Geology professor John Valley used a customized instrument known as a secondary ion mass spectrometer to detect the chemical signatures of life that lived 3.5 billion years ago.
Biological life appeared shortly after the Earth formed 4.5 billion years ago, according to research by UW-Madison geologist John Valley.
Although evidence has suggested for some time that life emerged earlier than had been believed, Valley’s decade-long study of 3.5 billion-year-old microfossils is the most compelling to date.
Valley discovered biochemical remnants that were from advanced species’ of microbes, meaning single-celled life must have appeared within Earth’s first 1 billion years.
“It’s completely possible life has existed the entire time and we just don’t have the evidence to prove it,” he says.
Valley says there has been “a long and focused search for evidence of early life,” beginning in 1953, when UW-Madison geologist Stanley Tyler found the first microfossils in a 1.8-billion-year-old gunflint chert on the shore of Lake Superior.
“Prior to that,” Valley says, “the oldest fossils were believed to be 550 million years old.”
The latest discovery was a joint effort by Valley and J. William Schopf, the director of UCLA’s Center for the Study of Evolution and the Origin of Life.
Schopf found the microfossils 25 years ago in western Australia’s Pilbara Craton, one of just two areas of undisturbed crust dating back to the eon in which the Earth had cooled down enough to allow continents to form. The fossils were found in Quartz-rich sedimentary rocks that have never been recrystallized, so the sediment layers are pristine.
The research, funded by NASA’s Astrobiology Institute, relied on a $3 million instrument known as a secondary ion mass spectrometer (SIMS). The SIMS — manufactured by Paris-based CAMECA Instruments, which has a factory in Madison — was customized to allow the team to count carbon isotopes.
By concentrating an ion beam on targets within the fossils, the team detected the chemical signatures of life that lived 3.5 billion years ago.
Initially, Schopf believed the microfossils represented 11 microbial species, but the new analysis has brought that number down to three. By counting the ratios of specific carbon isotopes — the basis of all known life — researchers found the evidence of life.
“We could see very delicate chemical textures,” Valley says. “If you laid out the fossils in a row, eight of them would fit across the diameter of a human hair, so they’re very small.”
Although microscopic, they were not simple. “Not only were these microbes living, but it was a complex community of microbes, there was an ecosystem,” says Valley, adding that that complexity took time to develop. “So this isn’t the first life; the first life had to have been much earlier.”
Perhaps as early as 200 million years after the Earth formed, meaning that microbes survived the Late Heavy Bombardment period, when leftovers from the birth of solar system pelted the planet and remelted most of its original crust.
For 3.5 billion years, life was single-celled. Not until the atmosphere was sufficiently oxygenated did animals emerge, around 500 million years ago.
Over the last decade, Valley’s team developed unique protocols for UW-Madison’s SIMS instruments. It’s being put to good use already, Valley says. Several paleo-research projects are underway. Among them is a layer-by-layer analysis of stalagmites in Cave of the Mounds.
As far as life goes, the question now is to pin down the conditions that made the first life possible. With the eight necessary elements present, and an energy source available, Valley’s money is on the appearance of liquid water.
“Water is the universal solvent for life,” he says. “For carbon-based life you need water as a catalyst to live in. If you don’t have water, none of the lifeforms we know of exist, and if it’s too hot, it destroys DNA.”
Valley is confident the study will withstand the scrutiny to come.
“We’ve made our case,” says Valley, whose lab is home to a 4.4 billion-year-old grain of zircon — the oldest rock ever found. “We’ll be publishing others, and people will poke at it. Anything that’s important and interesting gets criticized; that’s how science should work.”
