On NPR's Cosmos and Culture blog biologist Ursula Goodenough has been writing some terrific posts on evolution. Two that recently caught my eye:
Unicellularity Vs. Multicellularity: Why We Bother With More Than One Cell
Time and Life
Both are worth spending some time on.
Coming back to the topic of the evolution of multicellularity the latest issue of Geology has a paper that does some analysis on continental reconfiguration during the early Cambrian and proposes that there was a major movement and rotation of Gondwana resulting in establishment of a new ecological landscape, new conditions that may have provided the impetus for the rapid radiation of metazoans otherwise known as the Cambrian "explosion".
Explanations for the Cambrian explosion have occupied two extremes. One view proposes that the fuse was a biological one. Some crucial biological innovation in terms of gene regulation and molecular cascades remained to be discovered until complex metazoans could evolve. According to this view these changes likely occurred beginning around 600 mya. The other view proposes that practically all the complex genetic machinery necessary for metazoans to function already existed in unicellular eukaryotes. The reason for the delay in the advent of metazoans (complex unicellular eukaryotes date back to more than a billion years) were ecological constraints such as a lack of enough oxygen in the atmosphere.
Lately I sense that the ecological argument is winning out not least because earlier experiments in multicellularity are coming to light. Life getting organized into agglomerates of cells is a theme that has been independently invented several times and as far back as a couple of billion years ago if recent findings in Gabon and in India's Vindhyan basin hold true.
The thought of genetic potential waiting to be unshackled into new and varied forms is fascinating and one doesn't have to venture into the unfamiliar Cambrian terrain for an example. The transformation of the wolf into the myriad morphs of dogs is a strong example. A recent study (via Panda's Thumb) showed that the morphological variation in skull shape across the entire order of Carnivora is less that the variation seen within dogs. The ecological landscape experienced by ancestral wolves did shift dramatically in recent times not by continental movements as proposed for the Cambrian radiation but by the imposition of new selection pressures by humans. Wolves apparently always had the genetic potential to diversify into what could be described as downright weird forms. The changing human ecology provided the trigger and helped maintain these forms, many who may not have survived in the wild.
The evolution of complexity seems to be open ended. There must have been occasions when there was a significant change in genetic architecture. John Maynard Smith and Eors Szathmary document these steps in their book The Major Transitions in Evolution. Other times it was an exogenous influence that triggered a change, a matter of filling a new ecological niche.