Darwin's Great Worry

When Charles Darwin wrote On the Origin of Species, he and most paleontologists believed that the oldest animal fossils were the trilobites and brachiopods of the Cambrian Period, now known to be about 540 million years old. Many paleontologists believed that simpler forms of life must have existed before this but that they left no fossils. A few believed that the Cambrian fossils represented the moment of God's creation of animals, or the first deposits laid down by the biblical Flood.

Darwin wrote:

"the difficulty of assigning any good reason for the absence of vast piles of strata rich in fossils beneath the Cambrian system is very great," yet he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy."

He pondered the sudden appearance of many groups (i.e., Phyla) in the oldest known fossiliferous strata:

"Consequently, if my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer. (note: Silurian was then what Cambrian is now)"

And worried about the implications for the validity of his theories:

". . . . . these difficulties and objections may be classed under the following heads:- First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?

Since Darwin's time, the fossil history of life on Earth has been pushed back to 3.5 billion years before the present. Most of these fossils are microscopic bacteria and algae. However, in the latest Proterozoic - a time period now called the Vendian, or the Ediacaran, and lasting from about 650 to 540 million years ago - macroscopic fossils of soft-bodied organisms can be found in a few localities around the world such as the Burgess Shale in Canada, Chengjiang in China, and in Utah, confirming Darwin's expectations.