Maybe the Hadean Eon Wasn't So Bad
 

The very earliest era of Earth's history is called the Hadean Eon, a forbidding name that reflects the long-standing view that back then, the Earth was not a very welcoming place for life. The accepted view has been that for the first 800 million years or so of Earth's 4.5 billion year history, there was very little solid crust. Early Earth was seen as either a water world, or perhaps an inferno of volcanic activity run amok. (Land, and particularly the shallow waters surrounding land, are vital environments for the origin and evolution of life.) However, an online article in the journal Science paints a different picture. (There's a summary in this this press release from the University of Colorado.) The key conclusion is that the earth had a significant amount of solid crust as early as 100 million years after it formed. Although the surface of the Earth is constantly undergoing erosion and tectonic changes due to earthquakes, volcanoes, and mountain-building, some of these very earliest rocks still exist in their original form, in western Australia, and researchers have been able to use the element hafnium in these rocks to date them back to 4.4 billion years ago.

The chemical composition of the oceans and the atmosphere was much different then, and the solid land was likely scattered around in large islands rather than being the continents we have today, but still, the early Earth might have had a much more hospitable environment for life much earlier in its history than anyone had previously thought. This jibes with the results of work published in May (described in this article from Live Science), which examined zircon crystals and concluded that the early environment on Earth likely included solid land as far back as 200 million years after Earth formed. The zircon crystals are very impervious to physical or chemical damage, so some are preserved from very early in Earth's history.

This research has implications for the origins of life on Earth, and elsewhere in the universe as well. Perhaps life arose much earlier than previously thought. Even if the early Earth wasn't exactly hellish, it still wasn't a place where you and I would have been able to survive. It might, however, have been able to host something like the extremophile bacteria that live today in the acidic and hot conditions of Yellowstone. So if life can begin very early in a planet's history, this affects estimates of how likely the universe is to harbor at least simple life forms elsewhere.

The road to complex life forms on Earth, and even the continuance of existing forms, has never been a sure thing, though. During the Hadean period, there was still some bombardment from bits and pieces left over from the formation of the solar system, and the Earth is still vulnerable to impact, although to a much lesser degree. Also, the Earth has possibly gone through some climate extremes like the "snowball Earth" scenario that some believe happened 2.3 billion years ago and perhaps at other times as well. So it's hard to draw any conclusions about the overall likelihood of life forms, simple or complex, in the universe as a whole. For the moment, we've got only a single sample of a life-bearing planet, and it's hard to extrapolate from that. All we can say for sure is that we're here now, and we are getting some fantastic looks back into deep time and the earliest history of our planet.