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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Ice age could help predict oceans' response to global warming

A team of scientists led by a Tulane University oceanographer has found that deposits deep under the ocean floor reveal a way to measure the ocean oxygen level and its connections with carbon dioxide in the Earth's atmosphere during the last ice age, which ended more than 11,000 years ago.

The findings, published in Science Advances, help explain the role oceans played in past glacial melting cycles and could improve predictions of how ocean carbon cycles will respond to global warming.

Oceans adjust atmospheric CO2 as ice ages transition to warmer climates by releasing the greenhouse gas from carbon stored within the deep ocean.

The research demonstrates a striking correlation between global ocean oxygen contents and atmospheric CO2 from the last ice age to today -- and how carbon release from the deep sea may rise as the climate warms.

"The research reveals the important role of the Southern Ocean in controlling the global ocean oxygen reservoir and carbon storage," said Yi Wang, lead researcher and an assistant professor of Earth and Environmental Sciences at Tulane University School of Science and Engineering.

Wang specializes in marine biogeochemistry and paleoceanography.

"This will have implications for understanding how the ocean, especially the Southern Ocean, will dynamically affect the atmospheric CO2 in the future," she said.

Wang conducted the study with colleagues from the Woods Hole Oceanographic Institution, the world's leading independent nonprofit organization dedicated to ocean research, exploration and education.

She worked for the institute before joining Tulane in 2023.

The team analyzed seafloor sediments collected from the Arabian Sea to reconstruct average global ocean oxygen levels thousands of years ago.

They precisely measured isotopes of the metal thallium trapped in the sediments, which indicate how much oxygen was dissolved in the global ocean at the time the sediments formed.

"Study of these metal isotopes on glacial-interglacial transitions has never been looked at before, and these measurements allowed us to essentially recreate the past," Wang said.

The thallium isotope ratios showed the global ocean lost oxygen overall during the last ice age compared to the current warmer interglacial period.

Their study revealed thousand-year global ocean deoxygenation during abrupt warming in the Northern Hemisphere, whereas the ocean gained more oxygen when abrupt cooling occurred during the transition from the last ice age to today.

The researchers attributed the observed ocean oxygen changes to Southern Ocean processes.

"This study is the first to present an average picture of how the oxygen content of the global oceans evolved as Earth transitioned from the last glacial period into the warmer climate of the last 10,000 years," said Sune Nielsen, associate scientist at WHOI and co-author of the research.

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Endangered Devils Hole pupfish is one of the most inbred animals known

As its name implies, the Devil's Hole pupfish lives in a truly hellish environment.

Confined to a single deep limestone cave in Nevada's Mojave Desert, 263 of them live in water that hovers around 93 degrees Fahrenheit year-round, with food resources so scarce that they are always on the edge of starvation, and with oxygen levels so low that most other fish would die immediately. The pupfish, Cyprinodon diabolis, live in the smallest habitat of any known vertebrate.

New research now documents the extreme effect that these harsh and isolated conditions have had on this fish's genetic diversity.

In a paper published this week in the journal Proceedings of the Royal Society B, University of California, Berkeley, biologists report the first complete genome sequences of eight pupfish species from the American Southwest -- 30 individuals in all, including eight Devils Hole pupfish. Astoundingly, the Devils Hole pupfish is so inbred that 58% of the genomes of these eight individuals are identical, on average.

"High levels of inbreeding are associated with a higher risk of extinction, and the inbreeding in the Devils Hole pupfish is equal to or more severe than levels reported so far in other isolated natural populations, such as the Isle Royale wolves in Michigan, mountain gorillas in Africa and Indian tigers," said lead researcher Christopher Martin, UC Berkeley associate professor of integrative biology and curator of ichthyology in the campus's Museum of Vertebrate Zoology. "Although we were not able to directly measure fitness, the increased inbreeding in these pupfish likely results in a substantial reduction in fitness."

Other pupfish species are also inbred, the researchers found, but only between 10% and 30% of their genomes are identical.

Graduate student David Tian, lead author of the study, said that the level of inbreeding in the Devils Hole pupfish is equivalent to what would happen if four to five generations of siblings mated with one another. This tends to burn in or fix, rather than weed out, harmful mutations, potentially dooming a population to extinction by mutational meltdown. The Devils Hole pupfish species is currently doing well in the wild and in captive or "refuge" populations, but such low genetic diversity could spell trouble as the climate changes and human impacts become greater.

