Animal origins shift to comb jellies

Complex marine creatures called comb jellies should replace brainless, gutless, simple sponges at the base of the evolutionary animal tree of life, a new report asserts.

Scientists have long assumed that the ancestor of all living animals vaguely resembled sponges. Now, biologists must reformulate hypotheses on the evolution of the first animals more than 550 million years ago. “This finding makes people very uncomfortable,” says Joseph Ryan, an evolutionary biologist at the National Human Genome Research Institute in Bethesda, Md., and lead author of the study in the Dec. 13 Science.

For over a century, biologists have painted a picture of early animal evolution marked by the gradual addition of indispensible features. In that scenario, colonies of single-celled organisms called choanoflagellates gave way to multicellular predecessors of sponges; millions of years later, an offshoot of these organisms formed nerve cells. Later on, muscles developed. Scientists thought that comb jellies emerged after this point because they possess both a nervous system (complete with a rudimentary brain) and muscles.

Ryan and his colleagues questioned this scenario at a meeting in January when they announced that comb jellies may descend from an ancestor that evolved before sponges. They had sequenced the genome of the comb jelly species Mnemiopsis leidyi and compared it to genomes of sponges and dozens of other organisms. However, a hint of uncertainty in their results caused many biologists to reject the rearrangement of the evolutionary tree’s branches (SN: 5/18/13, p. 20).

Since then, the team solidified the conclusion with additional analyses. They used algorithms to take stock of the presence or absence of specific genes across animals and single-celled organisms such as choanoflagellates, whose ancestors predate animals. Comb jellies and the single-celled organisms turn out to lack many genes that animals typically have. This analysis, plus the genome and the comparisons, is published for the first time in the report. ­

“The new analysis together with earlier ones is very strong,” says Claus Nielsen, a biologist at the Natural History Museum of Denmark in Copenhagen who doubted the rearrangement until he saw this report. If he writes a new edition of his textbookAnimal Evolution, he says he will put comb jellies at the base of the animal tree.

Thomas Bosch, a biologist at the University in Kiel in Germany, agrees. But he adds, “I don’t understand the result in terms of biology.” The rearrangement suggests that the most recent common ancestor of animals may have had nerves and muscles, since comb jellies do. However, if that is true, sponges must have lost the parts without a trace.

An alternative that Bosch and Nielsen prefer is that animals’ common ancestor had most of the genes to develop nerves and muscles, but it did not actually possess those complex parts. Ryan’s study supports the idea. The researchers found that comb jellies lack some of the genes that other animals use to grow muscles, which means that comb jellies probably evolved muscles independently over millions of years with a different set of genes. It’s possible that comb jellies also evolved their nervous system after diverging from the common ancestor. In these scenarios, the comb jellies use genes in ways that no other living animals do.

Still, some biologists remain unconvinced of the new tree’s structure. Mansi Srivastava, an evolutionary biologist at MIT who helped sequence the sponge genome, says that both comb jellies and sponges might have lost many genes since they originated, scrambling the record of which group came first.  “It’s a really tough problem to solve,” she says.