Simplify the following text for a Grade 13 native English-speaking person.
Original: Complex organisms gradually became more common in the millions of years immediately preceding the Cambrian , but it was not until this period that mineralised â '' hence readily fossilised â '' organisms became common .
Simplified: Complex organisms became gradually more common in the Proterozoic , but it was not until the Cambrian that mineralised â `` hence readily fossilised â `` organisms became common .