Scientists have long thought the humble sea sponge, an animal that feeds by filtering water through itself, forms the oldest group of animals on earth. But a new study claims that the comb jelly phylum is in fact older and carries genetic material from distant, non-animal ancestors.
Comb jellies, which look like miniature jellyfishes, use rows of cilia hairs to swim through the ocean and catch prey with tentacles that release a sticky, mucous-like substance. Like other animals, they meet the standard of having developed from a fertilized egg into a multicellular organism.
Early Lineages and a Smoking Gun
To determine whether comb jellies or sponges branched off the animal family tree first, researchers from the University of California-Berkeley and the University of Vienna tried genetic analysis, but their initial attempts led nowhere thanks to the extreme age of each group.
They next turned to chromosomes, specifically the arrangement of genes on each chromosome, hoping to track patterns across time. After they had determined the chromosome structure for Hormiphora californiensis, a jelly comb species, they could see things more clearly: The jelly comb had a unique arrangement that paralleled non-animals, whereas the sponge fell in line with the other animals.
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“That was the smoking gun,” says Daniel Rokhsar, a biologist at the University of California-Berkeley and one of the study authors, in a press release.
Jelly combs had branched off first, before animal chromosomes had changed on a wide scale. This meant the jelly comb arrangement constituted “a relic of a very ancient chromosomal signal,” Rokhsar says. “It took some statistical detective work to convince ourselves that this really is a clear signal and not just random noise.”
The results may lay the groundwork for new research into how animal muscles and other organs evolved, and the methodology may prove useful in other areas, says Darrin Schultz a biologist and one of the study authors, in a press release. “We developed a new way to take one of the deepest glimpses possible into the origins of animal life.”
The Ancient Ancestor to All Animals
While the comb jelly’s lineage is the oldest yet living, extending to at least 500 million years ago, even older animals once existed some 600 to 700 million years ago and passed down genetic material to today.
“It's hard to know what they were like because they were soft-bodied animals and didn't leave a direct fossil record,” says Rokhsar. “But we can use comparisons across living animals to learn about our common ancestors.”
How Simple Can an Animal Be?
The ocean is full of simple animals such as the sea sponge, comb jelly, jellyfish and sea anemone that lack a defined brain, the researchers say, if not muscles and nerves.
“Sponges don't have a nervous system, they don't have muscles, and they look a little bit like colonial versions of some unicellular protozoans,” says Rokhsar.
After a larval stage, in which they swim around in a form that resembles a flower bulb, they settle down in one spot and rarely leave. From then on, they subsist on whatever food particles they catch in their pores.