Life on Earth Came Earlier, Crystal Evidence Suggests

By SPACE.com Staff

10 January 2001

Earth may have been cool enough to retain oceans and develop continents 4.4 billion years ago, according to a new study, possibly pushing back by several million years the time when our planet could have supported life.

The finding could also challenge the common view of when and how our Moon formed.

An international group of scientists probed a single tiny grain of zircon, a mineral commonly used to determine the age of rocks. The chemistry of the mineral and the rock in which it developed could only have formed in a low-temperature environment on Earth's surface, the researchers say.

They found evidence that 4.4 billion years ago, about 100 million years after Earth gathered itself together from the leftovers of the Sun's formation, temperatures had cooled to near or below the boiling point of water -- 212 degrees Fahrenheit (100 degrees Celsius). Previously, the oldest evidence for liquid water on Earth, a precondition for life, was from a rock estimated to be 3.8 billion years old.

"Conventional wisdom would not have predicted a low-temperature environment," said John Valley, a geology and geophysics professor from the University of Wisconsin. "These results may indicate that the Earth cooled faster than anyone thought."

The findings are reported in the Jan. 11 issue of the journal Nature.

Violent early days

The accepted view on an infant Earth is that shortly after it first formed 4.5 to 4.6 billion years ago, the planet became little more than a swirling ball of molten metal and rock. Scientists believed it took a long time, perhaps 700 million years, for Earth to cool to the point that oceans could condense from a thick, Venus-like atmosphere.

Complicating the picture is that for 500 million to 600 million years after Earth was formed, the young planet was pummeled by intense meteorite bombardment. About 4.45 billion years ago, a Mars-size object is believed to have slammed into Earth, creating the Moon by blasting pieces of Earth into space.

"This early age restricts theories for the formation of the Moon," Valley said. "Perhaps the Moon formed earlier than we thought, or by a different process."

Another intriguing question is whether or not life may have arisen at that early time. The earliest known evidence for life and for a hydrosphere is estimated at 3.85 billion years ago, and the oldest microfossils are 3.5 billion years old.

"This appears to be evidence of the earliest existence of liquid water on our planet," said Margaret Leinen, assistant director of geosciences at the National Science Foundation, which helped fund the study. "If water occurred this early in the evolution of earth, it is possible that primitive life, too, occurred at this time."

Life may have evolved and been extinguished several times in catastrophic, meteorite-triggered extinction events in those early years, Valley said.