Sea level changes shaped early life on Earth, fossil study reveals

A newly developed timeline of early animal fossils reveals a link between sea levels, changes in marine oxygen, and the appearance of the earliest ancestors of present-day animals.

The study reveals clues into the forces that drove the evolution of the earliest organisms, from which all major animal groups descended.

A team from the University of Edinburgh studied a compilation of rocks and fossils from the so-called Ediacaran-Cambrian interval -- a slice of time 580-510 million years ago. This period witnessed an explosion of biodiversity according to fossil records, the causes of which have baffled scientists since Charles Darwin.

The early animals found from this era were all sea-dwellers, at a time when oxygen levels in the air and ocean were much lower than today.

While the very first lifeforms before this time were mostly single-cell, and simple multi-celled organisms, creatures in the Ediacaran Period started to become more complex, with multiple cells organised into body plans that allowed them to feed, reproduce, and move across the ocean floor.

This era also marked the emergence of so-called bilaterian animals -- which display symmetrical body plans, in common with most present-day species including humans.

By compiling data from different sources -- including radioactive dating and geochemical information about the layers of rock in which fossils were found -- the team mapped all major fossil finds and various environmental datasets onto a single timeline.

The new chronology allowed the team to study trends in biodiversity for the period in question with more detail than before.

They combined these insights with further chemical clues from the geological record -- confirming a link between major changes in global sea levels, intervals when shallow marine environments gained more oxygen, and the appearance and diversification of early animal groups.

This dynamic set the stage for several significant bursts in biological diversity, known as the Avalon, White Sea, and Cambrian assemblages, each marking the arrival of new animal groups and the decline of others.

By reconstructing environmental conditions in deepest time, the study unlocks new insights into the ancient forces and pressures that shaped the earliest life on our planet.

The team also identified gaps in the fossil record, suggesting that current knowledge about early animals is biased by the clusters of sites worldwide where fossils have been found and studied.

Dr Fred Bowyer of the University of Edinburgh's School of Geosciences, said: "Constructing a timescale of early animal evolution using the rock record is a daunting task, only made possible through international and interdisciplinary research. But an integrated global approach is crucial. It exposes biases in our records, while also revealing patterns in fossil appearances, sea level cycles, and environmental oxygen."

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Scientists investigate the evolution of animal developmental mechanisms, show how some of Earth's earliest animals evolved

Lacking bones, brains, and even a complete gut, the body plans of simple animals like sea anemones appear to have little in common with humans and their vertebrate kin. Nevertheless, new research from Investigator Matt Gibson, Ph.D., at the Stowers Institute for Medical Research shows that appearances can be deceiving, and that a common genetic toolkit can be deployed in different ways to drive embryological development to produce very different adult body plans.

It is well established that sea anemones, corals, and their jellyfish relatives shared a common ancestor with humans that plied the Earth's ancient oceans over 600 million years ago. A new study from the Gibson Lab, published in Current Biology on June 13, 2023, illuminates the genetic basis for body plan development in the starlet sea anemone, Nematostella vectensis. This new knowledge paints a vivid picture of how some of the earliest animals on earth progressed from egg to embryo to adult.

"Studying the developmental genetics of Nematostella is sort of like taking a time machine into the very distant past," said Gibson. "Our work allows us to ask what life looked like long ago -- hundreds of millions of years before the dinosaurs. How did ancient animals develop from egg to adult, and to what extent have the genetic mechanisms that guide embryonic development endured across millennia?"

Most contemporary animals, from insects to vertebrates, develop by forming a head-to-tail series of segments that assume distinct identities depending on their position. Within a given segment, there is a further axis of polarity that informs cells whether they are at the front or back of the segment. Collectively, this is referred to as segment polarization.

Shuonan He, Ph.D., a former predoctoral researcher from the Gibson Lab, uncovered genes involved during development of the sea anemone, Nematostella vectensis, that guide the formation of segments and others that direct segment polarity programs strikingly similar to organisms higher up the evolutionary tree of life, including humans.

"The significance is that the genetic instructions underlying the construction of extremely different animal body plans, for example, a sea anemone and a human, are incredibly similar," said Gibson. "The genetic logic is largely the same."

