The Cambrian explosion
explosion is a period of time from around 540 to 490 million years
ago when some of the 35 or so animal phyla (basic body plans) known
today started to appear in the fossil record. The term "explosion" is somewhat misleading as the
Cambrian period stretches over 50 million years, but it is still fairly
rapid in geological and evolutionary timescales.
Before the Cambrian period, the only multi-cellular organisms known from fossil records are the strange shapes from the Ediacaran period. These may be a dead end as they appear not to have left decendents, with the possible exceptions of Bomakellia kelleri and Spriggina.
Although evidence from DNA molecular clocks and trace fossils indicate that multi-cellular creatures were living up to 400 million years before the Cambrian period (Ref 1) they have not been found in the fossil record yet, possibly due to being too small and/or fragile to fossilise (Ref 2.)
Another possible reason for the lack of earlier fossils is that bilateral animals may have evolved in an isolated geographical range (for example, a mid-ocean sea mount) only to out-compete the previously dominant Edicardian fauna when they reappeared from their isolation. This is roughly the idea behind Stephen Gould's theory of punctuated equilibrium.
It is unknown as to what actually triggered the sudden increase in diversity and size during the Cambrian period but possibilities include increased oxygen levels (also Ref 1) or an arms race due to the appearance of shells and teeth (Ref 3) or simply that they found ways to make better use of the resources that existed in their environment.
It is interesting to note that the bacteria mats which dominated the fossil record up to this point started to go into decline around the start of the Cambrian period, possibly because of the new animals grazing on them.
'The Cambrian radiation of bilaterians: Evolutionary origins and palaeontological emergence; earth history change and biotic factors',
Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008): 182