Gabrielle Walker has written a book about the beginnings of complex life on earth (and about the crusty old Geologists who have suggested and defend the so called Snowball Earth theory). The talk left me with a very strong impression that Gabrielle is as in love with crusty old Geolagists as she is with the theory itself.
Briefly, the theory is that the precambrian explosion (500 million years ago when all complex life seems to have begin on earth) was caused by an extraordinary ice age which resulted in all of the earth being covered in ice. The proof is in the rocks. In the juxtopositioning of ice-made rocks and rocks which were assembled in warm climates; in the magnetic allignment of the elements in the rocks suggesting that were created near the equater;