The tree of animal life must start somewhere. However, finding the original branch in such a tangled canopy is harder than it sounds.
A new analysis of genomic data shows that the longest-running branch has been deceiving us.
Since the 19th century, many scientists have shown sponges-one of the simplest creatures in existence-as the world’s first animal.
However, modern genomics research has compared this muscle-free, nerve-free, and organ-free spot with more complex organisms.
The comb jelly contains a small door called Ctenophora, but unlike Porifera-the door sponge belongs to-these globular organisms exhibit more advanced features, including neurons and muscle cells to detect and eat prey, and an intestine for digestion.
If these jellies are really the first batches, then it means that many of their characteristics subsequently disappeared in Porifera and later developed. Although this may sound completely illogical, it is not completely impossible, although it does threaten to change our understanding of early animal evolution and the development of the nervous system itself.
Anthony Redmond, a geneticist at Trinity College in Dublin, Ireland, explained: “It seems impossible for this complex feature to evolve twice independently, but evolution does not always follow a simple path.”
“For example, there is a distant connection between birds and bats, but they have independently evolved wings to fly.”
In recent years, some models that divide genes into smaller groups for comparison have found that jelly has greater phylogenetic signals than sponges, indicating that these animals evolved and existed first.
Using the same data, other models that did not divide genes but relied on a larger super matrix also revealed completely opposite timelines, with sponges arriving on the scene earlier than comb jelly.
Both of these methods have their limitations, but checking them together helps to alleviate some of the errors and biases that we have previously seen when viewing the data.
Now, a reanalysis using a more integrated model shows that we are starting in the right direction-that is, the sponge.
Redmond said: “Our method bridges the gap between the two different methods and provides strong evidence that sponges rather than jellies are our most distant animal relatives.”
“This means that our last common animal ancestor is simple in morphology, and shows the possibility of repeated evolution and/or loss of complex features (such as the nervous system) compared to comb jelly, which is our most distant animal relative Smaller.”
In evolution, natural selection tends to maintain the function and shape of certain proteins by substituting other amino acids with similar biochemical properties for specific amino acids. However, these characteristics may vary from site to site within and between genes.
Partitioning models, in which genes are split, and substitution patterns between sites are compared in groups, usually cannot explain this diversity, which means they may lose multiple “hidden” substitutions, especially in those Choose new amino acids in species that are relatively fast.
Therefore, most zoning models point the comb jelly at the original sisters of other animals. However, when merging models that account for site differences, the researchers found that the sponge took over this role.
This is consistent with previous studies, which found that partitioned and unpartitioned models affect the length of branches and changing nodes on the tree of life, “sometimes.”
In other words, the researchers believe that it is our “bad fit” and “oversimplification” model that inhibited certain evolutionary changes in the long branch of the comb jelly.This makes Ctenophora phylum look like it evolved before In fact, when these jellies split from Porifera very early and then develop rapidly, they become sponges.
The results support a previous study in 2020, which found that the rapid development of comb jelly has brought bias to our model, making us mistakenly believe that comb jelly was the first to appear.
Both Porifera and Ctenophora phyla have long branches, but when we compare the two using similar amino acid substitution models, due to their rapid evolution, the jelly seems to have to be older. However, in fact, the timing of the gene sequence of comb jelly is much tighter, which makes their relationship with sponges much closer than they actually are.
The researchers said that future models need to consider this bias, that is, “long branch attraction,” otherwise these differences will continue to appear.
Evolutionary biologist Detlev Arendt (Detlev Arendt) said: “It is usually difficult to place long-branched fauna.” Quanta Magazine In 2015.
“So far, the phylogenetic data is still inconclusive. [comb jellies] belong. “
Given that this debate has become so intense, this new study is unlikely to end the conversation forever. That is to say, the results of this updated model are indeed combined with other recent advances in genome research, which indicates that the comb jelly genes have helped us to get rid of the smell to some extent.
The authors wrote: “The suspicion that the ancestry is sisters of other animals has had a major impact on the development of new methods and the reassessment of the quality of commonly used phylogenetic methods.”
“This seems to continue, and many of the research avenues highlighted in this debate have yet to be explored.”
The research was published in Nature Communications.