In the face of these potential threats, the new genome sequences will help scientists and conservationists assess the health of native pupfish populations and potentially intervene in refuge populations to increase the genetic diversity of these species -- the Devils Hole pupfish, in particular.

"With this new genomic data, there's a lot of potential to look not just at genetic diversity and how these species are related to each other phylogenetically, but also look at inbreeding and mutation load to get an idea of what their current status is, how evolutionary history may have influenced their current genetic variation, and think about where the population is going and what we should do, if anything, to preserve these species," Tian said.

Population decline and rescue

Pupfish species are scattered around the globe and tend to like isolated lakes and springs, often with extreme conditions that most fish would find unsurvivable. About 30 species inhabit warm, salty desert springs and streams in California and Nevada. Martin has studied various pupfish populations, including several on San Salvador Island in the Bahamas, to understand the genetics behind their adaptation to extreme conditions and unusual ecological niches.

The Devils Hole pupfish, however, is unique in its small range and perilous existence, Martin said, making its fluctuating population in the wild worrisome to conservationists.

"Part of the question about these declines is whether they may be due to the genetic health of the population," Martin said. "Maybe the declines are because there are harmful mutations that have become fixed because the population is so small."

The small population is partly a result of human incursions into their habitat, Martin noted. Local ranchers and developers pumped groundwater in the region in the 1960s and '70s that drastically reduced the water level in Devils Hole, leading to a drop in population levels. A 1976 Supreme Court ruling that allowed the federal government to limit groundwater pumping saved Devils Hole and the resident population, while captive breeding at a nearby 100,000-gallon pool in the Ash Meadows National Wildlife Refuge rescued the species. Nevertheless, a decline in the 1990s led the wild population to its nadir in 2013: 35 individuals. The wild population has since recovered, while the refuge population has ballooned to about 400, twice the wild population.

Humans are not totally to blame for the lack of genetic diversity in the Devils Hole pupfish, however. The UC Berkeley researchers also sequenced the genome of a pupfish collected in 1980 and held at the University of Michigan. It showed inbreeding and a lack of genetic diversity similar to that found in individuals collected recently, most of which died a natural death. This implies that the pupfish has likely seen population bottlenecks frequently over hundreds, if not thousands, of years.

One result of this, Martin and Tian found, is that 15 genes have disappeared entirely from the Devils Hole pupfish genome. Five of them seem to be involved in adaptation to living in low-oxygen or hypoxic environments.

"These deletions are a paradox, because this is a habitat where you're most exposed to hypoxia," Martin said. "It could have something to do with the stability of the habitat over time. But it looks to us like the hypoxia pathway is broken. Once you break one gene, it doesn't really matter if you break additional genes in that regulatory pathway. Our future work is to actually look at what these deletions do. Do they increase tolerance of hypoxia? Do they decrease tolerance of hypoxia? I think those two scenarios are equally plausible at this time."

Selective breeding within a captive population of Devils Hole pupfish could help increase the diversity and perhaps save the species from eventual extinction, he said. And to restore genes already lost, CRISPR genome editing could add them back.

The fact that the genome of the fish collected in 1980 was about as inbred as today's fish is "maybe good news," Martin said, "in that the population has historically been highly inbred with very low genetic diversity, suggesting that the recent decline in the '90s, with population bottlenecks to only 35 fish in 2013 and 38 fish in 2007, doesn't seem to have had much of an effect."

Tian is currently analyzing about 150 complete genome sequences of nine species of American pupfish to get a more complete picture of the deleterious mutations and gene deletions in the various Southwestern populations. He sees the study as an example of what conservation genomics can do for endangered and possibly inbred populations around the world.

"We're on a really cool cusp when it comes to using genomic data and applying it to conservation, especially at a time where it's a problem that is likely only going to get worse with climate change and increased habitat fragmentation and just anthropogenic changes," he said.

Tian is leery of genetic interventions, however, since little is known about how genes influence the physical and behavioral characteristics of a species and how this relates to fitness and adaptation to a specific environment. Conservation should still be a priority.

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Earliest record of wildfires provide insights to Earth's past vegetation and oxygen levels

While wildfires over recent years have raged across much of the western United States and pose significant hazards to wildlife and local populations, wildfires have been a long-standing part of Earth's systems without the influence of humans for hundreds of millions of years.

"Wildfire has been an integral component in earth-system processes for a long time and its role in those processes has almost certainly been underemphasized," said Ian Glasspool, lead author of a study published yesterday in Geology that describes the earliest record of wildfire found yet to date.