This new study builds upon a 2018 study published in Science from the Gibson Lab that showed that sea anemones have an internal bilateral symmetry early in development with eight radial segments. The study demonstrated that Hox genes -- master development genes that are crucial for human development -- act to delineate boundaries between segments and likely had an ancient role in segment construction.

The team's latest finding explores how segments form and what accounts for differences in their identities. Using spatial transcriptomics, or the differences in gene expression between segments, the team discovered hundreds of new segment-specific genes. These include two crucial genes that encode transcription factors that govern segment polarization under the control of Hox genes and are required for the proper placement of sea anemone muscles.

The astonishing diversity of organisms on Earth can be compared to the assembly of Legos. "Whether you construct a dinosaur, a sea anemone, or a human, many of the core genetic building blocks are largely the same despite drastically different animal forms," said Gibson.

This is the first time that scientists have evidence of a molecular basis for segment polarization in a pre-bilaterian animal. While extensively studied in bilateral species like fruit flies and humans, the idea that cnidarian animals possess segmentation was unexpected. Now, the team has evidence that these segments are also polarized.

"This provides further evidence that investigating a broad diversity of animals can have direct implications for understanding general principles, including those which apply to human biology," said Gibson. "Going one step further, by understanding the logic of sea anemone development and comparing it to what we see in vertebrates, we can also extrapolate back in time to understand how animals likely developed hundreds of millions of years ago."

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Earth's first animals had particular taste in real estate

Even without body parts that allowed for movement, new research shows -- for the first time -- that some of Earth's earliest animals managed to be picky about where they lived.

These creatures from the Ediacaran Period, roughly 550 million years ago, are strangely shaped soft-bodied animals that lived in the sea. Researchers have long considered them enigmatic.

"It's not like studying dinosaurs, which are related to birds that we can observe today," said Phillip C. Boan, UC Riverside paleontology graduate student and lead author of the new study. "With these animals, because they have no modern descendants, we're still working out basic questions about how they lived, such as how they reproduced and what they ate."

For this particular research project, the researchers focused on understanding where in the sea the animals spent their lives.

The ancient sea was also a largely foreign place compared to today's marine environments. It was dominated by a mat on the sea floor composed of bacteria and layers of other organic materials. In addition, predatory creatures were uncommon.

Given the alien nature of Ediacaran Earth, the researchers were surprised to find an animal that lived much the way barnacles do today. A new Paleobiology paper details how Obamus coronatus, named for the former U.S. president, opted to live on specific parts of the sea floor in the company of other Obamus.

The animal averaged about a half-inch in diameter and was "shaped like a French cruller donut with ribbons on top," Boan said. It did not move of its own accord, and likely spent its entire life embedded in its preferred spot on the sea floor.

"We think about the very oldest animals and maybe you wouldn't expect them to be so picky. But Obamus only occurs where there is a thick mat, and it's a pretty sophisticated way of making a living for something so very old," said Mary Droser, UCR distinguished professor of paleontology and study co-author.

In 2018, Droser's laboratory named the Obamus in honor of Barack Obama's passion for science. Her group discovered it at an extraordinarily well-preserved fossil site in the Australian Outback, at what is now called Nilpena Ediacara National Park.

A series of storms buried the Ediacaran sea floor at Nilpena in layers of sediment, helping preserve sandstone impressions of entire animal communities that lived together there. "This way, we're able to piece together whole ecosystems," Droser said. "Looking at them is like snorkeling around on the ancient sea floor, instead of looking at a single animal in a fish tank."

For this project, the research team selected three animals found in relatively large numbers at Nilpena, and examined how they were geographically distributed.

The other two animals, Tribrachidium and Rugoconites, are also immobile creatures with no modern descendants. "They are tri-radially symmetrical, like the Mercedes Benz logo," Boan said. "And they would have lived their entire lives embedded in the sea floor, as Obamus did."

Distribution for these other two animals was varied. Sometimes they could be found living in the company of other organisms like themselves, but not in every instance. However, Obamus displayed a clear preference.

"This is really the first example of a habitat-selective Ediacaran creature, the first example of a macroscopic animal doing this," Boan said. "But how did they get where they wanted to go? This is a question we don't yet know the answer to."

