The Hadean eon is an era akin to Hades — a time of fiery volcanoes, crashing meteors and a hot atmosphere thick with carbon dioxide. It began 4.6 billion years ago with the big bang and lasted for around 600 million years. Geologists previously did not think that the Hadean era could have hosted the incipience of life — until last month.
An article published in September presented data from Jack Hills, Australia, that shifts the timing of life’s inception back into the Hadean era, 4.1 billion years ago — 300 million years earlier than previously thought. The authors’ evidence: an ancient grain of graphite.
The earth is currently thought to be 4.5 billion years old. The earliest undisputed fossils preserved in rock are about 3.2 billion years old — any fossils older than that were likely destroyed over the ensuing millennia. Luckily, geologists do not need fossils to infer the existence of life on earth — carbon isotope ratios also stand as evidence of biogenic activity. A month ago, the earliest materials inferred to be biogenic by their carbon isotope ratios, thus suggesting life, were around 3.8 billion years old.
Jack Hills, Australia, is home to 4.1-billion-year-old rocks, dated using zircon crystals. A geologist’s best friend, zircons are minerals that crystallize as igneous rocks cool. When they first crystallize, zircons contain uranium. Over time, the uranium decays into lead, so any zircon crystal found in a rock can give the rock’s age by the ratio of uranium to lead. Bell and colleagues collected 10,000 ancient zircon crystals in Jack Hills and sifted through them to find a single 4.1-billion-year-old crystal which would hold groundbreaking evidence of early life on earth: a graphite inclusion.
The 4.1-billion-year-old graphite is a form of carbon with an isotopic signature rich in carbon 12 (C-12), which suggests that the carbon is biogenic — most likely cycled through plants via photosynthesis. In photosynthesis, plants take in carbon dioxide and convert it into carbohydrates to store as energy. Carbon exists in nature in several forms with different atomic weights; C-12 is lighter and rarer, while C-13 is heavier and more common. Plants take in C-12 preferentially because it travels into their stomata more quickly. As a result, organic materials and biogenic sedimentary rocks have a higher amount of C-12.
The researchers have only found one zircon, and thousands are necessary for robust results. They are currently requesting $15 million from the U.S. government for such a hunt.
