Scientists have announced that the oldest living creature on our planet is a jellyfish-like organism called a ctenophore. It evolved from the same primordial animals that humans did.
This fascinating creature first emerged 700 million years ago, a significant time before the dinosaurs, which appeared only 230 million years ago. The study found that ctenophores are the closest relatives of the first animals and can still be spotted in modern-day oceans and aquariums.
A team from the University of California, Berkeley embarked on a quest to decipher the relationships within the animal tree of life. They wanted to broaden our understanding of the origins and evolution of life on Earth.
This exploration has not only provided a new perspective on the timeline of animal evolution but also quashed a long-held belief that sea sponges were the first animals. Sea sponges were considered the earliest due to their fossils dating back about 600 million years.
Ctenophores are unique creatures. They have eight sets of cilia, akin to tentacles, which they use to propel themselves through the ocean depths, some reaching more than four miles below the surface.
Study co-author Professor Daniel Rokhsar remarked: “The most recent common ancestor of all animals probably lived 600 or 700 million years ago.” He noted that it’s challenging to know what these ancestors were like as they were soft-bodied creatures and didn’t leave a fossil record.
“But we can use comparisons across living animals to learn about our common ancestors. It’s exciting – we’re looking back deep in time where we have no hope of getting fossils, but by comparing genomes, we’re learning things about these very early ancestors.”
When most people consider animals, they think of creatures like worms, flies, mollusks, sea stars, and vertebrates. For instance, worms evolved approximately 500 million years ago, and vertebrates appeared about 450 million years ago.
Together, these creatures are known as bilaterians, which have evolved to have a centralized brain, a gut running from mouth to anus, muscles, and other shared features. These features were present by the time of the well-known ‘Cambrian Explosion’ around 500 million years ago.
Jellyfish are classified as bona fide animals, despite their lack of many bilaterian features. They do not have a defined brain and may not even possess a nervous system or muscles. Nevertheless, they still exhibit the hallmarks of animal life, such as developing multicellular bodies from a fertilized egg.
The evolutionary relationships among these diverse animals, specifically the order in which each lineage diverged from the main trunk of the animal tree of life, have been controversial.
“Traditionally, sponges have been widely considered to be the earliest surviving branch of the animal tree,” Rokhsar told SWNS. He explained how the sponge lineage seemed to preserve many features of the animal ancestor on the branch leading to all other animals, including humans.
Ctenophores, also known as comb jellies, were the other candidate for the earliest animal lineage. While they look superficially like jellyfish, they are only distantly related. Unlike common jellyfish, ctenophores propel themselves with eight rows of beating cilia arranged down their sides like combs.
The research team had previously found that the chromosomes of sponges, jellyfish, and many other invertebrates carry similar genes, despite over half a billion years of independent evolution. This discovery suggested that the chromosomes of many animals evolve slowly. It 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’d just changed a lot over hundreds of millions of years,” said Rokhsar. The team needed to determine whether the differences were due to extensive evolution or because the ctenophores branched off before other animal lineages appeared.
To investigate this, the researchers combined efforts to sequence the genomes of another comb jelly and sponge, as well as three single-celled organisms outside the animal lineage: a choanoflagellate, a filasterean amoeba, and a fish parasite known as an ichthyosporean.
While rough genome sequences of these non-animal organisms existed, they lacked critical information about chromosome-scale gene linkage – where genes sit on the chromosome.
In an impressive finding, the team discovered that ctenophores and non-animals shared particular gene-chromosome combinations. In contrast, the chromosomes of sponges and other animals exhibited distinctly different rearrangements. This evidence suggested that ctenophores had branched off before these rearrangements occurred in other lineages.
“That was the smoking gun – we found a handful of rearrangements shared by sponges and non-ctenophore animals. In contrast, ctenophores resembled non-animals,” explained Professor Rokhsar. “The simplest explanation is that ctenophores branched off before the rearrangements occurred.”
This groundbreaking study has reshaped our understanding of the animal tree of life, shedding light on the origins and evolution of life as we know it. It’s a remarkable testament to the complexity and diversity of life on Earth.
It’s also a reminder of how much we still have to learn about our planet’s rich and diverse past. The humble ctenophore, a jellyfish-like creature, is a living testament to the long evolutionary journey that has resulted in the diverse array of animals we see today.
Ctenophores, commonly known as comb jellies, are a group of marine animals that are characterized by their unique features and fascinating biology. Despite their name, they are not jellyfish, though they share a similar appearance. Here’s a comprehensive overview of what we know about these creatures as of my last update in September 2021.
Ctenophores belong to the phylum Ctenophora, which contains around 100 to 150 known species. They are considered one of the earliest branches of the animal evolutionary tree, possibly predating other simple multicellular organisms like sponges.
Ctenophores are generally translucent, gelatinous organisms that range in size from a few millimeters to 1.5 meters in length. They are radially symmetrical and have soft, flexible bodies. Most species are bioluminescent, capable of producing light through a chemical reaction.
A defining feature of ctenophores is their ‘combs’, or rows of cilia, which they use for locomotion. These cilia, the largest known in the animal kingdom, refract light as they beat, creating a beautiful, shimmering effect. The ctenophores move by beating these cilia, which propels them through the water.
Ctenophores are carnivorous, feeding on a variety of small marine organisms such as larvae, plankton, and sometimes other ctenophores. They use their sticky tentacles to catch prey. Some larger species do not have tentacles and instead swallow their prey whole.
Interestingly, despite their predatory habits, ctenophores lack a true digestive system. Instead, they have a network of canals that distribute nutrients throughout their bodies.
Most ctenophores are hermaphroditic, meaning each individual possesses both male and female reproductive organs. They can reproduce both sexually, by releasing eggs and sperm into the water for external fertilization, and asexually, through a process known as budding.
Ctenophores are found in a wide range of marine environments around the world, from polar to tropical waters. They inhabit both surface waters and the deep sea. Some species are pelagic (living in the open ocean), while others live close to the seafloor.
In some areas, ctenophores have had a significant ecological impact. For example, the introduction of the ctenophore Mnemiopsis leidyi into the Black Sea in the 1980s led to a severe decline in fish populations, as the ctenophores consumed the same food as the fish larvae.
Ctenophores possess a nerve net, a simple type of nervous system where neurons are spread throughout the body. However, they do not have a centralized brain. Some research suggests that ctenophores have developed their nervous systems independently of other animals, a phenomenon known as convergent evolution.
The conservation status of most ctenophore species is unknown. Their gelatinous bodies do not fossilize well, making it difficult to study their evolutionary history and population trends.
The study of ctenophores has provided interesting insights into early animal evolution. Their unique biology and diverse array of behaviors make them a fascinating group of organisms to study.
Image Credit: UC Berkeley
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