In a prehistoric version of "the chicken or the egg" question, researchers have long debated which animal group came first. A traditional view pegs sponges—marine creatures that look more like rocks or corals—as our ancient ancestors.

But a new genetic study is stirring the waters, suggesting comb jellies, gelatinous marine animals that look similar to jellyfish, are actually the first animals to have evolved over 600 million years ago.

While an argument over ancient ancestry may seem academic, it's an important question to answer because it influences how researchers think about the nature of animal evolution, said study co-author Andy Baxevanis, a geneticist at the National Human Genome Research Institute in Bethesda, Maryland.

Sponges are simple creatures, lacking muscles or a nervous system, he said, while comb jellies have both. (See a video on how comb jellies hunt.)

"There's been this long-standing tenet in evolutionary biology," Baxevanis explained, that once evolution led to "some kind of complex cell type—like musculature or an eye—you wouldn't lose it."

That's because keeping muscles or a nervous system would be a survival advantage.

But if comb jellies did in fact come before the simpler sponges, that view would be turned upside down, suggesting instead that animals started off with genes for complex biology and some groups then lost them over time.

A Complete Sequence

Baxevanis and colleagues came to this conclusion after producing the first complete genome sequence of a comb jelly using a species called the sea walnut (Mnemiopsis leidyi).

Of the four ancient animal groups in contention for the title of first animal ancestor—sponges, comb jellies, jellyfish, and a group called the placozoa—only the comb jellies lacked a complete genome sequence from one of its members. (See jellyfish pictures.)

Complete genome sequences are important when trying to compare relationships between groups of organisms.

When Baxevanis and colleagues started their project, they were mainly interested in producing that comb jelly genome sequence and filling the gap in the data.

But when they plugged their genome into a computer program that helps researchers determine the evolutionary relationships between organisms—a field of study called phylogenetics—they were in for a shock.

Although their model came up with several possible scenarios for how comb jellies were related to all the other animal groups, some of the most likely scenarios put the gelatinous animals at the base of the animal family tree. (See video of the blood belly comb jelly.)

"The deepest evolutionary split [about 600 million years ago] involved the comb jellies splitting off from the rest of the animals," said Baxevanis. He and his colleagues reported their findings today in the journal Science.

Stirring the Pot

The new study has caused quite a stir among experts in the field.

A study in 2008 first suggested comb jellies were the ancestral group to all other animals, said Mansi Srivastava, an evolutionary developmental biologist at the Massachusetts Institute of Technology in Cambridge.

"That was pretty shocking to the community," said Srivastava, who was not involved in either study. "A lot of people didn't believe the results."

The problem was that fast-evolving genomes like the ones in comb jellies could sometimes trip up the computer programs used to help determine evolutionary relationships, she explained.

That would result in some groups getting lumped together with other organisms they weren't necessarily related to.

"Some people thought the 2008 paper was the result of this," Srivastava said, "and with more analysis, we could answer this question."

That's where the current Science study comes in: The authors provided more data by sequencing a complete comb jelly genome, enabling researchers to take another run at the "who first" question.

But Srivastava and others are still hesitant about the study's claims that comb jellies are indeed the ancestors to animals.

Troublemakers

One of the issues has to do with timing, said Hervé Philippe, a researcher with the National Center for Scientific Research in Moulis, France.

"You have to look over the history of organisms that occurred more than 600 million years ago," said Philippe, who was not involved in the current study.

A lot of mutations can happen over hundreds of millions of years, he explained. Separating out the earlier mutations in an ancestral group that gave rise to all the other animals from more recent genetic changes gets tricky.

Some of the methods Baxevanis and colleagues used in their analysis couldn't resolve more recent relationships between modern-day animal groups, said Philippe. "If you can't recognize recent events, it's unlikely that you'll recognize ancient events."

The study authors acknowledge that some of the evolutionary family trees they came up with leave the door open for more debate. Some still place the sponges as the ancestral group to animals.

But Baxevanis seems excited about the coming debate.

"We're being the troublemakers here," he said. When the study authors set out on this project, they didn't expect to get the results they did.

"I think it's a great example of not knowing where science is going to take you when you start," Baxevanis said.