In the study, Glasspool and co-author Robert Gastaldo document 430-million-year-old charcoal produced by wildfires found in samples from Wales and Poland. Their discovery pushes back the earliest record of wildfire by an additional 10 million years.

Glasspool explained that wildfire has three essential ingredients: a source of fuel, a source of ignition (which comes in the form of lightning strikes), and sufficient atmospheric oxygen.

"It looks now as though our evidence of fire coincides closely with our evidence of the earliest land plant macrofossils. So as soon as there's fuel, at least in the form of plant macrofossils, there is wildfire pretty much instantly," said Glasspool.

However, the types of plants that existed 430 million years ago during the Silurian period would have looked starkly different from the plants we see and are familiar with today. Instead of grasses, trees, and flowers, flat-lying plants barely even an inch tall would have covered much of the landscape, with the occasional waist-height or knee-height plant. In contrast to much of the diminutive plant cover, the ancient fungus Prototaxites would have stood nearly 30 feet (9 meters) tall, towering over the landscape. These Silurian plants would have been strongly dependent on water for their reproduction and likely would not have been found in seasonally dry areas.

"The Silurian landscape had to have enough vegetation across it to have wildfires propagated and to leave a record of that wildfire," said Gastaldo. "At points in time that we're sampling windows of, there was enough biomass around to be able to provide us with a record of wildfire that we can identify and use to pinpoint the vegetation and process in time."

In addition to a sufficient source of fuel, which Silurian plant life was able to provide, the other crucial factor in producing early wildfires is atmospheric oxygen levels. At the present day, oxygen makes up approximately 21% of the gasses in the planet's atmosphere. Atmospheric oxygen levels have changed greatly over Earth's history, with essentially zero oxygen in Earth's atmosphere for the first part of the planet's history.

As the research study describes, modern burn experiments indicate that wildfires are unlikely to occur below levels of 16% atmospheric oxygen.

"If you drop below that level you might initiate a fire but it's not going to propagate," said Glasspool. "So when you look at the probability of finding charcoal in the record, you're really only going to find charcoal if that fire was able to propagate, and you can put a minimum threshold value on atmospheric oxygen when you find charcoal."

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Scaling new heights with new research showing how plants can grow at altitude

A new study has found that plant species are adapted to the altitude where they grow by 'sensing' the oxygen levels that surround them.

Altitude is an important part of plant ecology with at least 30% of plant species diversity contained in mountains and climate change is leading to the retreat of alpine species and some crops to higher altitudes.

Research led by scientists at the University of Nottingham has identified a mechanism through which plants can sense atmospheric oxygen levels (that decrease with altitude) that will help to understand how plants live at high altitude. The work was carried out in collaboration with scientists in Spain and Ecuador and was funded by the Leverhulme Trust. Their findings have been published today in Nature.

Researchers analysed plants growing at low and high-altitude locations. The team, working in Nottingham, Ecuador and Spain was able to identify how oxygen-sensing controls the pathway of chlorophyll synthesis, permitting plants to match the levels of a key toxic chemical to surrounding oxygen levels.

Climate change is leading to the displacement of wild species and crops (for example coffee) to higher altitudes, this research offers new insights into the underlying genetic mechanisms controlling their ability to survive at different altitudes. This new understanding of the genetic changes plants go through at altitude could lead to approaches to help plant breeders enhance the capacity of crops to grow at higher altitudes.

The research was led by Professor Michael Holdsworth from the University of Nottingham in collaboration with Professor Karina Proaño at ESPE University in Sangolquí, Ecuador and Professor Carlos Alonso Blanco from the Spanish National Centre for Biotechnology CSIC.

Professor Holdsworth commented: "Altitude is a key component of ecology with different altitudes subjecting plants to changing environments, some components of which are fixed by altitude and others that are not. For life at high altitude, it was previously considered that plants need to adapt to many variables, including high UV light and lower temperatures usually present at high altitude but this study is the first time that perception of atmospheric oxygen levels has been shown to be a key determinant of altitude adaptation in plants. "

He continues: "Exploring this novel finding allowed us to show that atmospheric oxygen level is the key determinant of altitude perception. We define the molecular pathway through which oxygen-sensing results in an adapted phenotype and we find that distinct species of flowering plants are adapted to absolute altitude through conserved oxygen-sensing control of chlorophyll synthesis and hypoxia gene expression. Showing that this mechanism works in diverse species provides a new paradigm for plant ecology."

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