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