World's Earliest Animals: Candidates, Criteria, Debate | Sea Life, Islands and Oceania

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START OF THE TREE OF LIFE

Dickinsonia

Sarah Moore wrote in Live Science: All animal life is descended from a single common ancestor — a multicellular organism that most likely lived more than 600 million years ago. This ancestor had two offspring; one that led to the evolution of all animal life, and another that is referred to as the sister to all animals. In the quest to identify which living animals are most closely related to this sister group, scientists have narrowed down the possibilities to two candidates: sea sponges and comb jellies (ctenophores).[Source: Sarah Moore, Live Science, May 24, 2023]

A study published May 17, 2023 in the journal Nature has resolved this long-running debate with the novel use of chromosomal analysis. The solution came while Darrin T Schultz, lead author and current postdoctoral researcher at the University of Vienna, and a multi-institutional team were sequencing the genomes (the complete set of genetic information) of comb jellies and their close relatives to understand more about their evolution.

Rather than comparing individual genes, the team looked at their positions on chromosomes across species. While changes to DNA occur over the course of evolution, genes tend to remain on the same chromosome. On rare occasions of fusion and mixing, genes transfer from one chromosome to another in an irreversible process. Schultz compares this to shuffling a deck of cards. If you have two decks of cards and you shuffle them, they become mixed. "Once mixed, you can’t unmix it in the way it was before, the probability of that is almost impossible," Schultz told Live Science.

In other words, once a gene has moved from one chromosome to another, there is almost zero chance of it appearing in its original position again further down the evolutionary line. By looking at the large-scale movement of groups of genes across animal groups, Schultz and the team were able to gain important insights into the family tree of these animals. The team found 14 groups of genes that appeared on separate chromosomes in comb jellies and their single-celled, non-animal relatives. Interestingly, in sponges and all other animals, these genes had rearranged into seven groups. Given that the DNA of the comb jellies holds the gene groups in their original position (prior to rearranging into the seven groups) it is indicative that they are descendants of the sister group that broke from the animal family tree, before the mixing occurred. Further, the gene location rearrangements that were common to both sponges and all other animals suggest a shared ancestor from which these rearrangements were inherited. The findings, therefore, resolve the controversial question over the lineage of the entire animal tree of life.

Since the ancestors of comb jellies and sponges branched off from the family tree, their modern descendants have continued to evolve, so we cannot use this information to indicate what the first animals exactly looked like. However, scientists believe there is significant value in studying these modern animals in light of this new information about their lineage. "If we understand how all animals are related to one another, it helps us understand how animals evolved the things that make them animals," Schultz said.

Does the Presence of Cholesteroids Prove Dickinsonia are the World’s Earliest Animal?

In a paper published in Science in September 2018, researchers argued that a creature called Dickinsonia was was the oldest known animal. The segmented, pancake-shaped fossils of these life forms date to 575 to 541 million years ago during the Ediacaran period, and were first discovered in 1947 in Australia. They can can reach one meter in length and were first thought to an an early form of jellyfish. [Source: Shannon Hall, Scientific American, September 20, 2018]

Shannon Hall wrote in Scientific American: Fossilized Dickinsonia and other Ediacarans exhibit no obvious characteristics such as appendages, a mouth or a gut that would link them to anything in the animal kingdom. As such, their place on the family tree of life has been quite contentious: If Dickinsonia were not jellyfish, perhaps they were instead annelid worms or mushrooms, or enormously oversize lichens or single-celled organisms.

The problem is that although Dickinsonia fossils have now been spotted at dozens of sites across the globe, they are typically found solely as two-dimensional imprints in sandstone. “It would be like trying to judge the structure of our modern world if all you had was footprints,” says Guy Narbonne, a paleontologist at Queen’s University in Ontario. Then in 2016, Ilya Bobrovskiy, a graduate student at Australian National University, made a startling discovery, stumbling upon Dickinsonia fossils in Russia that were essentially mummified in a mixture of clay and sandstone.

The potential was enormous. “I’m in awe of this study because it’s a spectacular opportunity to get molecular information about a fossil that has been so enigmatic,” says Roger Summons, a geobiologist at Massachusetts Institute of Technology. When samples were analyzed, scientists uncovered cholesteroids: the molecular fossils of cholesterol, a distinctive signature of animal life. Whether animal, vegetable or otherwise, every Earthly organism is composed of cells bounded by layers of lipid molecules; only animals, however, have cholesterol in their cell membranes. So spotting cholesteroid meant Dickinsonia were in fact animals.

650-Million- Year-Old Australian Reef May Hold Fossils of Earliest Animal Life

890 million year old sponge fossils?

