Comet and Asteriod Impacts

The early Earth was bombarded by comets, but geological processes (erosion, lava flows, etc.) have hidden almost all of the scars. The moon was bombarded just as heavily as the Earth, however, and if you look at it through binoculars you can easily see many of the larger craters. It is believed that the biggest thing ever to hit the Earth was actually a planet as big as Mars (which is half the size of the Earth), and that the impact melted the Earth's crust completely. The impact threw out a lot of material which gradually came together to form the moon (which is a quarter the size of the Earth). We are extremely lucky to have a big moon like this, because it helps to keep the Earth stable in the way it spins: other planets such as Mars occasionally tip over so much that their poles can point straight at the sun, and if the Earth was able to behave that way it would probably have been impossible for advanced life to evolve here because of the extreme changes in climate that would have resulted.
indentThe bombardment of the Earth by comets ended long ago, but there are still some comets left which might yet hit us. The most recent one to hit us appears to have broken up into several chunks before slamming into the Earth: this happened well over two hundred million years ago, and it's not clear whether that had any effect on the mass extinction of the world's species of that time: they were already being wiped out by extreme climate change caused by volcanic activity. The dinasaurs were wiped out in the same manner just sixty five million years ago: volcanic activity was again the main factor, though an asteroid impact may have finished the job. Comets are made primarily of water, whereas most of the asteroids are lumps of rock which failed to join up to make a planet, and these normally hang out in the asteroid belt between Mars and Jupiter. The volcanic activity which ended the dinasaur age involved enormous amounts of liquid rock flowing onto the Earth's surface in the region of India, and there is a temple cut into that rock where you can go to look at some of the layers of this solidified lava. There is a weak, very narrow, dusty layer in the middle caused by the asteroid impact, and we can tell this because it contains high levels of iridium, a material common in asteroids but very rare on the Earth. A row of elephant heads are carved into the rock, and the weak layer caused by the asteroid impact happens to run right through them: rather sadly, all the trunks have broken off along this line as a result.