Intelligent Design proponents act as though the fossil record is empty and then suddenly full when they point to the Cambrian Explosion. This is false. Ongoing paleontological study of the Vendian Period, approximately 650 to 540 million years ago, which precedes the Cambrian, shows a marked development of macroscopic life of increasing complexity and which belies the argument that evolutionary theorists are searching for a non-existent explanation for the Cambrian Explosion. They are not.
There is a problem in collecting and identifying large bodies of fossil evidence for the Vendian Period, because most Vendian rocks are igneous and of the few sedimentary deposits, which is where we find fossils, most are either near subduction zones or buried deep beneath other younger sedimentary rocks. This means that most Vendian fossils have already been destroyed by the natural processes of eithter plate tectonics, in which sedimentary rocks approaching subduction zones are "metamorphosed" into compressed metamorphic rocks and/or destroyed completely when suducted and converted into volcanic magma. Or they are destroyed by the pressures of over 500 million years of deposits of younger sedimentary rocks above them, which causes them to metamorphose into various types of metamorphic rock.
But there are Vendian fossils that have been found in a few sites which do demonstrate the existence of more complex life forms than mere protozoan bacteria and single-celled organisms. Included among these are the Ediacara Hills in Australia, the White Sea in the northernmost reaches of Russia, and Mistaken Point in Newfoundland.
So what has been found in the Vendian Period. Here are some examples:
A Cyclomedusa found on the Winter Coast of the White Sea in Northern Russia
A Cyclomedusa is usually considered to be a bottom-dwelling polyp similar to a Sea Anenome, though some believe it may have been more similar to a floating jellyfish.
An Eoporpita, also from the White Sea fossil beds in northernmost Russia
An Eoporpita is also generally considered to be a bottom-dwelling polyp, but is noted for its advanced development of tentacles.
A specimen of Spriggina from the Ediacara Hills in Australia
Spriggina was originally thought to be an annelid, similar to a worm, but most paleontologists now consider it to have been an arthropod and perhaps an ancestor of the Trilobite, which many who comment on what they consider to have been an unlikely development of complex life in the Cambrian Explosion use as an example.
The Intelligent Design proponents who continually return to the argument that the Cambrian Explosion is so sudden and unexpected in its breadth and complexity need to rethink their argument. The evidence from the fossil record grows every day to establish a body of evidence that says there WAS a long period spanning over a hundred million years in which increasingly varied and complex forms of life developed as a precursor to the Cambrian
You can read more at An Introduction to the Vendian Period