The research team theorizes that Obamus were likely motivated by the need to reproduce.

"There are a limited number of reproductive strategies, especially for animals like these," Droser said. "There are more strategies today, and they're more elaborate now. But the same ones used today were still being used 550 million years ago."

Obamus likely spread itself via selective larva that preferred locations with thick microbial mat and near other Obamus. "We don't entirely understand how Obamus offspring spread out, but we know that when they picked a place to live, it was very specific," Boan said.

A deeper understanding of how life on Earth developed over time can give researchers insight into how life could develop on another planet. For this reason, Droser's lab is funded by NASA's Exobiology program.

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World's oldest meal helps unravel mystery of our earliest animal ancestors

The contents of the last meal consumed by the earliest animals known to inhabit Earth more than 550 million years ago has unearthed new clues about the physiology of our earliest animal ancestors, according to scientists from The Australian National University (ANU).

Ediacara biota are the world's oldest large organisms and date back 575 million years. ANU researchers found the animals ate bacteria and algae that was sourced from the ocean floor. The findings, published in Current Biology, reveal more about these strange creatures, including how they were able to consume and digest food.

The scientists analysed ancient fossils containing preserved phytosterol molecules -- natural chemical products found in plants -- that remained from the animals' last meal. By examining the molecular remains of what the animals ate, the researchers were able to confirm the slug-like organism, known as Kimberella, had a mouth and a gut and digested food the same way modern animals do. The researchers say it was likely one of the most advanced creatures of the Ediacarans.

The ANU team found that another animal, which grew up to 1.4 metres in length and had a rib-like design imprinted on its body, was less complex and had no eyes, mouth or gut. Instead, the odd creature, called Dickinsonia, absorbed food through its body as it traversed the ocean floor.

"Our findings suggest that the animals of the Ediacara biota, which lived on Earth prior to the 'Cambrian Explosion' of modern animal life, were a mixed bag of outright weirdos, such as Dickinsonia, and more advanced animals like Kimberella that already had some physiological properties similar to humans and other present-day animals," lead author Dr Ilya Bobrovskiy, from GFZ-Potsdam in Germany, said.

Both Kimberella and Dickinsonia, which have a structure and symmetry unlike anything that exists today, are part of the Ediacara biota family that lived on Earth about 20 million years prior to the Cambrian Explosion -- a major event that forever changed the course of evolution of all life on Earth.

"Ediacara biota really are the oldest fossils large enough to be visible with your naked eyes, and they are the origin of us and all animals that exist today. These creatures are our deepest visible roots," Dr Bobrovskiy, who completed the work as part of his PhD at ANU, said.

Study co-author Professor Jochen Brocks, from the ANU Research School of Earth Sciences, said algae are rich in energy and nutrients and may have been instrumental for Kimberella's growth.

"The energy-rich food may explain why the organisms of the Ediacara biota were so large. Nearly all fossils that came before the Ediacara biota were single-celled and microscopic in size," Professor Brocks said.

Using advanced chemical analysis techniques, the ANU scientists were able to extract and analyse the sterol molecules contained in the fossil tissue. Cholesterol is the hallmark of animals and it's how, back in 2018, the ANU team was able to confirm that Ediacara biota are among our earliest known ancestors.

The molecules contained tell-tale signatures that helped the researchers decipher what the animals ate in the lead up to their death. Professor Brocks said the difficult part was differentiating between the signatures of the fat molecules of the creatures themselves, the algal and bacterial remains in their guts, and the decaying algal molecules from the ocean floor that were all entombed together in the fossils.

"Scientists already knew Kimberella left feeding marks by scraping off algae covering the sea floor, which suggested the animal had a gut. But it was only after analysing the molecules of Kimberella's gut that we were able to determine what exactly it was eating and how it digested food," Professor Brocks said.

"Kimberella knew exactly which sterols were good for it and had an advanced fine-tuned gut to filter out all the rest.

"This was a Eureka moment for us; by using preserved chemical in the fossils, we can now make gut contents of animals visible even if the gut has since long decayed. We then used this same technique on weirder fossils like Dickinsonia to figure out how it was feeding and discovered that Dickinsonia did not have a gut."

