Impacts of large asteroids/comets can lead to the extinction of species which are otherwise very successfull. The last major impact occured about 65 million years ago and killed the dinosaurs. An important piece evidence we have for an asteroid impact being the reason for the mass extinction that took place at that time is a layer of iridium rich soil on the boundary between the Cretaceous and the Tertiary layers (iridium is much rarer in the Earth's crust than it is in asteroids). A few years ago a huge impact crater was found (by gravitational mapping) near the Yukatan peninsula, which is now burried under sediments. It is suspected to be the site of the asteroid impact which occured 65 million years ago.
What is intelligence? The ability to communicate seems to be necessary. The kind of complex language developed by humans does not seem to provide an evolutionary advantage (for early humans in the African savannah) - so how did it evolve? It has been suggested that the skill to throw a rock (at another human / an animal) may have bee the starting point for the ability to put a number of actions into a sequence, which is necessary for complex language. This skill does produce the evolutionary advantage of being able to better defend oneself and develops the required ability to do things in a precise sequence.
Astronomy 201 Homepage
Astronomy Mac Lab Homepage