TO HELL AND BACK
Was life on early Earth as bad as all that? And what does
that mean for life on other planets? Robert S. Boyd reports
A scientific quest called “Mission to Really Early Earth” has unearthed evidence that our planet had an ocean, a continent and an atmosphere suitable for life half a billion years earlier than previously thought.
Since the requirements for life – land, water and air – were established so soon on Earth, some scientists say the finding makes it more likely that living creatures could also have arisen on other worlds.
“If it happened so early on Earth, why couldn’t it happen elsewhere in the universe as well?” said Stephen Mojzsis, a geoscientist at the University of Colorado in Boulder.
According to the traditional view of its infancy, Earth formed between 4.5 and 4.6 billion years ago from a disk of dust, rocks and gas circling the sun.
It then took 700 million years for the young planet to settle down and cool off enough for the first microscopic organisms to appear around 3.8 billion years ago, paleontologists believed.
This early period was named the Hadean (“hellish”) Eon, because it was presumed to be totally hostile to life. During much of that time, the planet was bombarded by giant meteorites like those that blasted the craters on the moon. Any early life would have been wiped out.
Now, however, researchers report evidence that conditions were much more benign when the Earth was only 150 million to 200 million years old – three to four per cent of its present age.
“The stage was set 4.3 billion years ago for life to emerge on Earth”, Mojzsis told a conference on astrobiology – the study of life on other worlds – here last month.
“There was probably already in place an atmosphere, an ocean and a stable crust within about 200 million years of the Earth’s formation”, said Mojzsis, chairman of the conference. “Water was gushing out of the Earth.”
This picture of a comfortably warm, wet young world “contrasts with the hot, violent environment envisioned for our young planet by most researchers”, Bruce Watson, a geochemist at Rensselaer Polytechnic Institute in Troy, N.Y., declared in a recent online edition of the journal Science. “It opens up the possibility that life got a very early foothold.”
“If there was surface water, then life presumably could exist”, said Don Brownlee, an astronomer at the University of Washington in Seattle.
“We don’t know when life began on Earth,” cautioned Mark Harrison, an Australian geoscientist who was at the astrobiology conference. “But it could have emerged as early as 4.3 billion years ago. Within 200 million years of the Earth’s formation, all of the conditions for life on Earth appear to have been met.”
Two hundred million years sounds like an awfully long time, but it’s relatively brief on the geologic scale.
For comparison, suppose Earth’s 4.5 billion-year-old lifespan ws shrunk to one year, with 1 January marking the beginning and 31 December representing today. By that yardstick, life could have begun on Earth as early as 12 January. Under the older, traditional view, it would have taken until 26 February to get started.
The evidence for a very young habitable Earth consists of a collection of tiny crystals called zircons dug up in the Jack Hills of Western Australia over the last 20 years. New technology pioneered by Mojzsis and John Valley, a geochemist at the University of Wisconsin-Madison, has made it possible to determine how and when they formed.
For example, zircons contain uranium, which decays at a known rate. The Jack Hills zircons also enclose bits of shale, a sedimentary rock that must have previously been created by erosion by liquid water. In addition, the zircons contain a rare type of “heavy” oxygen that forms only in the presence of water.
“These zircons tell us that they melted from an earlier rock that had been to the Earth’s surface and interacted with cold water”, Mojzsis said. “There is no other known way to account for that heavy oxygen.”
Sonia Esperanca, an earth scientist at the National Science Foundation in Washington, D.C., called the Jack Hills zircons “time capsules of processes happening in the earliest times in Earth’s history.”
“The estimated ages for the oldest evidence of an early crust have been getting progressively older as geologists seek out and analyze new samples”, said Douglas Erwin, a paleontologist at the National Museum of Natural History in Washington, D.C., who isn’t involved in the Mission to Really Early Earth. Erwin agreed that primitive microorganisms could have existed that long ago. “But I expect it will be very difficult to get any real evidence on the matter”, he said in an e-mail message.
“It’s certainly possible that life arose before the great bombardment, then was extinguished and arose again afterward, but we have no evidence either way”, said University of Washington geochemist Roger Buick in an e-mail message.
Another note of skepticism comes from Samuel Bowring, a planetary scientist at the Massachusetts Institute of Technology in Cambridge. “It’s a bit of a leap from a few grains of zircon to continents and oceans,” Bowring said, but he acknowledged that “it is consistent with most people’s view of early planetary evolution.”
The Mission to Really Early Earth is supported by the National Science Foundation and NASA’s Astrobiology Institute, which studies the origin of life on our planet and its possible existence on other heavenly bodies.
“We’re beginning to get the tools to test the Hadean world”, said Mojzsis. “Hell wasn’t as bad as we thought.”