As our living ark swings around the Sun, its current loop is fairly circular. But Earth's orbit isn't as stable as you may think.
Every 405,000 years, our planet's orbit stretches out and becomes 5 percent elliptical, before returning to a more even path.
We've long understood this cycle, known as orbital eccentricity, drives changes in the global climate, but exactly how this impacts life on Earth was unknown.
Now, new evidence suggests that Earth's fluctuating orbit could actually impact biological evolution.
A team of scientists led by paleoceanographer Luc Beaufort, from the French National Centre for Scientific Research (CNRS) have found clues that orbital eccentricity is driving evolutionary bursts of new species, at least in plankton of the photosynthesizing variety (phytoplankton).
Coccolithophores are microscopic sunlight-eating algae that create plates of limestone around their soft, single-cellular bodies. These limestone shells, called coccoliths, are extremely prevalent in our fossil records – first appearing around 215 million years ago during the Upper Triassic.
Did you ever wonder how the White Cliffs of Dover were formed?
— The FOSSIL Project (@projectFOSSIL) December 13, 2018
They're made of #microfossils! Single-celled coccolithophores produce calcium carbonate #coccoliths that fossilize and form chalk!
Watch this clip of coccolith formation courtesy of Alison R. Taylor!#paleontology pic.twitter.com/5N95rQxB20
These oceanic drifters are so abundant they contribute massively to Earth's nutrient cycles, so forces that alter their presence can have a huge impact on our planet's systems.
Beaufort and colleagues measured a staggering 9 million coccoliths across 2.8 million years of evolution in the Indian and Pacific oceans, with the help of AI automated microscopy. Using well-dated ocean sedimentary samples they were able to obtain an incredibly detailed resolution of around 2,000 years.
The researchers were able to use size ranges of the coccoliths to estimate species numbers, as previous genetic studies have confirmed different species in the Noelaerhabdaceae family of coccolithophores can be told apart through their cell sizes.
They discovered the average length of a coccolith followed a regular cycle in line with the 405,000 year orbital eccentricity cycle. The largest average coccolith size appeared a slight time lag after the highest eccentricity. This was irrespective of if Earth was experiencing a glacial or interglacial state.
"In the modern ocean, the highest phytoplankton diversity is found in the tropical band, a pattern probably related to high temperatures and stable conditions, whereas seasonal species turnover is highest at mid-latitudes because of a strong seasonal temperature contrast," Beaufort and colleagues explained in their paper.
They found this same pattern was reflected across the large time scales they examined. As Earth's orbit becomes more elliptical the seasons around its equator become more pronounced. These more varied conditions spurred coccolithophores to diversify into more species.
"A greater diversity of ecological niches when seasonality is high leads to a larger number of species because Noelaerhabdaceae adaptation is characterized by the adjustment of coccolith size and degree of calcification to thrive in the new environments."
Size variation of coccoliths across different time periods: Miocene (left), Pleistocene (right). (Weimin Si)
The most recent evolutionary phase the team detected started around 550,000 years ago – a radiation event in which new Gephyrocapsa species emerged. Beaufort and colleagues confirmed this interpretation using genetic data on the species alive today.
By using data from both oceans they were also able to distinguish between local and global events.
What's more, by calculating mass accumulation rates in the sediment samples the researchers untangled the potential impact morphologically different species had on Earth's carbon cycle, which they can modulate through both photosynthesis and the production of their limestone (CaCO3) shells.
"Lighter species (for example, E. huxleyi and G. caribbeanica) contribute the most to coccolith carbonate export," the team wrote, explaining that when mid-size opportunistic species dominate there is less carbon being stored away through shells from the dead animals sinking into the depths.
In light of these findings and other supporting research, Beaufort and team suggest the lag seen between orbital eccentricity and changes in climate could hint that "coccolithophores may drive – rather than just respond to – carbon cycle changes."
In other words, these minuscule little organisms, along with other phytoplankton, may help change Earth's climate in response to these orbital events. But further work is required to confirm this.
This research was published in Nature.