Far back in geological time, about 600 or 700 million years ago, the first animals evolved on Earth. Their relatives, which are still alive today, include sponges, sea anemones, and comb jelly. However, which of these animals is closest to exactly the first animal remains one of the most controversial questions in evolutionary biology. With so few fossils of these early squishy animals, their history was inevitably obscure and difficult to reconstruct what happened.
The results of the study, published on May 17, Nature elucidated these early animal relationships by examining the chromosomes of three closely related unicellular animals: sponges, comb jelly, jellyfish, and animals. A team of researchers from the University of California, Berkeley, the University of Vienna, the Monterey Bay Aquarium Research Institute, and the University of California, Santa Cruz studied the patterns of chromosomal breaks and fusions at the base of the evolutionary tree of animals to help: It revealed that. Comb jellies, formally known as ctenophores, are actually the closest relatives of the first animals.
“Understanding these deepest relationships in the phylogenetic tree of animal life will reconstruct the origin and evolutionary history of many of the complex traits that most interest us, such as the nervous system and animal symmetry. It’s extremely important to do that,” says Casey. Yale University evolutionary biologist Dan was not involved in the study.
For more than 100 years, Dunn explains, there has been an implicit assumption that the evolutionary history of animals has largely been the gradual addition of complex features to animal lineages. One of the widely held hypotheses was that the corpus cavernosum was indeed primitive, as it lacked neurons and muscles. This gave rise to the idea that neurons and muscles must have diverged from the animal lineage before they developed. Because comb jelly has a network of muscle and nerve cells, it was thought to branch out later.
However, back in 2008, based on early information from the genomes of the first sponges and cinnamon, Dunn and his colleagues proposed that comb jellies diverged before sponges. The researchers found that the gene inventories of these animals were inconsistent with the idea that sponges were “a snapshot of this machinery before it evolved,” says Dunn. Sponges already had genes similar to those for neurotransmitters. Perhaps they had a special shape and function and were used for cell-to-cell communication long before neurons evolved.
Dozens of studies have been published since the 2008 paper. Some agreed with Dunn’s results, while others disagreed. “Personally, I’ve been neutral on this debate,” says Pauline Cartwright, an evolutionary biologist at the University of Kansas. somehow. ”
“So my conclusion was that this is a very difficult question,” added Cartwright, who was not involved in the 2008 paper or the new study. “Part of the reason this is so difficult is because we’re looking at something that happened more than 500 million years ago from now. It happened relatively quickly, so there isn’t much information available to reconstruct these very ancient events.” And cinte animals have evolved independently over 500 million years, making them unique to their lineage. has various characteristics of
in the Nature After publishing this paper, the research team took a new and creative approach to analyze the genomes of these early animals. Over hundreds of millions of years, gene sequences have mutated so much that all signals about the relatedness of different lineages were washed away. “So you need something that evolves very slowly that you can track,” says Dan Roxall, an evolutionary genomicist at the University of California, Berkeley, who oversaw the study. Instead of looking at nucleotide changes (single-letter changes in DNA), the method, developed by Roxal with Oleg Simakov and Darin Schultz at the University of Vienna, focuses on larger-scale features within the genome. Genes on chromosomes.
This technology is based on a simple idea. That is, in the process of evolution, the order of genes on the chromosome is shuffled by mutations. For example, by inversion, which reverses the order of genes within a chromosome. Although the order can change, genes on the chromosome form a kind of linkage group. It is not usually shuffled with genes on other chromosomes. However, in rare cases, chromosomes break and fuse, mixing their joining groups. These events are so rare that they can be traced back to the earliest animal origins.
A key insight is that the fusion and mixing of chromosomes is as irreversible as mixing milk into a cup of tea. The researchers therefore speculated that if fusion and admixture events shared between two lineages were observed, they must have occurred in the common ancestor of the two lineages. Because fusion and mixing events are irreversible, they are particularly well-suited for resolving relationships within the animal tree that have been resistant to traditional methods.
To unravel the relationships that underlie the animal tree, researchers assembled the sequences of each chromosome of the comb jelly. vorinopsis microptera, Two deep-sea sponges and three unicellular relatives, choanoflagellates, fish sporozoans, and filamentous amoebas. They also used existing chromosomal-scale genomes of cnidarians. (Sea anemones, jellyfish, corals, etc.), sponges, amphioxus, or amphioxus, are invertebrates that are very closely related to vertebrates and are bilaterians, animals with bilateral symmetry.
From this wealth of genomic data, the team found four fusion and admixture events common to bilaterians (lancelets), jellyfish and sponges, but not to cinteforms. If the sponges diverged before the cinnamon, then these very same four fusion and mixing events must have occurred independently in the two lineages, which is very unlikely. Thus, the researchers’ findings strongly support the idea that cinnamon were the first to diverge. “This paper is a game-changer in the debate about these relationships and their evolutionary implications,” says Dunn.
“I strongly believe that [the researchers] They settled this debate because of the type of letters they were using,” says Cartwright. “They have very strong data supporting the divergence of early cinnamon animals.”
The finding means that the ancestors of all animals, including sponges, already had well-developed nervous systems and were probably able to swim freely, Cartwright added. “We need to rethink the function and structure of our early ancestors in animals, which were probably more complex than simple sponges,” she says.
Another implication of this finding is that sponges, being filter-feeding devices attached to the bottom of the ocean floor, have lost many of the appropriate nervous and muscular components. Cartwright explained that the nervous system elements of the sponge genome may be remnants of a well-developed ancestral nervous system, rather than the beginning of an animal’s nervous system.
Evolutionary loss is clearly part of the story, rather than the evolution of animals gradually becoming more complex. It is also clear that early animals evolved abnormal neuronal features. Recent discoveries have shown that ctenophores lack synapses, the tiny connections between neurons. Instead, the cells of the primitive nervous system, known as the neuropil, fuse together to form a syncytium. This is “a whole new way of building the nervous system,” Dunn says. Also, sponges do not have neurons, but they do have cells with nerve cell characteristics in their digestive system called neuron cells.
One of the takeaways from this long quest is that as more information emerges, researchers may discover that the nervous system of early animals was more diverse and innovative than we can currently imagine. It means that there is sexuality. You now have a solid tree to anchor them in, providing a kind of roadmap for future discoveries about the evolution of essential traits in animals.
