Underground microbes survived the multiple impacts, which only scorched part of the planet's surface and went on to thrive as water temperatures increased, scientists found. This helped life's emergence and early diversification, according to researchers who have modelled how the Earth's crust changed during this time. Although many believe the bombardment, which is believed to have lasted for up to 200 million years, would have sterilised Earth, the new study shows it would have melted only a fraction of it and that microbes could well have survived in underground habitats, insulated from the destruction. Dr Oleg Abramov, of Colorado University, said the findings suggest the microbes could date back to well before the asteroid storm. "These new results push back the possible beginnings of life on Earth to well before the bombardment period 3.9 billion years ago. "It opens up the possibility that life emerged as far back as 4.4 billion years ago, about the time the first oceans are thought to have formed”. The researchers, whose findings are published in Nature, used data from Apollo moon rocks, impact records from the moon, Mars and Mercury, and previous theoretical studies to build three-dimensional computer models that replicate the bombardment. The 3-D models allowed them to monitor temperatures beneath individual craters to assess heating and cooling of the crust following large impacts in order to evaluate habitability. The study indicated that less than 25 percent of Earth's crust would have melted during such a bombardment.