The Earth is an extraordinary planet. Despite its continuously changing climates, from ice ages to global warming, Earth’s ability to support life has never wavered. The mechanisms by which life emerged and evolved on ancient Earth are the subject of Professor Timothy Lyons’ research at the University of California, Riverside. Understanding more about how oxygen levels increased in the atmosphere and oceans, and the implications for early life on Earth, is vital when determining whether the evolution of life on Earth is truly unique.
The remarkable persistence of life on Earth is exemplified by the fact that it has withstood periods of intense environmental change. Understanding the conditions that prevailed on ancient Earth is not only a fascinating challenge, it sheds light on the evolution of terrestrial life and is relevant to the search for life on other planets.
One of the key concepts underpinning the study of the Earth is that the present is the key to the past, and Professor Lyons’ research builds on that concept. Professor Lyons is based within the Department of Earth Sciences at the University of California, Riverside, and is also the team leader of the Alternative Earths Team of the NASA Astrobiology Institute. His research uses present-day analogues both as the key to the past to determine how life on Earth evolved and also to help in the search for life on other planets.
Studying the development of life on Earth is complex enough, but by developing new techniques to gather evidence on Earth, research findings can be applied elsewhere in the universe. For example, present-day Earth has environments that are useful analogues when studying other planets. The Atacama Desert in South America shares similarities with the extreme environments of Mars, and the conditions found in the cold waters of the Antarctic have been used to inform missions to Europa and Enceladus – moons of Jupiter and Saturn, respectively.
The modern world also informs our ability to reconstruct our own distant past. Through this modern calibration, biological and geochemical signatures from ancient rock samples taken from different periods of geological time expose the evolutionary processes and pathways of our long history. And the diverse chapters of sustained early habitability on our dynamic planet – what Lyons and his team call Alternative Earths – have become templates for exploration of life on extra-solar planets (exoplanets) far beyond our solar system. If we know what the early stages of life looked like on our own planet, it will make it easier to look for them on others.
To understand the development of life on Earth, we must consider the composition of the Earth’s atmosphere at that time. Oxygen is believed to have begun accumulating in the atmosphere around 2.4 billion years ago, culminating in the diversity of species that populate our planet today. The Earth’s atmosphere has evolved from initially having zero oxygen to our present-day atmosphere containing around 20%. Oxygen is essential for life, and as it became more concentrated in the atmosphere and shallow oceans, life could begin to further diversify and evolve.
Oxygen fuels evolution
In order to comprehend how important atmospheric oxygen was to the development of life on Earth, it is necessary to know its concentration in the shallow oceans where much of early evolution occurred. There is a link between the oxygen concentration of the atmosphere and that of the oceans due to exchange between the two, but the best approach would provide direct evidence for oxygen in those surface waters – the ideal habitat for the development of complex life, including animals. Professor Lyons and his colleagues are developing methods of chemical fingerprinting for past oxygen levels to decipher the geological record of oxygenation in the ancient oceans and its relationships with early evolving life –similar to the way a pathologist uses DNA evidence to solve a crime.
The energetic properties of oxygen are not only crucial for respiratory processes in land-based species, but they also provide cells with the ability to diversify and evolve. Oxygen also has a shielding effect on the Earth through its production of ozone in the Earth’s stratosphere. This protects the Earth’s surface and the life it hosts from the sun’s ultraviolet radiation.
A proxy for life
Iodine is an element best known for its antiseptic properties, yet it was present as a trace component in ancient oceans. Iodine is also found at low levels in present-day oceans, with around one thousand cubic metres of modern day seawater yielding around 60 grams of iodine.
Throughout the geological record, iodine has been preserved in sedimentary rocks deposited in the oceans. Limestone is one example of an iodine-containing carbonate rock typically forming in shallow tropical seas similar to environments where life is believed to have emerged. Through their research, Professor Lyons, his former student Dalton Hardisty, and their collaborator Zunli Lu at Syracuse University have revealed that iodine has a strong correlation with oxygen concentrations. Their focus has been to relate the iodine content of rocks to past oceanic oxygen concentrations. Crucially, this requires an understanding of how iodine becomes preserved in rocks in the first place.
A new approach
Life is understood to have begun in Precambrian oceans perhaps as long as four billion years ago. As species evolved from simple bacteria and archaea to more complex algae and other eukaryotes, including plants and animals, the transition from ocean to land was eventually made. Oxygen is essential to this complex life, although many questions remain about that relationship – including how much is required. Being able to map oxygen concentrations in the surface ocean throughout time is essential to our understanding of patterns and drivers of evolution during this transition.
Following the award of a National Science Foundation Grant in 2014, Professor Lyons and his team, including Dalton Hardisty and Professor Lu, took a new approach to investigating the relationship between iodine and oxygen concentrations. This included studying present day environments where sediments that ultimately form these rocks accumulate – a strand of research not previously attempted for iodine. The overall aim was to calibrate present day findings with ancient iodine and thus inferred oxygen concentrations. This could then provide a powerful tool for understanding the development of life on Earth.
