The search by researchers to uncover the ‘first’ animal – the ancestor for all other animals – has taken a new turn.
A new study says comb jellies are more likely to be the first animal ancestor, displacing an existing theory which says sponges are the world’s oldest.
700 million years ago when there were no rings of Saturn, no sharks and no trees, there might have been see-through swimming invertebrates known as ctenophores (better known as comb jellies), says the study, published in Nature.
“The most recent common ancestor of all animals probably lived 600 or 700 million years ago,” says Professor Daniel Rokhsar, a cell biologist at the University of California, Berkeley.
“It’s hard to know what they were like because they were soft-bodied animals and didn’t leave a direct fossil record, but we can use comparisons across living animals to learn about our common ancestors.
“It’s exciting. We’re looking back deep in time where we have no hope of getting fossils, but by comparing genomes, we’re learning things about these very early ancestors.”
The two contenders for the earliest animal are comb jellies and an equally primitive species – sponges. But unfortunately, because the two genetically diverged so long ago, it’s been difficult to untangle the two into a clear favourite.
“The results of sophisticated sequence-based studies were basically split,” said Rokhsar.
“Some researchers did well-designed analyses and found that sponges branched first. Others did equally complex and justifiable studies and got ctenophores. There hasn’t really been any convergence to a definitive answer.”
The researchers looked at the evolution of the chromosome of sponges, comb jellies and a number of single celled non-animals.
“Traditionally, sponges have been widely considered to be the earliest surviving branch of the animal tree, because 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,” Rokhsar says.
“And so, it was a nice story: First came the unicellular protozoans, and then sponge-like multicellular consortia of such cells evolved and became the ancestor of all of today’s animal diversity.”
But they found instead was that comb jellies, genetically, looked distinctly more non-animal than animal.
“That was the smoking gun — we found a handful of rearrangements shared by sponges and non-ctenophore animals. In contrast, ctenophores resembled the non-animals. The simplest explanation is that ctenophores branched off before the rearrangements occurred,” said Rokhsar. So, although this likely won’t be the last we hear on the matter, this does provide some compelling evidence that in the battle of the ancients, comb jellies came first.