The Cryogenian Period (720 million years ago to 635 million years ago) preceded the Ediacaran Period (635 million to 542 million years ago), which preceded the Cambrian Period, know for the Cambian Explosion of life forms. In 2008, scientists announced researchers that they had discovered a 650 million year old reef that was once underwater in the middle of the Australian outback along the Flinders Ranges, Discover magazine reported: Researchers say the tiny fossils they've already found in the ancient reef may be the earliest examples of multicellular organisms ever found, and may answer questions about how animal life evolved. Researcher Malcolm Wallace explains that the oldest-known animal fossils are 570 million years old. The reef in the Flinders Ranges is 80 million years older than that and was, he said, “the right age to capture the precursors to animals” [Source: Eliza Strickland, Discover magazine, September 26, 2008]

The first fossils discovered in the reef appear to be sponge-like multicellular organisms that resemble tiny cauliflowers, measuring less than an inch in diameter.The reef's discovery was announced at a meeting of the Geological Society of Australia. Unlike the Great Barrier Reef, the Oodnaminta Reef – named after an old hut near by – is not made of coral. “This reef is much too old to be made of coral,” Professor Wallace said. “It was constructed by microbial organisms and other complex, chambered structures that have not been discovered before.” Coral was first formed 520 million years ago, more than 100 million years after the Oodnaminta was formed.

The Oodnaminta Reef formed during a very warm period in the Earth's history, which was sandwiched between two intensely cold eras, when scientists believe ice extended to the planet's equator. Researchers say the tiny organisms found in the reef may have gone on to survive one of the most extreme ice ages in Earth history which ended about 580 million years ago, apparently leaving descendents in the later life-friendly Ediacaran. "It's consistent with the argument that evolution was going on despite the severe cold," said Professor Wallace. The Ediacaran saw an explosion of complex multicellular organisms, including creatures that resembled worms and sea anemones; the sponges could be the ancestors of those species. For more on the the strange critters that flourished in the Ediacaran,

890 Million Year Old Sponges? — the World’s Earliest Animals?

A study published in Nature in July 2021 suggested that mesh-like structures found 890-million-year-old rocks in the "Little Dal" limestones in northwest Canada were fossils of primitive sponges. If the claim holds up the sponges would predate the earliest undisputed animals by more than 300 million years. National Geographic reported: However, most claims of extremely old fossilized life kick up controversy. The creatures that flourished in ancient seas may have looked quite different than those that swim through oceans today, and scientists disagree about how much and which types of evidence can distinguish animals from other forms of life — or geologic structures. And the Little Dal fossils are no different. "What we have is essentially something a bit like a Rorschach inkblot test, where there are some squiggles in a rock," says Jonathan Antcliffe, a paleontologist specializing in early life at the University of Lausanne, Switzerland.[Source: Maya Wei-Haas, National Geographic Science News, July 29, 2021]

Elizabeth Turner, the sole author of the study, held up a mustard yellow natural bath sponge — a modern relative to the newly proposed fossil sponge. She pointed out the network of flexible tubes that give the sponge its squish, explaining that the mesh is "identical" to the newly analyzed fossils, as well as to several younger mesh-like fossils recently identified by other scientists. "It seems almost like a no-brainer," says Turner, a field geologist at Ontario's Laurentian University. But she acknowledges that the proposed animal identity will be controversial. "It's time for it to be published and go out to the community for discussion and challenge."

The newly described fossils were tucked in the nooks and crannies of the towering Little Dal reef. The structure formed at a time when warm, shallow seas flooded a vast tract of land through what is now North America Many sponges build their skeletons out of tiny rigid structures called spicules, which are made from calcium carbonate or silica and shaped like toy Jacks. In fossils, the structures provide telltale signs of early sponges, but keratosan sponges lack these rigid skeletons. Instead, they get their squishy structure from networks of the protein spongin, which has a soft, spring-like texture that is ideal for their modern use for bathing. By studying paper-thin sections of the rocks under a microscope, Turner documented the similarities of the tubular shapes and structures in the Little Dal samples to fossils that were previously identified as keratosan sponges, as well as to modern sponges.

The association of these sponges with the microbial reefs would make sense, Turner notes. Earth’s atmosphere was not always rich in oxygen, and the early date for the sponges places them before this life-friendly gas was common throughout the sea. But so-called "oxygen oases" would likely have existed around cyanobacteria reefs, where the photosynthetic microbes would have spit out oxygen that the sponges could have used. Other experts are less convinced of the case, noting that the sponge-like network isn't as unique to the group as Turner and others suggest. "Basically, every area of life — bacteria, algae, the fungi, the plants, the animals — they can all make things that look like this," Antcliffe says.

