Dupple writes ""If you look at how highly cratered the Moon is — the Earth used to look like that; parts of Mars still do look like that," explained Prof Maria Zuber, Grail's principal investigator from the Massachusetts Institute of Technology, US.
"This period of time when all these impacts where occurring — this was the time when the first microbes were developing.
"We had some idea from the chemistry [of ancient rocks] that Earth was a violent place early on, but now we now know it was an extremely difficult place energetically as well, and it shows just how tenacious life had to be to hang on," she told BBC News.
Prof Zuber was speaking in San Francisco at the American Geophysical Union (AGU) Fall Meeting, the world's largest annual gathering for Earth and planetary scientists.."
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