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The tree of animal life had to start somewhere. Finding that original branch among such a tangled canopy is more difficult than it looks.
New analysis of genomic data suggests that a phylum vying for the oldest branch has been deceiving us from the start.
Since the 19th century, many scientists have presented the sponge – one of the simplest creatures in existence – as the world’s first animal.
Modern genomic studies, however, have contrasted this muscle-less, nerve-less, organ-less drop with a much more complex creature.
Comb jellies include a small phylum known as Ctenophora, but unlike Porifera – phylum sponges belong – these orb-shaped creatures show much more advanced traits, including neurons and muscle cells to detect. and eat prey, as well as a gut for digestion.
If these jellies did come first, it means that many of their traits were later lost in Porifera, only to evolve again later. While this may sound downright illogical, it is not entirely out of the question, although it threatens to change our understanding of the early evolution of animals and the development of the nervous system itself.
“It may seem very unlikely that such complex traits can evolve twice, independently, but evolution does not always follow a simple path,” says geneticist Anthony Redmond of Trinity College Dublin in Ireland.
“For example, birds and bats are distant but have independently evolved wings to fly.”
In recent years, some models that have divided genes into smaller groups for comparison have shown that comb jellies hold a stronger phylogenetic signal than sponges, suggesting that these animals evolved and existed first. .
Using the same data, other models that don’t partition genes but rely on a larger super-array revealed the exact opposite timeline, with sponges arriving on the scene earlier than comb jellies.
Both of these approaches have their limitations, but looking at them together helps mitigate some of our earlier errors and biases when examining the data.
A reanalysis using a more integrative model now suggests we were on the right track to start – namely, sponges were everything.
“Our approach bridges the gap between two conflicting methodologies and provides strong evidence that sponges, not comb jellies, are our most distant animals,” says Redmond.
“This means that our last common animal ancestor was morphologically simple and suggests that repeated evolution and / or loss of complex features like a nervous system is less likely than if the comb jellies were our most distant animal relatives.”
During evolution, natural selection tends to maintain the function and shape of certain proteins by replacing specific amino acids with others that have similar biochemical properties. These properties, however, can differ from site to site, within and between genes.
Partitioned models, where genes are split and substitution patterns between sites are compared in groups, often ignore this diversity, meaning they might miss multiple “ hidden ” substitutions, especially in species where new amino acids have been selected relatively quickly.
As such, most partitioned models indicate that comb jellies are the original sister of other animals. But when models that account for site differences are incorporated, researchers have found that sponges take on this role instead.
This is consistent with previous studies which found that partitioned and unpartitioned models can affect branch length and change nodes on the tree of life, “sometimes dramatically.”
In other words, say the researchers, it is our “ill-fitting” and “overly simplistic” models that suppress some evolutionary changes on the long branch of comb jellies. This gives the impression that the phylum of Ctenophora has evolved before sponges, when in fact these jellies probably separated from Porifera quite early and then evolved incredibly quickly.
The results support a previous study in 2020 which found that the rapid evolution of comb jellies introduces a bias into our models that trick us into thinking they came first.
Porifera and Ctenophora phyla both have long branches, but when we compare the two using similar amino acid replacement models, the jellies appear to have to be older due to their rapid evolution. In reality, however, the genetic sequences of the comb jellies occurred within a much shorter time frame, making them appear more closely related to sponges than they actually are.
Two trees possible for the first existing phyla. (Telford and Kapli / The conversation)
Future models must account for this bias, known as “long-branch pull,” the researchers say, or else these gaps will keep showing up time and time again.
“Groups of animals with long branches are often difficult to locate,” said evolutionary biologist Detlev Arendt. Quanta Magazine in 2015.
“So far, phylogenetic data does not really allow us to determine where [comb jellies] to belong.”
Given the intensity of this debate, this new study is unlikely to end the conversation for good. That said, the results of this updated model join other recent advances in genomic research that suggest that the genes in comb jellies are somehow distracting us from the scent.
“The question of which line is sister to other animals has had a major influence on the development of new approaches and the reassessment of the quality of commonly applied phylogenetic methodologies,” the authors write.
“It looks set to continue, with many avenues of research highlighted by this debate yet to be explored.”
The study was published in Nature communications.
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