Big Time Discovery
Dramatically pushing back the emergence of life on Earth by nearly 400 million years, scientists Wednesday said they have found chemical traces in the world’s oldest known sediments that suggest that simple life forms thrived 3.85 billion years ago.
The discovery could be the first evidence of the primordial microbes that arose when inanimate matter organized itself into the earliest living molecules.
The finding, published today in the journal Nature, raises a host of provocative possibilities about life at the dawn of time on Earth--a period when many experts believe the primitive planet was still hot from the oven of creation. One scientist suggested that it may be the trace of primitive organisms that had a radically different biochemistry from that which drives all modern living things, while others said it bolsters the idea that life did not originate on Earth at all.
In an unusual rock formation on Akilia Island in west Greenland, researchers from the Scripps Institution of Oceanography in La Jolla and UCLA found what they believe is the distinctive signature of Earth’s earliest life: traces of carbon isotopes and other organic evidence of primitive metabolisms, locked inside microscopic grains of a mineral called apatite.
The carbon isotopes were discovered in Greenland rocks that may have formed the floor of Earth’s first ocean between 3.8 billion and 3.87 billion years ago. Earth itself is about 4.5 billion years old, and the oldest known physical fossils, which resemble modern bacteria, are about 3.46 billion years old.
Almost nothing is known for certain about Earth’s earliest history or the conditions under which life arose. In the time since the Greenland rocks formed, their sediments have been so churned, broiled and squeezed in the mold of time--at pressures up to 5,000 times that of Earth’s current atmosphere and temperatures above 900 degrees Fahrenheit--that no direct physical fossil evidence of the early life forms survives. But if the scientists are correct, the traces left by their body chemistry have been preserved.
“What we are seeing can be thought of as chemical fossils,” said Stephen J. Mojzsis, a Scripps research fellow who led the NASA-funded team. “The organic matter trapped in the rocks preserves a biological signal. It seems that life gets started very early.”
William Schopf, director of the UCLA Center for the Study of Evolution and the Origin of Life, who discovered the earliest known physical fossils, called the new research “very significant.” Norman Pace, a microbial biologist at UC Berkeley, described the finding as “wonderful.”
John M. Hayes, a leading authority at the Woods Hole Oceanographic Institution on the chemistry of early Earth, termed the research “very notable.” But he said it “should also inspire caution.”
“It is evidence for an isotope effect, not evidence of life,” he said. “It is great if it is true, but we have to know a lot more before we can take this to the bank. There are questions.” And the largest of those questions, he said, may be the simplest: ‘If this is true, what does it mean?”
Gustaf Arrhenius, a senior Scripps scientist involved in the project, said the chemical fossils could be remnants of the living sludge whose biochemistry preceded modern DNA-based life forms.
If that is true, the organisms were spawned in the so-called “RNA world” in which the chemical recipe used by living things to reproduce, change and evolve was carried out by a single, primitive molecule, rather than today’s more sophisticated chemical systems.
In this theoretical world, the DNA system that today serves as the blueprint for all living things had yet to evolve. Instead, its molecular functions were handled entirely by a simpler RNA-based system. Today RNA serves only as a messenger in the more complex DNA system.
“It might be the first trace of the RNA world known to us,” Arrhenius said.
While some experts found that idea far-fetched, others like Hayes said the discovery bolsters the idea that life did not originate on Earth at all. The fossils may push the origin of life on Earth into a period when the planet is thought to have been under a lethal rain of meteors that could have obliterated any fledgling native life easily.
“If we believe that . . . the Earth was being clobbered up to 3.8 billion years ago, then this makes it all that much more likely that life was imported,” Hayes said.
“Anyone who says this evidence doesn’t turn their mind in this direction is probably not being candid. It is one of the lines of significance of this work,” he said.
The Greenland rocks were analyzed with a new high-resolution ion microprobe at UCLA that allows researchers to learn the composition of microscopic samples like the apatite grains with remarkable precision. The $3-million device shoots a beam of charged atoms at a sample that releases a “sputter” of ionized particles that can be analyzed in a mass spectrometer.
“There is nothing voodoo about the concept of this,” said Mark Harrison, director of UCLA’s W.M. Keck Foundation Center for Isotopic Geochemistry, who performed the microprobe analysis.
“I can understand the knee-jerk criticism,” Harrison said. “This is an isotopic fossil. It is unconventional to accept chemical and isotopic arguments rather than morphological evidence” of bones, shells or other more physical fossil remains.
However, he said, “We have consistent results. I think we are stuck with the simplest interpretation, which is this reflects the emergence of life by 3.85 billion years.”
