For the first time, researchers have found a powerful answer to the centuries-old mystery of the existence of a multicellular animal on Earth.
The jellyfish-like creature known as the comb jelly first appeared 700 million years ago – compared to dinosaurs born 230 million years ago.
The study found that ctenophores are the closest relatives of the first animals and can be found swimming in modern oceans and aquariums.
A team from the University of California at Berkeley set out to find out the relationship between the tree of life of animals in order to better understand the origin and evolution.
Perhaps we will finally find out what the very first animals looked like https://t.co/jbHoSGsGi5
— ScienceAlert (@ScienceAlert) May 18, 2023
The finds also put an end to the longstanding argument that sea sponges were the first animals, as their fossils are about 600 million years old.
Ctenophores have eight sets of tentacle-like cilia running down their sides, which they use to propel themselves across oceans, traveling over four miles below the surface.
“The most recent common ancestor of all animals probably lived 600 or 700 million years ago,” Daniel Rohsar, a professor at the University of California and co-author of the study, said in a statement. “It’s hard to know what they looked like because they were soft-bodied animals and left no direct trace fossils, but we can use comparisons between animals.”
The team explained that most people think of worms, flies, clams, starfish and vertebrates when they think of animals.
In comparison, worms evolved about 500 million years ago and vertebrates evolved about 450 million years ago.
Together they are called bilaterians, they have a head with a central brain, intestines running from the mouth to the anus, muscles and other common features that had already developed by the time of the famous “Cambrian explosion” about 500 million years ago.
Jellyfish are classified as lacking many binary traits—for example, they lack a specific brain and may not even have a nervous system or muscles—but they still share common features of animal life, in particular the development of multicellular bodies from a fertilized egg.
The evolutionary relationships between these diverse beings—in particular, the order in which each lineage branches off from the main stem of the animal tree of life—is a matter of controversy.
Traditionally, sponges are considered the oldest surviving branch of the animal tree because sponges don’t have a nervous system, they don’t have muscles, and they look a bit like colonial versions of some single-celled protists, Rohsar said. SWNS. .
So, it was a beautiful story: first, unicellular protozoa appeared, then spongy multicellular consortia evolved, which became the ancestors of the entire variety of modern animals.
In this scenario, the sponge line retains many traits of the animal ancestor on the branch that gives rise to all other animals, including us.
and specializations evolved that gave rise to neurons, nerves, muscles, and all the things we know and love as hallmarks of the rest of animal life.
And sponges seem primitive because they don’t have these properties. Another candidate for the closest animal origin is the group of comb jellyfish, which are popular animals in many aquariums.
Unlike ordinary jellyfish that scurry through the water, ctenophores move with the help of eight rows of beating cilia, located on the sides like scallops. And along the California coast grows a one-inch sea gooseberry, a lectinophore. Each species has a different number of chromosomes—humans have 23 pairs—and a different distribution of genes across chromosomes.
Researchers have previously shown that the chromosomes of sponges, jellyfish and many other invertebrates carry similar genes despite more than half a billion years of independent evolution.
This discovery showed that the chromosomes of many animals evolve slowly and allowed the team to mathematically reconstruct the chromosomes of the common ancestor of these diverse animals.
“At first, we couldn’t tell if the chromosomes of ctenophores are different from those of other animals, simply because they have changed so much over hundreds of millions of years,” Rohsar said.
On the other hand, they may have been different because they first branched out before all other breeds of animals appeared. We needed to find out.
The researchers joined forces to sequence the genomes of another jellyfish and a comb sponge, as well as three single-celled creatures outside the animal lineage: flagellates, flagellated amoebae and a fish parasite called ichthyosporia.
Approximate sequences of the genomes of these non-animals existed, but they did not contain the critical information needed to link genes across the entire chromosome.
Remarkably, when the team compared the chromosomes of these diverse animals and non-animals, they found that ctenophores and other animals had certain combinations of genetic chromosomes, while the chromosomes of sponges and other animals were rearranged quite differently.
Source: Daily Mail
