What creatures first appeared on Earth: scientists get to the bottom of the mystery

Yulia LoseynkoLife
Scientists have named the most ancient multicellular animals as ctenophores

Life on Earth began with not much diversity, scientists are convinced. And they persist in searching for the organism that was first and gave rise to all living things.

According to the publication Science Alert, researchers have announced that they have found a reliable answer to a centuries-old riddle, which multicellular animal first appeared on Earth. For a long time, sponges (type Porifera) claimed the title. They were given the lead because of their anatomical simplicity, such as their lack of a nervous system. But new research has pointed to another contender.

New evidence suggests that ctenophores, distant relatives of jellyfish, may be the very first multicellular organisms. This is despite the fact that they have a more complex differentiated nervous system. They are a perfect example of how evolution is not a direct route to greater complexity.

"The oldest common ancestor of all animals probably lived 600 or 700 million years ago. It's hard to know what they were like because they were soft-bodied animals and left no direct trace from the fossil record. But we can use comparisons with living animals to learn about our common ancestors," explains molecular biologist and study author Daniel Rohsar of the University of California, Berkeley.

Previous analyses focusing only on gene sequences have yielded conflicting results. Some researchers used it to suggest that sponges were the first, while others pointed to ctenophores.

By comparing the location of highly conserved gene sequences in chromosomes, Darrin Schulz and colleagues, a bioinformatician from the University of Vienna, discovered patterns that clearly indicate the order of evolutionary events between organisms. The researchers compared their recently sequenced genomes of a ctenophore, two marine sponges, two unicellular animals (choanoflagellates and amoebae) and a microbial parasite related to both animals and fungi (ichthyosporium) to other more modern animals.

Linking highly conserved gene sequences found in the same chromosome in all animals revealed an obvious pattern. Sponges and more modern animals share the same traits of a rare type of chromosome fusion and rearrangement. But this was not the case in ctenophores, whose genomes are arranged more similarly to those of unicellular animals.

"We found several rearrangements common to sponges and animals that are not combinivores. Conversely, ctenophores resembled non-animals. The simplest explanation is that the crested sponges branched off before the rearrangements occurred," explained Rohsar.

So, these are the animals that most likely evolved first, and the sponges followed. These sponges then passed on their new messed-up chromosome arrangement to the animal descendants. "It took some statistical detective work to make sure it was really a clear signal and not just random noise," Rohsar added. He said the researchers are dealing with relatively small groups of genes and perhaps a billion years of divergence between animals and non-animals.

However, he said, the data strongly suggest that the crested beetles came first. The alternative hypothesis that sponges originated earlier can only be confirmed if multiple convergent rearrangements occurred in both sponges and non-ctenophore animals, which is very unlikely.

The new methods that enabled the development team to make this discovery will allow more accurate exploration of other evolutionary mysteries in the past. "The fingerprints of this long-standing evolutionary event are still present in animal genomes hundreds of millions of years later," Schultz says. Ahead, scientists see the prospect of discovering how to properly understand the basic common functions of animals. For example, how they sense their environment, how they eat and how they move. -

Previously OBOZREVATEL told about an ancient creature with teeth "screwdriver", which puzzled scientists.

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