The first single-celled organisms drifted at sea. Then, some 700 million to 800 million years ago, clusters of cells joined together to form Earth’s first multicellular animal. More recently, the animal family tree split into two branches. One lineage gave rise to all other animals on Earth, from brontosauruses to badgers. The other, dubbed “sister of all other animals,” continued its separate evolutionary journey. For more than a decade, scientists have debated which animals alive today hail from that earliest sister, fingering two phyla as the most likely candidates: sponges and comb jellies.
A study published today in Nature offers some of the strongest evidence yet that comb jellies are the true descendants of that sister.
“If it’s true, it’s extraordinary,” says Max Telford, a zoologist at University College London who was not involved in the study. “From a biological point of view, it’s very surprising.”
It’s surprising to Telford and others because, on paper, sponges seem like the obvious descendant. Anatomically, they are incredibly simple. Sponges lack both muscle and nerve cells, which all other animals, including comb jellies, have. Sponges also have a type of specialized cell called a choanocyte, which resembles choanoflagellates—the single-celled relatives of more complex life. In contrast, comb jellies—translucent, oval-shaped, gelatinous creatures that look like spaceships of the sea—are relatively complex, suggesting they evolved after sponges.
Despite that complexity, in 2008 scientists using computer programs to compare the genes of dozens of species concluded that comb jellies, and not sponges, were the true descendants. But the evidence wasn’t conclusive, so the debate raged on. And later genetic studies using different models pointed the finger at sponges, as many originally suspected.
The whipsawing results left many scientists feeling “like they were banging their heads against the wall,” says Darrin Schultz, the lead author of the new study and a biologist at the University of Vienna.
So, Schultz’s team devised a new approach. Instead of comparing individual genes, they looked at large-scale patterns of gene arrangements on chromosomes. As animals evolve, bits and pieces of DNA get swapped around, but genes often stay on the same chromosome—a trend known as synteny. But occasionally chromosomes fuse and mix, allowing genes to move irreversibly to a new chromosome. Schultz compares it with shuffling cards from two separate decks to make a new deck. “Once you fuse and mix, then you can’t unmix,” he says.
Schultz’s team looked at examples of synteny in comb jellies, sponges, and some unicellular relatives. In comb jellies and the unicellular relatives, they identified 14 groups of genes on separate chromosomes. But in sponges, those same genes “fused and mixed” into just seven groups, mirroring a pattern seen in all other animals. That suggests the ancestor of comb jellies was the first to split from the common ancestor of all living animals—making comb jellies the true sister. All other animals, including sponges, evolved later, after those chromosomes fused.
“We’re getting a hint of what the genome of something that was alive a billion years ago was like,” Schultz says.
Anthony Redmond, an evolutionary geneticist at Trinity College Dublin, agrees. “We’ve really lacked tools like that up to now,” says Redmond, who in the past backed sponges as the earliest animal to branch off. “I think it’s fair to say it’s the best support we’ve ever had for comb jellies as the sister.”
If the results stand, Schultz says, scientists will have to rethink a number of issues including how characteristics such as neuronlike cells evolved. “It means that neuronlike cells probably evolved 100 million years or so earlier than people believed,” he says, and that sponges lost them along the way.
The new approach could give scientists a workable alternative to using individual genes to study evolution in very early life, Telford adds. “[They’ve] come at this in a completely different way,” he says. “You don’t need a clever model. The data are just really in your face.”
The first single-celled organisms drifted at sea. Then, some 700 million to 800 million years ago, clusters of cells joined together to form Earth’s first multicellular animal. More recently, the animal family tree split into two branches. One lineage gave rise to all other animals on Earth, from brontosauruses to badgers. The other, dubbed “sister of all other animals,” continued its separate evolutionary journey. For more than a decade, scientists have debated which animals alive today hail from that earliest sister, fingering two phyla as the most likely candidates: sponges and comb jellies.
A study published today in Nature offers some of the strongest evidence yet that comb jellies are the true descendants of that sister.
“If it’s true, it’s extraordinary,” says Max Telford, a zoologist at University College London who was not involved in the study. “From a biological point of view, it’s very surprising.”
It’s surprising to Telford and others because, on paper, sponges seem like the obvious descendant. Anatomically, they are incredibly simple. Sponges lack both muscle and nerve cells, which all other animals, including comb jellies, have. Sponges also have a type of specialized cell called a choanocyte, which resembles choanoflagellates—the single-celled relatives of more complex life. In contrast, comb jellies—translucent, oval-shaped, gelatinous creatures that look like spaceships of the sea—are relatively complex, suggesting they evolved after sponges.
Despite that complexity, in 2008 scientists using computer programs to compare the genes of dozens of species concluded that comb jellies, and not sponges, were the true descendants. But the evidence wasn’t conclusive, so the debate raged on. And later genetic studies using different models pointed the finger at sponges, as many originally suspected.
The whipsawing results left many scientists feeling “like they were banging their heads against the wall,” says Darrin Schultz, the lead author of the new study and a biologist at the University of Vienna.
So, Schultz’s team devised a new approach. Instead of comparing individual genes, they looked at large-scale patterns of gene arrangements on chromosomes. As animals evolve, bits and pieces of DNA get swapped around, but genes often stay on the same chromosome—a trend known as synteny. But occasionally chromosomes fuse and mix, allowing genes to move irreversibly to a new chromosome. Schultz compares it with shuffling cards from two separate decks to make a new deck. “Once you fuse and mix, then you can’t unmix,” he says.
Schultz’s team looked at examples of synteny in comb jellies, sponges, and some unicellular relatives. In comb jellies and the unicellular relatives, they identified 14 groups of genes on separate chromosomes. But in sponges, those same genes “fused and mixed” into just seven groups, mirroring a pattern seen in all other animals. That suggests the ancestor of comb jellies was the first to split from the common ancestor of all living animals—making comb jellies the true sister. All other animals, including sponges, evolved later, after those chromosomes fused.
“We’re getting a hint of what the genome of something that was alive a billion years ago was like,” Schultz says.
Anthony Redmond, an evolutionary geneticist at Trinity College Dublin, agrees. “We’ve really lacked tools like that up to now,” says Redmond, who in the past backed sponges as the earliest animal to branch off. “I think it’s fair to say it’s the best support we’ve ever had for comb jellies as the sister.”
If the results stand, Schultz says, scientists will have to rethink a number of issues including how characteristics such as neuronlike cells evolved. “It means that neuronlike cells probably evolved 100 million years or so earlier than people believed,” he says, and that sponges lost them along the way.
The new approach could give scientists a workable alternative to using individual genes to study evolution in very early life, Telford adds. “[They’ve] come at this in a completely different way,” he says. “You don’t need a clever model. The data are just really in your face.”