How a Single Act of Evolution Nearly Wiped Out All Life on Earth
A single gene transfer event may have caused the Great Dying
Evolution giveth, and, 252 million years ago, evolution nearly tooketh away.
The power of natural selection and random mutations have, over time, created the amazing diversity of life on Earth, from the little lice that live on your lashes to the mighty blue whale. But, once, a single act of evolution—the transfer of two genes from one type of bacteria to one type of archaea—nearly wiped out all life on this planet, suggests a team of researchers in a new study.
Roughly 252 million years ago, the Permian-Triassic extinction, known as the Great Dying, saw 90 percent of marine life and 70 percent of terrestrial life snuffed out in a relative blink of an eye. The functional cause was a disruption of the planet's carbon cycle, which transfers carbon between air, sea and land and keeps a certain portion in long-term storage. Something—scientists don't know for sure—caused a burst of carbon to come out of storage. When it did, the temperature soared, the ocean acidified and life on Earth nearly collapsed.
Previously, scientists have tried to pin the shift in the carbon cycle and the ensuing extinction on everything from meteorites to volcanoes. Some scientists say the Great Dying happened all at once, while others suggest it happened in waves.
In the new study, led by geophysicist Daniel Rothman, the researchers noticed something important about the rate of the disruption. If the extinction had been caused by a meteorite or volcano, the changes likely would have come as a burst before slowly tapering off. But that's not what they saw. Instead, the disruption of the carbon cycle appeared to be exponential—growing faster and faster with time. To them this suggests one thing: rampant microbial growth.
Though we tend to think of evolution as a particular individual organism having a genetic mutation that works out, in microbes, evolution can also happen when microbes of different types trade genes.
The scientists posit that, around the time of the extinction, a type of archaea known as Methanosarcina gained two genes from a bacteria. These genes gave them the ability to eat the organic wastes that litter the sea floor. As they ate, the archaea would have pumped out methane gas—rushing carbon that had long been stored in the organic materials back into the water. Through a genetic analysis, the scientists calculated that Methanosarcina gained this ability some time from 200 to 280 million years ago.
Whether Rothman and colleagues' speculations pan out will be seen with time, but that this scenario is even plausible is a testament to the power of microbial evolution. From the onset of photosynthesis to outbreaks of disease and who knows what's next, it's a reminder that Earth is the microbes' world. We just live in it.