple clusters observed in vertebrate genomes) originated from a single primordial cluster. Analysis of sequence homology between genes within single clusters further demonstrates that the entire Hox system descended from one "ProtoHox" gene.
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