Professor Lyons and his team sampled iodine concentrations in modern settings in the Bahamas. These results were then linked with previous and ongoing studies that measured iodine concentrations in ancient rocks collected around the world, including Glacier National Park (previous spread). In addition to carbonate rocks, more organic-rich sediments and the equivalent rocks they would eventually form, such as shales, are also studied. In fact, the modern ocean provides the framework required to understand the uptake into sediments and the potential for preserving those records for millions and even billions of years.
The group has focused on a time period roughly 1.8 to 0.8 billion years ago known for harbouring the earliest known records of eukaryotic life and culminating with the first traces of animals. Related research using a different chemical fingerprint suggests oxygen levels in the atmosphere were quite low during this middle chapter of Earth’s history, and the latest iodine results confirm that oxygen availability in the surface ocean was likewise low and variable – perhaps creating a challenging environment for the earliest complex life.
Through their research, Professor Lyons, Dalton Hardisty, and their colleagues have found that ancient Earth’s evolution did not follow a steady path. Instead, Earth’s species may have struggled to evolve due to restrictions from the fluctuating oxygen concentrations within ancient oceans. They also note spatial variability in the evolution of life, suggesting that oceans contained ecological hotspots where conditions were more suitable for the emergence and evolution of life.
Most importantly, a comprehensive picture has emerged for Earth’s essential middle chapter that bridges evolving life in the oceans with the composition of the atmosphere above. The models for that atmosphere offer the team, through their ties to NASA Astrobiology Institute, a special opportunity to explore how possible biosignatures, such as oxygen, may one day soon identify life on an exoplanet light years away.
Q&A with Professor Timothy Lyons
What drives you to research the origins of life on Earth?
I have long been motivated by the challenges and opportunities that lie with understanding the early evolution of life on Earth and its cause-and-effect relationships with the co-evolving environment. Now, thanks to support from NASA and NSF, we are able to explore almost four billion years of persistent habitability on a dynamic early Earth with the aim of guiding the search for life on distant worlds.
How do you think life managed to keep evolving through periods of global environmental change?
Life is resilient, and change, while creating challenges, also creates opportunity. The great diversity of life we have on our planet today, from humans to microbes, reflects the dynamic conditions on and within our planet and the diverse niches and ecological possibilities that have resulted. Life, viewed broadly, has a great capacity to adapt by finding opportunity in the face of obstacles.
How do the present-day analogues you have studied help you to understand ancient settings?
Rocks provide the geochemical records of past conditions and processes and their relationships to life. Often those details for the past, such as levels of dissolved oxygen in the oceans, are transient features. We are left searching for creative solutions to reconstructing past oceanic and atmospheric chemistry, such as elemental, molecular, or isotopic chemical fossils that are controlled by and can capture ephemeral properties and preserve them in rocks for millions and billions of years.
Do you believe that life exists elsewhere in the universe?
First, such searches are never a question of ‘belief’. However, given what we know about the diversity of life on our planet over its long history and its ability to adapt and even thrive under change, I am optimistic. Research like ours is providing a catalogue of life signatures that could be detected in the atmospheres or on the surfaces of distant exoplanets. These advances, in combination with the nearly limitless number of planets that are and will be discovered in habitable zones around other stars, and the truly remarkable technological advances, mean that the odds are very good. I sincerely hope our research will contribute to what I am sure will be a success story in the decades to come.
How do you plan on building upon this research in future?
We will continue to refine our understanding of the diverse chapters of Earth history at even higher sensitivity and temporal resolution with a very specific focus on the compositions of the related atmospheres and oceans during these various ‘Alternative Earths’. The grand challenge is helping to facilitate the remote detectability and identification of these biosignatures using an ever-improving and still emerging suite of instruments and telescopes. Our specialty and continuing motivation is a holistic view of wide-ranging planetary systems and states. From that platform, our goal is to recognise the full suite of factors necessary to generate and sustain life and its products on planets like and unlike Earth – and to uniquely identify the signatures of a habitable world and the life on and below its surface. Such identifications demand that we can distinguish the products of abiotic processes from those that require life, which is no small task. Even oxygen has biological and abiological sources that can be confused from a distance.
- Research Objectives
Professor Timothy Lyons’ research focuses on exploring the evolving ocean and atmosphere and their cause-and-effect relationships with the origin and evolution of life.
National Science Foundation (NSF) and the NASA Astrobiology Institute (NAI)
Zunli Lu, Syracuse University
Peter Swart, University of Miami
Professor Timothy Lyons received a BS from Colorado School of Mines followed by an MS in Geology from University of Arizona. He later earned a PhD in Geology and Geochemistry from Yale University. He is currently a Distinguished Professor of Biogeochemistry at the University of California, Riverside.
Dalton Hardisty received a BS and MS from Indiana University in Environmental Science and Geology. He completed a PhD in Geology from UC Riverside studying under Professor Lyons. Hardisty is a postdoctoral fellow at Woods Hole Oceanographic Institution and will be starting as an Assistant Professor of Global Change Processes at Michigan State University in 2018.
Professor Timothy Lyons
Distinguished Professor of Biogeochemistry
Department of Earth Sciences
University of California, Riverside
900 University Ave.
Riverside, CA 92521
- Uncovering secrets of life on ancient Earth