The closest living relative of the first animal has finally been found

The article discusses a standing evolutionary biology debate: Which living animals are closest relatives to the very first animals on Earth? What animal is the common ancestor to all living animals? For over 100 years, scientists have assumed sponges were the most primitive due to their lack of muscles and neurons. It fit the current idea at the time that complexity was added step by step. However, a recent Nature study used chromosome comparisons to show that comb jellies branched off first, not sponges. Previously, it was believed that sponges evolved into worms and then into future species, but knowing that comb jellies came first allows us to fill in the gaps in the known species and fossil record. Also, knowing which group split first helps reconstruct traits of the first animals. Whether they were as simple as sponges or complex swimming organisms

Earlier studies compared DNA sequences of sponges and crenophores. Results were not linear, and the outcome varied depending on the models used. Now, instead of small DNA sequence changes, researchers examined large-scale chromosome structures across animals and other unicellular relatives. Chromosomes can shuffle genes internally, but occasionally, whole chromosomes break and fuse, making them strong markers for evolutionary relationships. They disclose cross-transferred events shared by bilaterians, jellyfish, and sponges, but not ctenophores. These rare events never occur independently in two different lineages, the data strongly support ctenophores like comb jellies branching first because it doesn't share the cross-transferring genetic trait.

Ctenophores were shown to branch before sponges by analyzing entire chromosomes, indicating that Ctenophora branched before acquiring the characteristics from the fused genomes of future species. The first animals were more complex than once thought, and are suspected to have had muscular and nerve structures. This discovery also suggests that sponges may not be primitive at all, but organisms that lost complexity over time. Evolution doesn’t always move like a staircase toward greater sophistication; sometimes species simplify, and sometimes they get more complex only if needed. Recognizing this can help scientists better understand that evolution is a branching process with both beneficial and detrimental traits, not just a linear progression toward complexity.