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First animals developed complex ecosystems before the Cambrian explosion

Early animals formed complex ecological communities more than 550 million years ago, setting the evolutionary stage for the Cambrian explosion, according to a study by Rebecca Eden, Emily Mitchell, and colleagues at the University of Cambridge, UK, publishing May 17 in the open-access journal PLOS Biology.

The first animals evolved towards the end of the Ediacaran period, around 580 million years ago. However, the fossil record shows that after an initial boom, diversity declined in the run-up to the dramatic burgeoning of biodiversity in the so-called "Cambrian explosion" nearly 40 million years later. Scientists have suggested this drop in diversity is evidence of a mass extinction event roughly 550 million years ago -- possibly caused by an environmental catastrophe -- but previous research has not investigated the structure of these ancient ecological communities.

To evaluate the evidence for an Ediacaran mass extinction, researchers analyzed the metacommunity structure of three fossil assemblages that span the last 32 million years of this geological period (between 575 to 543 million years ago). They used published paleoenvironmental data, such as ocean depth and rock characteristics, to look for metacommunity structure indicative of environmental specialization and interactions between species. The analysis revealed increasingly complex community structure in the later fossil assemblages, suggesting that species were becoming more specialized and engaging in more inter-species interactions towards the end of the Ediacaran era, a trend often seen during ecological succession.

The results point to competitive exclusion, rather than mass extinction, as the cause of the diversity drop in the late Ediacaran period, the authors say. The analysis indicates that the features of ecological and evolutionary dynamics commonly associated with the Cambrian explosion -- such as specialization and niche contraction -- were established by the first animal communities in the late Ediacaran.

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Research shows we're surprisingly similar to Earth's first animals

 The earliest multicellular organisms may have lacked heads, legs, or arms, but pieces of them remain inside of us today, new research shows.

According to a UC Riverside study, 555-million-year-old oceanic creatures from the Ediacaran period share genes with today's animals, including humans.

"None of them had heads or skeletons. Many of them probably looked like three-dimensional bathmats on the sea floor, round discs that stuck up," said Mary Droser, a geology professor at UCR. "These animals are so weird and so different, it's difficult to assign them to modern categories of living organisms just by looking at them, and it's not like we can extract their DNA -- we can't."

However, well-preserved fossil records have allowed Droser and the study's first author, recent UCR doctoral graduate Scott Evans, to link the animals' appearance and likely behaviors to genetic analysis of currently living things. Their research on these links has been recently published in the journal Proceedings of the Royal Society B.

For their analysis, the researchers considered four animals representative of the more than 40 recognized species that have been identified from the Ediacaran era. These creatures ranged in size from a few millimeters to nearly a meter in length.

Kimberella were teardrop-shaped creatures with one broad, rounded end and one narrow end that likely scraped the sea floor for food with a proboscis. Further, they could move around using a "muscular foot" like snails today. The study included flat, oval-shaped Dickinsonia with a series of raised bands on their surface, and Tribrachidium, who spent their lives immobilized at the bottom of the sea.

Also analyzed were Ikaria, animals recently discovered by a team including Evans and Droser. They were about the size and shape of a grain of rice, and represent the first bilaterians -- organisms with a front, back, and openings at either end connected by a gut. Evans said it's likely Ikaria had mouths, though those weren't preserved in the fossil records, and they crawled through organic matter "eating as they went."

All four of the animals were multicellular, with cells of different types. Most had symmetry on their left and right sides, as well as noncentralized nervous systems and musculature.

Additionally, they seem to have been able to repair damaged body parts through a process known as apoptosis. The same genes involved are key elements of human immune systems, which helps to eliminate virus-infected and pre-cancerous cells.

These animals likely had the genetic parts responsible for heads and the sensory organs usually found there. However, the complexity of interaction between these genes that would give rise to such features hadn't yet been achieved.

"The fact that we can say these genes were operating in something that's been extinct for half a billion years is fascinating to me," Evans said.

The work was supported by a NASA Exobiology grant, and a Peter Buck postdoctoral fellowship.

Going forward, the team is planning to investigate muscle development and functional studies to further understand early animal evolution.

