Earliest animals: Comb jellyfish were the first

comb jellyfish

Comb jellies (here the species Hormiphora californensis) are the sister group of all other multicellular animals. © Darrin Schultz / 2021 MBARI

What was the earliest evolution of animals? Which group formed the first branch in the animal family tree? Using chromosome analysis, a study now shows that comb jellyfish were the first to split off around 700 million years ago – even before sponges, which were previously thought to be the most primitive representatives of the animal kingdom. Comb jellyfish are therefore of all modern animals at the genetic level still most closely related to the protozoa from which the first animals developed.

Several hundred million years ago, the first multicellular animals developed in the oceans from unicellular microorganisms. But which of the animal groups that can still be found today is the most primitive? So far, science has generally assumed that it is the sponges. They have neither organs nor a nervous system and act like a collection of unicellular organisms. Another candidate was comb jellyfish (Ctenophora), which look similar to jellyfish but are only distantly related to them. Although genetic analyzes showed that both sponges and comb jellyfish are very primitive, they could not clarify which of the two groups formed the first branch of the animal family tree.

Sponges, comb jellyfish and protozoa were examined

A team led by Darrin Schultz from the University of Vienna in Austria has now solved the mystery. Instead of analyzing how the genes evolved over time, Schultz and his colleagues focused on the order in which the genes are arranged on the chromosomes. This trait changes very little over the course of evolution. In a previous study, the research team had shown that the chromosomes of sponges, jellyfish and many other invertebrates carry very similar chromosomal gene patterns, despite having evolved independently for more than half a billion years.

For the current study, the team compared the chromosome structure of two species of sponges, two species of comb jellyfish and three non-phylum unicellular creatures. The result: the arrangement of the genes in comb jellyfish was more similar to that of protozoa in key points. "That was the crux of the matter," says co-author Daniel Rokhsar of the University of California at Berkeley. "We found a handful of rearrangements that sponges and all other animals except comb jellyfish have in common. In contrast, the comb jellyfish resembled the non-animals. The most plausible explanation is that the comb jellyfish branched off before the reorganizations took place.”

window into the past

"The fingerprints of this ancient evolutionary event can still be found in the genomes of animals hundreds of millions of years later," says Schultz. The chromosome analyzes thus open a window into a hitherto largely unknown epoch of the earliest development of animals. "The most recent common ancestor of all animals probably lived 600 or 700 million years ago," explains Rokhsar. "It's hard to tell what they looked like because they were soft-bodied animals that left no direct fossil traces. But we can make comparisons between living animals to learn about our common ancestors. With our analysis we are looking deep into the past where we have no hope of finding fossils - but by comparing genomes we are learning about these very early ancestors.”

The study thus provides an important cornerstone for understanding how animal phyla are related to one another - from primitive organisms such as comb jellyfish and sponges to worms and arthropods to vertebrates such as humans. It can also help to understand how important features such as the nervous system, muscles, and digestive tract evolved. "This research gives us context for understanding what makes animals animals," says Schultz. "It will help us understand the basic functions we all have in common, such as environmental awareness, nutrition and locomotion."

Source: Darrin Schultz (University of Vienna, Austria) et al., Nature, doi: 10.1038/s41586-023-05936-6

Recent Articles

Related Stories

Stay on op - Ge the daily news in your inbox