Scientists confirm 700-million-year-old ctenophores oldest living animal on Earth


Scientists have confirmed that the oldest living animal on earth evolved from the first animals that also gave rise to humans.

The jellyfish-like creature, known as a ctenophore, 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 they swim in today’s oceans and aquariums.

A team from the University of California, Berkeley, set out to solve the relationships of the animal tree of life to better understand its origins and evolution.

The findings also have put an end to the long-held debate that sea sponges were the first animals, as their fossils date back about 600 million years.

Ctenophores have eight sets of cilia running along their sides, which they use to propel themselves through the oceans as they travel more than four miles below the surface

Ctenophores have eight sets of cilia, similar to tentacles, running down their sides, which they use to propel themselves through the oceans as they travel more than four miles below the surface.

Daniel Rokhsar, a professor at the University of California and co-corresponding author of the study, said in a rack: ‘The most recent 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 gentle animals and left no direct fossil record. But we can use comparisons between living animals to learn more about our common ancestors.

“It’s exciting — we’re looking way back in time where we have no hope of finding fossils, but by comparing genomes, we’re learning things about these very early ancestors.”

The team explained that most people think of worms, flies, molluscs, starfish and vertebrates when they think of animals.

By comparison, worms evolved about 500 million years ago and vertebrates appeared about 450 million years ago.

Together they are called bilaterians, which have a head with a centralized brain, a gut that runs from mouth to anus, muscles, and other common features that had already evolved by the time of the famous “Cambrian explosion” some 500 million years ago .

Jellyfish are categorized as bona fide animals, which lack many bilateral features – for example, they lack a defined brain and may not even have a nervous system or muscles – but still share the features of animal life, most notably the development of multicellular bodies from a fertilized egg .

The evolutionary relationships between these diverse creatures—particularly the order in which each lineage branched off from the main trunk of the animal tree of life—are controversial.

‘Traditionally, sponges are generally considered to be the earliest 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 protozoa. Rokhsar told SWNS.

The findings also end the longstanding debate over whether sponges (pictured) appeared first

The findings also end the longstanding debate over whether sponges (pictured) appeared first

“And so, it was a fun story: First came the unicellular protozoa, and then spongy multicellular consortia of such cells evolved and became the ancestors of all of today’s animal diversity.”

“In this scenario, the sponge line retains many of the characteristics of the animal ancestor on the branch that leads to all other animals, including us.

“Specializations evolved that led to neurons, nerves and muscles and guts and all those things that we know and love as the defining features of the rest of animal life.

“Sponges seem primitive because they lack those features.”

The other candidate for the earliest animal lineage is the group of comb jellyfish, popular animals in many aquariums.

Although superficially similar to jellyfish, the two are only distantly related.

Unlike regular jellyfish that spout through the water, ctenophores propel themselves with eight rows of throbbing cilia arranged like combs along their sides. Along the California coast, a common ctenophore is the sea gooseberry with a diameter of 2.5 cm.

Each species has a characteristic chromosome number – humans have 23 pairs – and a characteristic distribution of genes along chromosomes.

The researchers had 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 suggested that the chromosomes of many animals evolve slowly, and enabled the team to computationally reconstruct the chromosomes of the common ancestor of these diverse animals.

“At first we couldn’t tell if ctenophore chromosomes were different from those of other animals simply because they had changed a lot over hundreds of millions of years,” Rokhsar said.

“It could also be that they are different because they first branched out before all the other animal lineages appeared. We had to find out.’

The researchers joined forces to sequence the genomes of another comb jelly and sponge, as well as three single-celled creatures that fall outside the animal lineage: a choanoflagellate, a filasterean amoeba, and a fish parasite called an ichthyosporean.

Raw genome sequences of these non-animals already existed, but they don’t contain the critical information needed for chromosome-scale gene linking: where they sit on the chromosome.

When the team compared the chromosomes of these diverse animals and non-animals, they found, remarkably, that ctenophores and non-animals shared certain gene-chromosome combinations, while the chromosomes of sponges and other animals were rearranged in a markedly different way.

Prof Rokhsar said: ‘That was the smoking gun – we found a handful of rearrangements shared by sponges and non-ctenophoric animals. Ctenophores, on the other hand, resembled non-animals.

“The simplest explanation is that ctenophores branched before the rearrangements happened.”