Richard Dawkins, who is also the author of The Blind Watchmaker made a point to consider that the Cambrian period shows up in the process of evolution as a "sudden show up", and by result, animals were basically put in this world without any previous genetic background.
However, he also points out that there is a perfectly normal explanation for the lack of evidence, and it is that the predecesors of what we know to date to be "animals" were soft shelled, and soft shell animals are very uneven leaving their fossilized reminds behind. With that said, it is not that there is a problem formulating a theory of an explosion, but there is indeed a problem looking into the existing records and forming a correlation on how the fossils of certain organisms imprint in rock, sand, or minerals better than others.
To make the long story short, these organisms had been evolving slowly, most of them with a dermis and shell that may have not been strong enough to survive fossilization. As a result, our records show inconsistencies in the pattern of evolution, leading us to believe that there was an onset of genetic material rather than a factual transformation.
The term "Cambrian Explosion" refers to a time, roughly 545 million years ago, when a huge number of species appear for the first time in the fossil record.
This is the cause of some controversy because evolutionary theory suggests (to many) that there should not be a relatively sudden "explosion" of diversity like that.
Theorists who believe that this was not really an explosion argue that there was really a gradual evolution of these species. They argue that there is no previous fossil evidence of that evolution simply because this was so long ago that the fossils have been lost.
So, in this theory, it is not an explosion because these species had already been evolving. All that happens in the Cambrian is that they form fossils that we have been able to find.
Paleontologists have found fossilized embryos that appear to date back to ancient times.
According to the journal "Science" a group of embryos from rocks Doushantuo deposit in China, have been aged as having 580-600 million years and are similar to annelids and molluscs embryos.
If confirmed this, then a complex world has existed already, before the Cambrian explosion . Another group of researchers have studied with X-rays, the internal structure of fossil embryos of more than 500 million years and determined that their internal structure was similar to modern arthropods.
Furthermore, embryos can be preserved if there is happening a quick burial, where oxygen and bacteria can not act. This means that layers were not formed by a gradually and slowly mechanism .