51¶¯Âþ

Not only can comets that hit the Earth damage the surface, but according to
Comets and the Origin and Evolution of Life (Springer-Verlag, £29, ISBN 0
387 94650 0), edited by Paul Thomas, Christopher Chyba and Christopher McKay,
they might even have seeded the planet with the very building blocks of life
itself. Or, later, interfered with the way that life evolved. Scary stuff.