Debate Over the World’s Earliest Animals

comb jelly

Maya Wei-Haas wrote in National Geographic Science News: Turner's study described above "joins the lengthy debate about when the earliest animals arose — and what evidence is necessary to confirm a fossil as an animal. Over the last several decades, the use of geochemical tracers for early life, known as biomarkers, became a common way to identify possible creatures, explains Keyron Hickman-Lewis, a geobiologist specializing in ancient microbes at London's Natural History Museum. The fossil remains of various types of lipids, for example, are commonly used as biomarkers. [Source:Maya Wei-Haas, National Geographic Science News, July 29, 2021

But since then, Hickman-Lewis says, much of this supposed evidence for early life has turned out to be false. Some of the would-be biomarkers were likely due to contamination, while other chemical traces were not surefire signs of animals. For example, scientists recently found that a combination of algae and geologic alteration could produce the same compounds that were previously identified as evidence of ancient sponges extracted from 635-million-year-old sediments in Oman. So after much early excitement, Hickman-Lewis says, "we became very suspicious of an early origin for animals."

In a 2014 review of the evidence for early sponges, Antcliffe and his colleagues found that the oldest convincing animal fossils are sponge spicules found in Iran dating to roughly 535 million years ago — and he says no recent studies have yet changed his mind. Many analyses have identified what he calls "hints and whispers" of earlier sponge-like structures. But none sport indisputable characteristics, such as spicules or pores. The latter of these features were key to confirming the identity of the much-debated Archaeocyathid sponges, another group that lacks spicules but has been identified in rocks as old as 523 million years.

Partly the challenge comes down to the difficulty of identifying ancient sponges compared to other animals, says Drew Muscente, a paleobiologist at Cornell College in Mount Vernon, Iowa. Dinosaurs, for example, have an array of distinctive boney features — sockets, skull sutures, and more — that can help scientists tell their fossils apart from nonliving objects. "When you have a sponge or a sponge-like organism, you're missing all of those little details," he says. Abiotic, or nonliving, chemical processes can also form structures that look surprisingly similar to life, adds Rachel Wood, a carbonate geologist at the University of Edinburgh. "She may be right. But I think you really have to explore and disprove all the other possibilities to make such a really strong claim like this." So for now, Wood says, "I don't think that she's really nailed that these are sponges."

Comb Jellies Not Sponges the World’s Oldest Animals?

There has long been a debate on which animal came first — comb jellies (ctenophore) or sponges. Sponges spend most of their lives in one spot, filtering water through their pores to collect food particles. Many have argued that due to the sponge's primitive features, it came first. Ctenophores (pronounced TEEN-oh-fours) are predators commonly known as comb jellies that travels through the ocean in search of food. Though they resemble jellyfish, they are distinctly different creatures that propel themselves through water using cilia (short vibrating or moving hairlike structure) instead of tentacles. They are still part of the marine ecosystem today and can be found in waters all over the world.

In December 2013, sponges lost their crown as the world’s oldest animals when scientists from the University of Miami announced in the journal Science that a species comb jelly known as sea walnuts and sea gooseberries represented the oldest branch of the animal family tree based on DNA research. Associated Press reported: All animals evolved from a single ancestor and scientists want to know more about how that happened. More than half a billion years ago, the first split in the tree separated one lineage from all other animals. Traditionally, scientists have thought it was sponges. [Source: Malcolm Ritter, Associated Press, December 12, 2013]

“The evidence in favor of comb jellies comes from deciphering the first complete genetic code from a member of this group. Scientists were finally able to compare the full DNA codes from all the earliest branches. The genome of a sea walnut, a plankton-eating creature native to the western Atlantic Ocean, was reported online Thursday in the journal Science by Andreas Baxevanis of the National Human Genome Research Institute with co-authors there and elsewhere. The work supports some earlier indications that comb jellies were the first to branch off.

Sorting out the early branching of the tree could help scientists learn what the ancestor of all animals was like. But despite decades of study and the traditional view favoring sponges, there is plenty of disagreement about which early branch came first. The question is "devilishly difficult" to answer, and the new paper is probably not the last word, said Antonis Rokas of Vanderbilt University, who did not participate in the new work. "The results need to be taken seriously," he said, but "I'm pretty sure there will be other studies that suggest something else."

In May 2023, scientists at the University of California Berkeley announced in a study published in Nature that the world’s first animal likely were comb jellies. "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 soft-bodied animals and didn't leave a direct fossil record," said Daniel Rokhsar, a UC Berkeley professor and co-author of the study, in a statement. "But we can use comparisons across living animals to learn about our common ancestors." [Source: Simrin Singh, CBS News, Fri, May 19, 2023]

CBS News reported: This new research has determined that while sponges came early, they were likely second to ctenophores. In order to make that determination, scientists looked at the organization of genes in the chromosomes of the organisms. The chromosomes of the ctenophore look very different than the chromosomes of sponges, jellyfish and other invertebrates — alerting researchers that the ctenophore could have either come much earlier than the others, or much later. "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," Rokhsar explained in the news release. "Alternatively, they could be different because they branched off first, before all other animal lineages appeared. We needed to figure it out."

