Sentence:  Alternatively, if animals had a relatively long Precambrian history, why do so many kinds appear abruptly and more or less synchronously in the fossil record during Early Cambran time? Was there an external environmental factor that either prevented animals from becoming fossilizable (i.e., large muscular, and/or mineralized) prior to the beginning of the Cambrian, or did some biological or environmental event trigger the metazoan radiation? Hypotheses that have been advanced to explain these questions tend to focus on either an external environmental control on, or an internal property of, the organisms that participated in the <WSD>Cambrian explosion</WSD>. 
Meaning of the word marked with <WSD> tags:  The relatively rapid appearance, during the Cambrian Period around 541 million years ago, of most major animal phyla, as demonstrated in the fossil record.