A sponge essentially carves organs out of negative space, using its layers and jelly to delineate a complex network of channels and pores, which transport nutrients and waste much like a human kidney or bloodstream. This Spartan anatomy is so efficient that a single sponge can filter up to a thousand times its body volume of water in one day. Off the coast of Canada, reefs of glass sponges (so named for their silicate skeletons) can clean more than five hundred vertical feet of overlying water. And, if they take in dirt or toxins, sponges can clear themselves out with a languorous sneeze.
Even some professional biologists disregard sponges as lowly, primitive proto-animals, sitting at the bottom of an evolutionary ladder with us on the top rung. We treat their biology as an impoverished subset of our biology. We relegate their existence to a checklist of missing traits: no limbs, muscles, nerves, or organs, and none of the tiger’s fearful symmetry. But these creatures, according to Dunn, Leys, and Haddock, are not primitive relics; they are modern animals that excel at their own particular life styles. By ignoring them, we blind ourselves to a wondrous hidden biology and get a misleading view of evolution.
There are no primitive life forms. There are only relative degrees of nearly unfathomable complexity.
This revised tree, with ctenophores on the earliest branch, complicates several once tidy stories about the evolution of animal traits, notably the nervous system. Sponges lack neurons entirely, but their genes seem to allow for chemical signalling of some kind. Ctenophores have nervous systems but lack the genes that other animals use to build neurons and neurotransmitters. If sponges are the earlier of the two clades, the story unfolds neatly: they had the genetic building blocks for a nervous system, which ctenophores elaborated and bilaterians went to town on. But this narrative shatters if ctenophores branched off first. It could mean that they evolved nervous systems independently from all other animals, including us. Meanwhile, sponges either never developed true nerves or started off with nerves and lost them (after all, what need does a sedentary filter feeder have for such an extravagance?).
This is a much tougher reality to accept. The idea of one group of supposedly primitive animals going off-script and inventing a different nervous system, and then a second group actually losing theirs, is practically unconscionable. More.
Mmmmm, not unconscionable, just unDarwinian.
Oh, wait! [added] That amounts to the same thing.
Oh, not to worry, relax and remember that Darwin’s followers are surely thinking up answers, or fresh abuse for doubters. And in the mean time, New Yorkers, DON’T get any ideas. Or you’ll never party with the airheads again. Ever.
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