The problem with Cambrian explosion is that it seems to have occurred in way too many separate lineages simultaneously. The most recent common ancestors for animals from Cambrian explosion seems to be go quite far back (that however is another controversial issue, molecular clocks are always controversial), and then suddenly all at once multiple independent lineages undergo period of extremely fast evolution. It’s a problem unresolved since 19th century. Wikipedia—Cambrian explosion
The problem with Cambrian explosion is that it seems to have occurred in way too many separate lineages simultaneously. The most recent common ancestors for animals from Cambrian explosion seems to be go quite far back (that however is another controversial issue, molecular clocks are always controversial), and then suddenly all at once multiple independent lineages undergo period of extremely fast evolution. It’s a problem unresolved since 19th century. Wikipedia—Cambrian explosion