A Battle Is Raging in the Tree of Life
Trilobites
Which came first, the sponge or the comb jelly?
Poriferans, better known as sponges, are squishy, stationary and filled with holes. Ctenophores, also called comb jellies, are soft blobs wreathed by feathery cilia.
For the past decade, the two groups have been caught up in a raging battle, at least in the pages of scientific journals. At stake is a noble place in evolutionary history: closest living analogue of the first-ever animal. A new analysis, published Wednesday in Royal Society Open Science, hands victory back to the sponges, although more bouts are sure to come.
All animals, from sponges and comb jellies to humans and other mammals, have particular characteristics in common: We’re multicellular, our cells have nuclei and we consume food rather than make it. We also all have a common ancestor, which inhabited the oceans at least 550 million years ago and shared those traits. But no one knows for sure what that first animal looked like, or how it lived.
That’s why scientists look at existing species to shed light on the founder of the animal kingdom.
“By comparing modern animals, we’re trying to infer what the ancestor was like,” said Antonis Rokas, an evolutionary biologist at Vanderbilt University. “How complex was it? What kind of genes did it have, and what kinds of traits?”
For years, most researchers agreed that sponges were the sister group of the first animals, suggesting a simpler, spongier forebear. But in 2008, a genetic analysis pointed to the comb jellies instead. Since then, a number of experts, including Dr. Rokas, have also found support for the comb jellies theory, while other new genetic studies still point to the sponges.
Claus Nielsen, a morphologist affiliated with the Natural History Museum of Denmark and author of the new paper, has followed this controversy, which has unfolded entirely during his retirement, with great interest.
“I’m 81 years old, and I’ve been a zoologist since my 20s,” he said. “My whole concept of the animal kingdom would crash with this ctenophores-first idea.”
Dr. Nielsen decided to take a morphological approach to the question.
“I think about living organisms,” he said. “I think about how they function, how they react to the environment, how they eat, how they reproduce and so on.” He combed through about a thousand papers on poriferans, ctenophores and their relatives, and squeezed out relevant information about their anatomical and molecular characteristics. He then took two possible evolutionary scenarios — one in which poriferans branched off first, and the other in which ctenophores did — and elucidated which traits the relevant groups would have gained and lost on each path.
The two tell two very different stories. Sponges don’t move quickly, and are only able to eat tiny particles. So the poriferans-first scenario involves a gradual increase in complexity, as later groups gain things like digestive and nervous systems.
By contrast, comb jellies already have nervous systems, and can eat krill, amphipods and even each other. If the ctenophores evolved first, that would mean the sponges gave those things up.
“To lose the ability to digest larger organisms — to me, that is nonsense,” said Dr. Nielsen.
Many disagree. Joseph Ryan of the University of Florida, who thinks ctenophores came first, argued that evolutionary trees based on even the most careful morphological studies are often less rigorous and objective than ones based on genetic data, and that the scenarios that seem impossible to Dr. Nielsen are as feasible as those presented in his study.
“All lineages are continuously evolving,” Dr. Ryan said, adding that loss of traits is common in evolution.
Dr. Rokas agreed: “What appear as major morphological transitions” can require “relatively simple changes in molecular terms.”
Dr. Nielsen replied that phylogenetic studies also involve subjective choice, and that the loss of a useful trait goes against natural selection.
As the battle rages, deeper issues rise to the surface. The controversy is “bigger than the usual questions that biologists investigate,” said Davide Pisani, a professor phylogenomics at the University of Bristol in England. “Take the nervous system: this is the key system that is ultimately responsible for our own consciousness. When and how many times did it evolve?” The answer, he said, “has ramifications that transcend science and move to philosophy.”
Dr. Nielsen, too, is fueled by a broader concern.
“If you only look at the molecules and don’t think about morphology, and the function of organisms, then I would say it’s not biology any longer,” he said. “I know some people won’t like me to say that. But that’s the way I think.”
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