Nearly four billion years ago, the Earth was pummeled by asteroids — some as large as the state of Kansas — during an episode known as the “Late Heavy Bombardment.” Now, scientists believe that bombardment phase may have jump-started early microbial life. The results also lend support to the possibility of extreme microbial life on other planets like Mars, and perhaps even on Earth-like planets in other solar systems that may have undergone similar bombardment phases.

Evidence from lunar samples, meteorites, and the scarred surfaces of the Moon and the inner planets show that our solar system was a violent environment during the geologic time phase known as the Hadean Eon, which occurred 4.5 to 3.8 billion years ago. During this time, a large number of impact craters formed on the Moon and the inner planets. Although these craters are visible today on the Moon and on Mars, tectonic forces and weathering have erased physical evidence of the bombardment on Earth.

Many researchers believed that the repeated asteroid impacts during the bombardment would have sterilized the Earth, making it impossible for existing life to survive or new life to emerge. But a new study at the University of Colorado shows that the bombardment would actually have created an abundance of subsurface habitats — such as networks of underground cracks and hydrothermal vents — where heat-loving microbes would have thrived.

Using data from Apollo moon rocks and impact records from the moon, Mars, and Mercury, the researchers built computer models to reconstruct the bombardment. They concluded that habitats for subterranean microbes that live in temperatures ranging from 175-230° F increased on Earth as a result of the impacts, so extreme microbial species would have flourished during the Late Heavy Bombardment. Similar species on Earth today live at temperatures of 250° F, such as the ones discovered in Yellowstone National Park’s hot springs and vents.

Geologic evidence indicates that life was present on Earth at least 3.83 billion years ago, but the researchers say the new findings push back the possible beginnings of life on Earth to well before the bombardment period that occurred 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,” said Oleg Abramov, astrobiologist at the University of Colorado and one of the lead authors of the study.

A paper on the study appears in the May 21 issue of the journal Science.

Image: Timelapse of Asteroid 2004 FH’s flyby (NASA/JPL)