This is so familiar a layout for animals that it takes a while to get used to the fact that animals can be laid out differently. Much stems from our bilaterial symmetry – our forms of motion, the ways our sensing and behavior are organized.
We are bilaterians, while jellyfish and sponges are not. Some time after the earliest days, an important event was the invention of bilaterally symmetrical animals – or bilaterians – those with a left and right as well as a top and bottom. The sea is a truly three-dimensional world. The first animals may have floated about as semi-organized balls of cells. That setting brings with it many consequences – a particular chemical environment, and physical features such as ease of motion.
This thinking was prompted by a pair of of conferences I went to over the last few years, one in Tübingen and one in London. This post is a continuation of a series I started a few months ago, looking at old and new scenarios for the early evolution of animals.
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