Life on Earth may have sparked into existence as early as 4.4 billion years ago, hundreds of millions of years sooner than previously thought possible, according to a study.
Until now scientists assumed that no life forms could have survived the so-called Late Heavy Bombardment, a 100-million-year fusillade during which our young planet was pummeled by meteorites that blasted craters the size of Thailand and France.
All told, billions of tonnes of space rock rained down on the planet, according to the new study.
Heat generated by this relentless pounding was intense enough to melt much of Earth's surface, rendering it uninhabitable even for primitive life forms that thrived at high temperatures.
That, at least, has been the conventional wisdom, bolstered by the fact that the earliest traces of life discovered so far appeared shortly after the extraterrestrial onslaught tapered off 3.9 billion years ago.
Fossils reveal microscopic life forms 3.5 billion years old, and geochemical clues point to more primitive organisms -- thought by some to be the common ancestor to all things living -- 300,000 million years before that.
But Stephen Mojzsis and Oleg Abramov of the University of Colorado argue that early Earth wasn't so hellish after all.
Many life forms that might have arisen earlier could well have survived the bombardment, according to their study, published in Nature.
Using numerical models of impact-generated heat in Earth's crust, they show that no more than 37 percent of the planet's surface was sterilised at any given time, and that only 10 percent reached temperatures above 500 degrees Celsius (930 degrees Fahrenheit).
That is very hot, but much of Earth was cool enough to accommodate different families of microbes able to take the heat, 20C to 50C (70F to 120F) for some, and beyond the boiling point -- up to 110C (230F) -- for others.
Some microbes would also have been able to live several kilometres below Earth's surface, much as some simple life forms do today, the study points out.
"Our analysis shows that there is no plausible situation in which the habitable zone was fully sterilised on Earth," said Mojzsis.
"All the criteria necessary for life" -- liquid water, energy sources such as sunlight, and chemical building blocks from meteors or Earth itself -- "were present at least since 4.38 billion years ago," he told AFP by e-mail.
"The antiquity of life may not be very much less than the Earth itself," he said.
Research published last week shows how a series of chemical reactions on early Earth could have produced ribonucleic acid, or RNA, a single-stranded cousin of the DNA that is the blueprint for all life.
Pushing back the date at which self-replicating life forms first emerged from the primordial soup would also explain how the relatively complex organisms that fossils records show existed 3.5 to 3.8 billion years ago could have had time to evolve.
In a commentary, also published in Nature, Lynn Rothschild of the NASA Ames Research Center in California says the new study convincingly shows how life may have emerged sooner rather than later.
"Moreover, it opens the possibility that life arose on Earth only once, and that the planet has been continuously inhabited ever since," she said.
Mojzsis is less inclined to think that everything within the realm of biology can be traced back to a single so-called "last universal common ancestor," or LUCA.
"Instead, they might have been a community of co-evolving proto-organisms that crystallised into a population" of primitive cells, he said.
Answering once-and-for-all the question of whether life happened more than four billion years ago will be extremely difficult, scientists say.
So much of the evidence -- if there is any -- was destroyed during Earth's fiery baptism that finding intact traces may be futile. The earliest sedimentary rock ever found is "only" 3.83 billion years old.