Life

Did Supernovae Help Push Life to Become More Diverse?

Life on Earth has been around for a long time—at least 3.8 billion years. During that time, it evolved significantly. Why has biodiversity here changed so much? A new study proposes a startling idea. Some major diversity changes are linked to supernovae—the explosions of massive stars. If true, it shows that cosmic processes and astrophysical events can influence the evolution of life on our planet.

The idea of cosmic catastrophes having an effect on life is not new. Usually, people think about such events happening to us in modern times. But, there’s a long history of Earth being affected by past cosmic events. It’s likely, for example, that shock waves from supernova explosions set the birth process of our Sun in motion. We experience solar flares and outbursts and how they interfere with our technology. We also know that impacts have shaped the planet throughout its history, as well. So, why couldn’t supernovae also play a role in the evolution of life? There are a lot of ideas about that, involving both astronomical and biological research.

Linking Supernovae to Life Changes

A team of scientists at DTU Space (Denmark’s largest space research institute) think there’s a strong correlation between changes in the diversity of marine life in the past half a billion years and the occurrence of nearby supernova explosions. According to Henrik Svensmark, author of a paper describing the team’s research, it’s possible that one effect of a supernova is a change in Earth’s climate. “A high number of supernovae leads to a cold climate with a large temperature difference between the equator and polar regions,” he said. “This results in stronger winds, ocean mixing, and transportation of life-essential nutrients to the surface waters along the continental shelves.”

The team’s paper points out some interesting specifics. It states, “In accordance with the cosmic ray theory, Earth experienced cold glacial periods when the local supernova frequency was high, i.e., high cosmic rays and warm climates when the flux was low. These results suggest that changes in supernovae frequency and, thereby, changes in cosmic rays have significantly influenced the Phanerozoic climate.”

This proposed influence of supernova explosions extends to the conditions for life. For example, the paper suggests a correlation between past supernova rates and the burial of organic matter in ocean sediments during the last 500 million years. The sequence goes like this: supernovae rates influence climate. Climate influences atmosphere–ocean circulation. That circulation brings nutrients to marine organisms. Nutrient concentrations control bioproductivity (how organisms thrive). Then, as they die, their remains settle into sea sediments, which fossilize and preserve the record of past biological activity.

All of this appears to correlate with changes in supernova rates. If this link turns out to be solid, then supernovae may well influence climate and the energy available to biological systems. And all that has an influence on marine life.

Searching the Fossil Record for Supernova Evidence

Variations in relative supernova history (black curve) compared with genera-level diversity curves normalized with the area of shallow marine margins (shallow areas along the coasts). The brown and light green curves are major marine animals’ genera-level diversity. The orange is marine invertebrate genera-level diversity. Finally, the dark green curve is all marine animals’ genera-level diversity. Abbreviations for geological periods are Cm Cambrian, O Ordovician, S Silurian, D Devonian, C Carboniferous, P Permian, Tr Triassic, J Jurassic, K Cretaceous, Pg Palaeogene, Ng Neogene. (Illustration: Henrik Svensmark, DTU Space).

So, what evidence is Svensmark’s team offering? They studied the fossil record of ancient shallow marine areas. These were along the edges of oceans and other bodies of water in the Phanerozoic period of Earth’s geologic history. That’s the period of time we’re in now. It began some 542 million years ago. These shallow marine shelves are relevant since most marine life thrives in these areas. By studying the rates of change in species of life they found clear evidence of explosions in biodiversity.

The team then looked at the astrophysical fossil record of supernovae. They studied supernova frequencies recorded in three data sets of open stellar clusters in the solar neighborhood. Those catalogs contain data about clusters within 850 parsecs of the Sun, with ages 520 million years and younger. The team then correlated the data from the two sets with each other to link higher-than-normal rates of past supernova explosions with climate-influenced changes in biodiversity in shallow marine environments.

How Can Supernovae Do This?

How does this proposed link between climate change and supernovae work? Let’s look at the chain of events that leads from star death to biodiversity changes on Earth. You start with a star at least 8 times the mass of the Sun. When this massive progenitor star reaches the end of its life, it collapses in on itself. The infalling material rebounds off the stellar core and rushes out to space. That cloud of debris scatters all the elements made by the star both before and during the supernova explosion. The event also emits huge amounts of cosmic rays. Those energetic particles eventually arrive in our Solar System. Some smash into Earth’s atmosphere and send showers of ions crashing through the atmosphere. There, they help create the aerosols that form clouds.

Clouds help regulate solar energy by controlling how much sunlight reaches Earth’s surface. The warmth of the sunlight is one part of the water-warmth-nutrient triad that enables life to form and thrive on the planet. So, in a very real sense, the influence of supernovae is part of the cycle of substantial climate shifts, thanks to the intensity of cosmic rays. According to Svensmark, those changes can be as much as several hundred percent over millions of years. “The new evidence points to a connection between life on Earth and supernovae, mediated by the effect of cosmic rays on clouds and climate”, he said.

If this idea Svensmark’s team is proposing stands, then it’s yet another link between distant astrophysical activities and the evolution of life on our planet.

For More Information

Supernovae and life on Earth appears to be closely connected
A Persistent Influence of Supernovae on Biodiversity Over the Phanerozoic

Carolyn Collins Petersen

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