The Sciences

Why We Suspect Earth's Journey Around the Sun Had a Say in How Life Evolved

It all began with a big debate: were the great extinctions and population explosions on Earth history driven by innate biological activities or were they prompted by environmental factors on the outside?

At various points in Earth’s history, life has been wiped out in cataclysmic extinction events and, at other times, life has evolved at a blistering pace thanks to a surplus of certain resources. According to a new study, one force that could have affected the coming and going of these biological flashpoints is – wait for it…

Changes in the way Earth moved around the Sun.

You read that right.

Scientists are still debating whether the great extinctions and (population) explosions were driven by innate biological activities or were prompted by environmental factors on the outside. One popular theory tries to broker a ceasefire: that while predators and competition affected species locally over smaller time scales, climate and tectonic events shaped life regionally over larger periods.

“We are interested in the general problem of what controls biodiversity,” James Crampton, lead author of the new study, told The Wire. “Is there a fixed limit on the number of species living at any time, or could this number just keep on increasing?” He’s a professor of palaeontology at the Victoria University of Wellington, New Zealand.

To answer these questions, we need to find out which force has the bigger effect: the internal biologicals or the external environmentals.

Now, apart from natural and anthropogenic influences, cosmic changes also affect Earth’s climate over several hundred thousand years. The planet’s path in its journey around the star isn’t fixed; it changes once over the course of a few to many lakh years. These changes are collectively called Milankovitch cycles, named for the Serbian geophysicist Milutin Milanković.

Each cycle accounts for three changes in Earth’s orbit: eccentricity, obliquity and precession. Eccentricity is how much the elliptical path around the Sun deviates from a perfect circle. Its value changes between every 100,000 and 413,000 years. Obliquity is how much the Earth’s rotation axis is tilted from the normal, which ranges from 22.1° to 24.5°, and cycles about every 41,000 years. Precession is the wobble of Earth’s axis; this movement has a periodicity of 19,000 to 23,000 years.

Apart from these three cycles, there have been ‘grand’ cycles every 2.4 million years for eccentricity and every 1.2 million year for the obliquity. These grand cycles have been already correlated with glacial cycles and sea-level changes on Earth.

And all together, they affect how much sunlight reaches Earth’s surface, thus altering the climate.

Crampton and his colleagues think the influence might extend further to terrestrial life as well. “However, this has never been demonstrated except in geologically recent mammal fossils in the last few million years,” he said. “So we set out to test this in our data.”

They looked at species diversity and extinction patterns in a group of now-extinct marine organisms called graptoloids. Hundreds of millions of years ago, these plankton-like creatures floated around in colonies, eating bacteria and single-celled algae. Although they have been extinct for a long time, there are very good fossil records of thousands of graptoloid species preserved around the world.

The researchers had previously recorded information about when each species of graptoloid evolved and when it went extinct, put together over a decade by two of the authors, Roger Cooper and Peter Sadler. Using this information, the new study examined about 1,800 species over 60 million years, between 480 and 420 million years ago, when the species diversity was highest. This span also includes the Great Ordovician Biodiversification Event, one of the most significant increases in the number of species Earth has ever seen.

From the data, they extracted rhythms in the rates of evolution and extinction and compared them to pulses in the Milankovitch cycles.

Both speciation and extinction – together called species turnover – displayed a 2.6-million-year pulse, close to the 2.4-million-year grand cycle for eccentricity. There was also a weak pulse every 1.3 million years, but which was stronger in the early part of period the researchers analysed. This correlated with the 1.2-million-year grand cycle for obliquity.

Together, 9-16% of species turnover in the 60-million-year span could be explained by the two grand cycles, suggesting a significant influence of external factors on how life changed on Earth.

Richard Bambach, a palaeontologist at the Smithsonian National Museum of Natural History, Washington DC, who studies how species diversity changes over time, said the study was “very well put together” and its conclusions, “unusually strong”.

An older study had unearthed extinction cycles among some ancient organisms, albeit with much larger periods of about 27 million and 62 million years, possibly caused by asteroids and other cosmic agents.

“Sedimentologists have suggested that several patterns in the formation of some types of sedimentary rocks may also reflect ancient Milankovitch cyclicity, but then haven’t had the precise dating system graptolites provide to establish such conclusions so certainly,” Bambach said.

The processed species turnover data also has a bonus for astronomers. Today, stargazers can study variations in Earth’s orbit back to about 50 million years. But the fossil data allows them to infer details of Milankovitch grand cycles up to about 480 million years ago.

However, this assumes that the Milankovitch cycles had the same modern timing about 450 million years ago, which may or may not be true, Bambach said.

For David Bapst, an analytical palaeobiologist at the Texas A&M University, College Station, the study raises more questions about which forces are important in driving species diversity. He pointed to another study published earlier this year using the graptoloid dataset, which suggested that about 12% of the change in species diversity arose from how many graptoloid species were present at the time. In other words, diversity increased slowly if variation in global species was higher.

“Given that they find in this new paper that the cyclicity is only responsible for 9-16% of the variation they see in the graptoloid diversification, I’m just left wondering which component is most important, and how accounting for the effect of one impacts the other,” Bapst said.

According to Crampton, this is only one more piece in the puzzle of life on Earth. “But there are still so many things about the richness of life that we don’t understand.”

One question his team is trying to answer is how fast ecosystems recover after major extinction events. The answer is  obviously important for today: some studies say we’re already in the middle of a sixth mass extinction. “Our data can help with this question and this is something that we will be working on,” Crampton said.

Lakshmi Supriya is a freelance science writer based in Bengaluru.