The "smoking gun" for researchers was when they compared the chromosomes of ctenophores to non-animals. "When the team compared the chromosomes of these diverse animals and non-animals, they found that ctenophores and non-animals shared particular gene-chromosome combinations, while the chromosomes of sponges and other animals were rearranged in a distinctly different manner," the news release said. According to researchers, the new insight is valuable to learning about the basic functions of all animals and humans today, such as how we eat, move and sense our surrounding environment.

Checking Out 555-Million-Year-Old Sea Life in South Australia

Cambrian Era Creatures

As we said before the Ediacaran Period (635 million to 542 million years ago) preceded the Cambrian Period, know for the Cambian Explosion of life forms. Many types of life evolved at this time. South Australia's 900-kilometer Mawson Trail travels through the Flinders range, passing by places with imprints of animals that lived 555 million years ago — likely the earliest human ancestor.Tracey Croke of the BBC wrote: The Flinders Ranges tell an unparalleled tale about the dawn of life, according to world-leading palaeontologists — one that forced scientists to rethink Earth's geologic time scale. An inkling was under our noses from the get-go on every Mawson Trail signpost: the illustration of a trio of creatures that resembled a feather, a slice of citrus fruit and the shed exoskeleton of a woodlouse. These are the best-guess recreations of what life looked like 550 million years ago – soft-bodied languid blobs (ranging in size from millimetres to more than a metre) known as Ediacaran Biota, named after the ancient hills in the Flinders Ranges, where their encrusted imprints were found. [Source: Tracey Croke, BBC, April 2022]

I slowly scanned the sedimentary layers of the gorge. If you know how to read it, this repository of the planet's evolution is one of the world's best exposure sites, according to Mary Droser, professor of geology at University of California Riverside. "The Flinders Ranges encompasses a huge swath of time that incorporates all of the really wacky environmental things that were going on, from Snowball Earth to global warming," said Droser. "We can see a 350-million-year window of time from a microbial world through to through to the early history of animals." This is because the shunting, subsiding and eroding activity of the Flinders left corridors through layers of time – revealing evidence of critical eras and events.

"There are places that have parts of the story, and there are places with phenomenal fossils, but the Flinders has this complete packaging that is really accessible. We can go back in time and see how life unfolded. The record is unparalleled," Droser said.

Significance of the 570-Million-Year-Old Fossils from the Flinders Range

Tracey Croke of the BBC wrote: One such chapter in Earth's story was recorded in the western ranges of the Flinders in 1946, when geologist Reg Sprigg was looking for mineral deposits in the low Ediacaran Hills. Sprigg, a keen palaeontologist turned over some of the flaggy sandstone slabs and found an entire community of fossilised imprints, which included five new genera and species. "He knew the age of the rocks, which were older than the Cambrian rocks [that] we know to have fossils with skeletons," said Droser, who is one of the world's leading researchers of Ediacaran fossils. That, she said, meant Sprigg knew these imprints were "very, very significant". [Source: Tracey Croke, BBC, April 2022]

Opabinia fossil

Sprigg's discovery solved one of the greatest mysteries in natural science, one that had kept Charles Darwin scratching his head his entire life. When Darwin wrote On the Origin of Species in 1859, he highlighted his concern about the apparent sudden appearance of Cambrian skeletonised fossils and the challenge it presented to his theory of evolution. He wrote: "… to the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer." This puzzle, known as Darwin's Dilemma, flummoxed scientists for almost a century. But Sprigg found concrete evidence of the missing piece.

Some 570 to 540 million years ago, these empty shapes in the rocks were occupied by the soft-bodied Ediacaran Biota creatures that were a step up from single cell organisms and a step down from animals running around eating each other – making them the earliest known complex animal life on Earth. Never before had so many been found in one place. The discovery revolutionised our understanding of how multicellular animal life evolved. In 2004, a new, globally recognised geological era that existed between 635 and 540 million years ago was formally created and ratified by the International Union of Geological Sciences. It was named, of course, the Ediacaran.

More recently, another missing link in the evolutionary puzzle sent revelations through the scientific community. From studying multiple miniscule fossilised burrows found in Nilpena in 2005, Droser and evolutionary biologists had long predicted that in the same period – around 555 million years ago – a more complex creature compared to other Ediacaran Biota was on the move, contracting muscles across its body to travel. In 2020, using 3D laser scanner technology, Droser and her team were able to recreate the creature – a plump, wormy blob, the size of a grain of rice. It had a notable difference compared to other life forms in existence at that time: it was the first animal ever to have a front and a back, a mouth, gut and rear end – called a "bilaterian". This meant Ikaria wariootia, as they named the blob, could possibly be the animal that ate and excreted its way on a long, transformative journey that, eventually, resulted in humans. "It's certainly the oldest bilaterian that we know of," Droser said.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated November 2025