In the final exams for our undergraduate zoology degrees, my fellow-majors and I were given an assortment of petri dishes, each of them containing an animal. Our task was to classify the creatures to the phylum level. Now, more than a decade later, I can conjure up only two of the test dishes. The first contained a dead cockroach (phylum: Arthropoda). The other contained a rock in a thin layer of water, with a green, slimy film on one of its faces. Midway through the allotted time, the invigilator observed aloud that many of us seemed to be trying to classify the rock. It was, he assured us, a rock. The unspoken corollary: we should perhaps focus instead on the slime.
The second dish turned out to contain a sponge (phylum: Porifera), an animal that looks like the antithesis of an animal. Sponges sit immobile, anchored to rock or rooted in sediment, filtering particles from water. There are around eighty-five hundred known species, and they live in both oceans and freshwater. Sponges exist in a rainbow of hues and can, in the case of the Caribbean giant barrel sponge, grow up to eight feet in diameter. All of them share the same basic body plan: a measly two layers of cells, enveloping a jelly-like filling. They lack digestive systems and circulatory systems. They have no left or right, no front or back. Good luck finding a sponge’s head—it doesn’t exist.
But this simplicity is deceptive. 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.
Sponges are smothered in a diverse coterie of bacteria and other microbes. All animals, including humans, form partnerships with such organisms, but a sponge’s partners produce an exceptionally vast pharmacopeia, perhaps to help it cope without an immune system of its own. Humans have repeatedly raided this chemical larder. Azidothymidine, the first drug approved by the U.S. Food and Drug Administration to fight H.I.V./AIDS, comes from a Caribbean demosponge. Other species gave rise to treatments for herpes, leukemia, and breast cancer.
Last month, in the journal Trends in Ecology & Evolution, a group of scientists published a tub-thumping defense of sponges and other supposedly simple animals. In their paper, Casey Dunn, Sally Leys, and Steve Haddock argue that humans have systematically underestimated these creatures, largely because of our innate bias against organisms outside our taxonomic clique. That clique, actually called a clade, includes all of the so-called bilaterians—animals with left-right symmetry that share a single ancestor. Tigers, hummingbirds, octopuses, scorpions, crocodiles, mantises, sharks, earthworms: all are bilaterians.
Dunn, Leys, and Haddock write that, as bilaterians ourselves, and rather narcissistic ones at that, we tend to look down on the other four animal clades: the placozoans (flat, creeping mats that are represented by just one known species); the cnidarians (jellyfish, sea anemones, corals, and their stinging kin); the ctenophores, or comb jellies; and the sponges. 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.
Take the ctenophores. There are only two hundred or so known species, but they are abundant in oceans from pole to pole. Because they are transparent and gelatinous, they are often mistaken for another animal. “People try to put them in boxes in their head, and the easiest one to put them in is the jellyfish box,” Haddock, who works at the Monterey Bay Aquarium Research Institute, in California, told me. That’s barely more accurate, he said, than putting a platypus “in the duck box or a beaver box.” Whereas jellyfish pulse through the water mouth downward, their umbrella-like bells opening and closing, ctenophores swim mouth forward; in most textbooks and scientific papers, they are depicted the wrong way up. And whereas jellyfish are radially symmetrical, like a flower, ctenophores are rotationally symmetrical, like a yin-yang symbol; their halves only match if you turn one a hundred and eighty degrees.
Ctenophores collar their prey with glue-secreting cells called colloblasts that are unique to them. And they are at their most innovative when it comes to the tiny beating hairs called cilia. We use cilia to clear mucus from our airways. Ctenophores use them as flippers, sense organs, and serrated teeth. One species, the sea walnut, uses its cilia to create imperceptibly subtle water currents that draw fish and other prey into its mouth. It is such an effective predator that, whenever it enters a new body of water, it throws the food webs into disarray. In the nineteen-eighties, it got into the Black Sea, where it devastated anchovy populations, to the detriment of the local dolphins.
Haddock says that ctenophores are now becoming trendy, in part because they appear to be the sister group of all other animals—that is, the earliest clade to have branched off from the main trunk of animaldom, hundreds of millions of years ago. For centuries, that honor had fallen to the sponges, but in 2008 Dunn, a Brown University biologist, compared genes from twenty-nine animals belonging to several phyla and concluded that ctenophores, not sponges, were the first to diverge. “That really raised some eyebrows,” he said. “Some people took that to mean that our analysis was intrinsically flawed.” But more and more evidence has since piled up in support of that view, including the first complete ctenophore genomes.
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. “There’s a tendency to think that, in the evolutionary lottery, humans lucked out,” Dunn told me. “We have cool articulated skeletons with complex muscles, and brains that we’re very proud of.” But the belief in an orderly, stepwise progression—sponge to ctenophore to bilaterian—is “complete rubbish,” he said. “It’s deeply flawed, but it’s there, inserting its imprint in how we talk and think.”
