Tiny zircons suggest life on Earth started earlier than we thought, UCLA researchers say
Los Angeles may be a city obsessed with youth, but it is also home to the world’s largest collection of Hadean zircons — the oldest known material on Earth.
Hadean zircons are tiny — about the same width as a strand of hair — but they contain a huge amount of information about the earliest days of our planet.
The Hadean period began 4.5 billion years ago, when Earth was just 40 million or so years old. Scientists have long presumed it was a time of hell-like conditions, characterized by scorching temperatures and molten rock.
But the zircons reveal a different story. Chemical signatures embedded in the crystals suggest the planet had water, continents and even interconnected oceans.
This month, researchers studying these zircons made an even more unexpected discovery: the first evidence that life may have existed 4.1 billion years ago. That’s 300 million years earlier than was previously thought.
“Open any textbook and you see all these assumptions about the early Earth — it had no water, it had no continents,” said UCLA geochemist Mark Harrison, the guardian of the zircons. “Yet every bit of evidence suggests it was much more like today than anyone imagined.”
Harrison has been studying zircons for 20 years, and over that time he has amassed a collection of more than 180,000 of the tiny crystals. The oldest specimens in his archive — the Hadean zircons — have remained chemically unchanged for at least 4 billion years. Some of them formed just 100 million years after Earth was born.
Finding zircon crystals is a long and painstaking process. The most ancient samples come from an area in Western Australia known as Jack Hills, where they are embedded in sandstone deposits.
To extract the zircons from the sandstone, scientists use a hammer to break the rock into chunks about 5 inches across. Then they grind the chunks into a wet sand. The sand goes into a toxic solution that is so dense, only zircon and other very heavy materials sink to the bottom.
The researchers collect that heavy material, put it under a microscope and look for pinky-purplish grains with a jewel-like glow. These are the zircons.
After the crystals have been identified, the scientists use tweezers to line them up in neat grids of 400. Each zircon is dated by measuring how much of its uranium has decayed into lead.
In every 400 zircons, about eight to 12 are old enough to be considered Hadean.
The evidence of possible life from 4.1 billion years ago did not come from the zircons themselves, but from tiny slivers of carbon embedded in the crystals.
“The zircon grew in magma where other small crystals were floating around,” said Elizabeth Bell, a post-doctoral researcher in Harrison’s lab. “So we see inclusions of other minerals in most grains.”
The research team went hunting for carbonaceous inclusions because they wanted to see what carbon was like at the time when the zircons formed — in particular, whether its signature was formed by biological or nonbiological mechanisms.
All carbon atoms have six protons, but the number of neutrons can vary. Carbon-12, which has six neutrons, and carbon-13, which has seven neutrons, are both abundant on Earth and across our solar system, but their ratio can vary. Biological processes like photosynthesis create a higher percentage of carbon-12, or what scientists call “light carbon.”
After years of examining individual zircons under the microscope, the researchers found one with two carbon inclusions that dated to 4.1 billion years. Both inclusions were buried so deep inside the crystal that they must have been closed off from the world since the zircon formed.
The geochemists zapped both of the inclusions with a minuscule ion beam. A mass spectrometer determined that the 4.1-billion-year-old carbon had the same ratio of carbon-12 to carbon-13 that you would find in plants today.
The study’s results were published this month in the Proceedings of the National Academy of Sciences.
“It’s not a smoking gun for there being life at 4.1 billion years,” Bell said. “But if you saw that same isotopic signature on the Earth today, you would say, ‘That is from a biogenic source.’”
The earliest microfossils suggest that there was life on Earth 3.5 billion years ago. Prior to this study, the earliest evidence of life based on a carbon signature was from 3.8 billion years ago.
Experts say that although the work doesn’t prove there was life on early Earth, it does offer an intriguing new way to approach the question of when life on Earth first came to be.
“Harrison and his team have challenged us now to think deeply about just how ancient the biosphere could be and to find new ways to explore for a cryptic record of it,” said Steve Mojzsis, a geologist at the University of Colorado in Boulder who was not involved in the study. “If life is responsible for these signatures, it arrives fast and early.”
Harrison agrees with this summation.
“The more important thing about our study is it provides a way forward,” he said. “With enough work, we will be able to get a smoking-gun answer.”
That would mean getting carbon fingerprints from 1,000 inclusions, rather than just the two detailed in the study, he added.
Harrison’s group is continuing the tedious work of extracting more zircons, dating them and scanning them for ancient bits of carbon. The team also plans to search the zircons for information about whether Earth had a magnetic field in its earliest days — a question that has important implications for the rise of life and the early forces affecting our atmosphere.
The zircons may be tiny, but they have plenty more to teach us.
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