There are good days and there are bad days, but by any measure the period around 3.9 billion years ago was the worst time to be living on Earth–or for that matter anywhere else in the solar system.
This is the period known as the Late Heavy Bombardment (LHB), when giant chunks of debris were let loose to batter the inner planets. No obvious traces of this terrifying episode remain on Earth, but we know it occurred thanks to the Moon. The circular features that give the man-in-the-moon its hollow-eyed expression are the marks of giant impacts that all date to around this time. Earth, with its larger surface area and stronger gravity, would have attracted even more big blows.
How bad was it? Estimates suggest about 50 objects at least 100 kilometers across pummeled the Earth during the LHB. (For comparison, it was a single 10 kilometer-diameter asteroid that is believed to have taken out the dinosaurs.) These are ocean-vaporizing events that would have liquefied large swaths of the planet’s surface.
Yet, despite all this, Oleg Abramov and Stephen Mojzsis say that life survived.
The two University of Colorado, Boulder researchers created a simulation of the Late Heavy Bombardment to study its effects on the temperature of the Earth’s crust down to a depth of four kilometers. (Today bacteria have been detected at comparable depths.) Not surprisingly they found that regions directly hit by an impact would have been sterilized of micro-organisms, the only possible life around at that time. But—surprisingly—a few thousand kilometers away, Earth’s crust would still be stable and capable of supporting ancient bacteria.
Some of those bacteria may even have thrived during the bad years. The giant impacts would have fractured Earth’s crust, creating passageways for hot magma and hydrothermal vents. This is the sort of environment that heat loving organisms known as thermophiles like best.
And it appears they would not have been alone. The study shows that even mesophiles, which require temperatures between 20° and 50° Celsius, could have found ample refuge to ride out the worst of the bombardment and survived to recolonize the planet.
There are at least two interesting implications to this. The first is that life on Earth may be older than we think. Because the Late Heavy Bombardment was so extreme, it’s been previously speculated that it wiped the planet clean of life. Now Abramov and Mojzsis say that’s not what happened.
The earliest (isotopic) evidence for life on Earth dates back to 3.83 billion years ago. That’s getting uncomfortably close to the Late Heavy Bombardment and it doesn’t give life much time to get started from scratch. But if life was already present well before the Late Heavy Bombardment, there’s less of a mystery about how it got going so fast.
The second implication is for life elsewhere in the solar system and beyond. If life on Earth could survive the Late Heavy Bombardment then presumably life on Mars could too. So could life on other planets in other solar systems where the same bombardment scenario may have occurred.
Life is funny that way. Getting it going must require just the right conditions and plenty of time—otherwise someone would have seen it spontaneously come to be in a lab somewhere. Yet, once life is established, it seems like you just can’t keep it down.
For my money, British rapper Michael Skinner (aka The Streets) says it best in his song “On the Edge of a Cliff”:
For billions of years since the outset of time
Every single one of your ancestors has survived?
Every single person on your mum and dad's side?
Successfully looked after and passed on to you life.
What are the chances of that, like?
It comes to me once in a while?
And everywhere I tell folk it gets the best smile.
The results of the new study were published last week in Nature. For more on this and other revelations about the origins of life, there’s an interview with Oleg Abramov in the next episode of The Universe in Mind podcast.
Ivan Semeniuk is host of The Universe in Mind podcast and a science journalist in residence at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto.