The late heavy bombardment is a period of highly intensified asteroid impacts taking place between 3920 and 3850 million years ago (mya). Life itself was formed either a few dozen million years before the late heavy bombardment or near its conclusion.
Evidence for the late heavy bombardment was found by Apollo astronauts visiting the Moon. Out of all the rock samples they brought back, many of them clearly remelted after asteroid impacts, and these remelting events were clustered with ages 3920 million years or a hundred or so million years younger. This period of time thereafter became called the "lunar cataclysm." Asteroids from the Moon have been shown all to have the 3920 million year age limit, but they are not clustered around the short time thereafter, having ages ranging from 2500 to 3900 mya. By extension, it was inferred that Earth, Venus, and Mercury would have also experienced a substantial increase in asteroid impacts during this period.
If the late heavy bombardment really happened, then this is the damage that likely would have been done to Earth:
- 22,000 or more impact craters with diameters of over 20 km
- about 40 impact basins with diameters of about 1000 km
- several impact basins with diameter of about 5,000 km
Serious environmental damage would have occurred every 100 years, making the planet a tough place to live, although early life did emerge during this time. Although the Earth had already cooled and solidified prior to this period, all elements of this geological era were erased, because the late heavy bombardment evidently destroyed most of the crust and therefore the oldest rocks to be dated have an age of 3850 million years. The period prior to that is known as the Hadean, after it, the Archean. The oldest bacterial fossils do not appear in the record until 3460 million years ago, but most who study early life believe that it originated several hundred million years before this.