This site on the origin of life on Earth is visually quite captivating. What's interesting, though, is the fact that, as reflected even on other sites, scientists seem relatively comfortable with the variety of possible situations that led to the appearance of the first cells, while the appearance of multi-cellularity just boggles everyone's minds. One of the most common explanations given for the existence of multi-celled organisms is that they evolved as a result of symbiotic relationships between types of cells that were already in existence. When it comes to providing details regarding the mechanism that led from symbiosis to full-fledged multi-cellularity, though, the data come up somewhat short. Once multi-cellularity appears, scientists breathe a sigh of relief and once again feel comfortable speculating on mechanisms and processes, invoking natural selection and other safety blankets. But that HUGE gap between the first cells and structurally complex organisms sticks out like a sore thumb and will have to be addressed sooner or later.
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