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Most close relatives of birds neared the potential for powered flight but few crossed its thresholds

 Uncertainties in the evolutionary tree of birds and their closest relatives have impeded deeper understanding of early flight in theropods, the group of dinosaurs that includes birds. To help address this, an international study led by HKU Research Assistant Professor Dr. Michael Pittman (Vertebrate Palaeontology Laboratory, Division of Earth and Planetary Science & Department of Earth Sciences) and co-first-authored by his former Postdoctoral Fellow Dr. Rui Pei (now an Associate Professor at the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing), produced an updated evolutionary tree of early birds and their closest relatives to reconstruct powered flight potential, showing it evolved at least three times. Many ancestors of the closest bird relatives neared the thresholds of powered flight potential, suggesting broad experimentation with wing-assisted locomotion before flight evolved.

"Our revised evolutionary tree supports the traditional relationship of dromaeosaurid ("raptors") and troodontid theropods as the closest relatives of birds. It also supports the status of the controversial anchiornithine theropods as the earliest birds," said Dr. Pei. With this improved evolutionary tree, the team reconstructed the potential of bird-like theropods for power flight, using proxies borrowed from the study flight in living birds. The team found that the potential for powered flight evolved at least three times in theropods: once in birds and twice in dromaeosaurids. "The capability for gliding flight in some dromaeosaurids is well established so us finding at least two origins of powered flight potential among dromaeosaurids is really exciting," said Dr. Pittman. Crucially, the team found that many ancestors of bird relatives neared the thresholds of powered flight potential. "This suggests that theropod dinosaurs broadly experimented with the use of their feathered wings before flight evolved, overturning the paradigm that this was limited to a much more exclusive club," added Dr. Pittman.

This study is the latest in the Vertebrate Palaeontology Laboratory's long-term research into the evolution of early birds and their closest relatives (see Notes). Asked about future plans, Dr. Pittman replied: "We have helped to better constrain the broader functional landscape of theropods just before flight evolved and in its earliest stages. We plan to now focus on the dromaeosaurids and early birds that we have shown to have the potential for powered flight to improve our understanding of what it took to fly and why."

 From Science Daily

Mystery of how early animals survived ice age

How did life survive the most severe ice age? A McGill University-led research team has found the first direct evidence that glacial meltwater provided a crucial lifeline to eukaryotes during Snowball Earth, when the oceans were cut off from life-giving oxygen, answering a question puzzling scientists for years.

In a new study published in the Proceedings of the National Academy of Sciences, researchers studied iron-rich rocks left behind by glacial deposits in Australia, Namibia, and California to get a window into the environmental conditions during the ice age. Using geological maps and clues from locals, they hiked to rock outcrops, navigating challenging trails to track down the rock formations.

By examining the chemistry of the iron formations in these rocks, the researchers were able to estimate the amount of oxygen in the oceans around 700 million years ago and better understand the effects this would have had on all oxygen-dependent marine life, including the earliest animals like simple sponges.

"The evidence suggests that although much of the oceans during the deep freeze would have been uninhabitable due to a lack of oxygen, in areas where the grounded ice sheet begins to float there was a critical supply of oxygenated meltwater. This trend can be explained by what we call a 'glacial oxygen pump'; air bubbles trapped in the glacial ice are released into the water as it melts, enriching it with oxygen," says Maxwell Lechte, a postdoctoral researcher in the Department of Earth and Planetary Sciences under the supervision of Galen Halverson at McGill University.

Around 700 million years ago, the Earth experienced the most severe ice age of its history, threatening the survival of much of the planet's life. Previous research has suggested that oxygen-dependent life may have been restricted to meltwater puddles on the surface of the ice, but this study provides new evidence of oxygenated marine environments.

"The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution. These harsh conditions could have stimulated their diversification into more complex forms," says Lechte, who is also the study's lead author.

Lechte points out that while the findings focus on the availability of oxygen, primitive eukaryotes would also have needed food to survive the harsh conditions of the ice age. Further research is needed to explore how these environments might have sustained a food web. A starting point might be modern ice environments that host complex ecosystems today.

"This study actually solves two mysteries about the Snowball Earth at once. It not only provides explanation for how early animals may have survived global glaciation, but also eloquently explains the return of iron deposits in the geological record after an absence of over a billion years," says Professor Galen Halverson.

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