The early Earth was hit by many comets. Why do some scientists believe that this was important? What is the problem with the theory?

A theory currently being debated by scientists is whether comets may have actually brought the basics of life with them when they hit the young Earth. This is in contrast to the theory based on the experiments of Miller and Urey that showed that the basic building blocks for life could have evolved in the conditions that existed on the early Earth.

Research is being done on comets and meteorites we already have on Earth, drilling into them to see if they are capable of delivering the organic compounds that developed into life. A major problem with the theory is whether the organics could be delivered in a way that left them intact--could they be transported through the cold of space, and survive the impact with Earth? Another area of investigation is whether much of Earth's water arrived in the same way. This is one of the reasons scientists are landing spacecraft on comets. The information obtained from comets while they are still flying through space could help answer the questions about their organic molecule and water content.